This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.
Chapter 5
Astronomical Circumstances
5.1 Introduction
The initiation of life and the progress of biologicalevolution are probably extremely rare things in theUniverse. First you need a planet upon which life cantake root. Then, a star provides the warmth needed tokeep planetary water from freezing. But some planetscan be so close to the star that their reserves of waterwill evaporate from the heat. While other worlds willorbit at such great distances that their icy coating willnever be melted into seas of life by a warming star. Thecombinations of circumstances that planets may findthemselves in are, well, astronomical. There is nothingwe can do about that. But we can find a reasonableway to cope with such complexity so we can find theanswers that concern us about a planet’s prospect forlife and evolution. That is the purpose of this chapter.
The ultimate value of this chapter is the presentation ofthe three following methods:
1. An integrated method for predicting best casescenarios for biological evolution on planets orbitingstars of different masses
2. An integrated method for predicting thehistorical profile of a planet in relation to its star’shabitable zone
3. A method for assessing a planet’s risk ofimpact from comets and asteroids
To a planet, the most influential outsider is its primarystar—the star it orbits. The star provides the planetwith radiant light and heat. And its powerful gravitykeeps the planet from drifting off and getting lost inspace. As part of this exposé, we must consider thesestellar properties if we are to consider planets ascandidates for life.
5.2 The life history of a star
Planetary biology theory ultimately wants to be able toestimate the maximum amount of time available forbiological evolution for planets under differentastronomical circumstances. For example, how muchtime would be available for a planet orbiting around alarge star compared to a planet orbiting around a smallstar? It turns out that there is a dramatic differencebecause of the properties of stars.
The stars that you and I see twinkling in the night skyare visible to us because they are ferociously ‘burning’hydrogen in unimaginably powerful internal nuclearfusion reactions. The result of those reactions is thebrilliant dots of light that decorate the night sky.Astrophysicists call this burning stage the ‘mainsequence’ stage of a star’s life. The main sequencestage is the longest of several possible stages in a star’soverall life. Stars go through the following stages duringtheir lifetimes (see fig. 1, Life stages of a star):
5.2.1 Protostar
This is a large and massive body of gas that hasaggregated into a collapsing ball, driven by gravity. It isdim although it may emit some infrared radiation latein its formation. As the collapse continues the internalpressure and temperature becomes high enough tostart nuclear fusion reactions. When they begin, thestar enters the next stage of its life, the main sequencestage.
5.2.2 Main Sequence Stage
For those of us who don’t study stars, this is the stagewith which we are most familiar. Our sun and the starswe see at night are in the main sequence stage. Duringthe main sequence stage, the star’s interior is occupiedby violent nuclear fusion reactions. The electromagneticenergy (mostly infrared radiation, visible light,ultraviolet radiation) that results from these reactionsmakes its way to the star surface, through the star’satmosphere and out into space. The result is aluminous body that radiates energy in all directions.This is the energy that can illuminate and warm thestar’s orbiting planets. The vigor of the internalreactions and the availability of hydrogen fuel limit theduration of the main sequence stage. Big stars tend toconsume their fuel quickly and have short mainsequence stages. Small stars burn their fuel slowly andhave long main sequence stages. Once the fuel isburned, the star enters into the next stage of its life.
This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.5.2.3 Giant Branch
Once its hydrogen fuel is mostly consumed, a medium-sized star like our own will expand outward to becomea red giant. During this stage (which will happen to oursun in about 5 billion years), our sun will expandoutward to engulf Mercury, Venus and possibly Earth.Compared to the main sequence stage, the giantbranch is short-lived.
5.2.4 Aftermath
At the end of the red giant stage, a medium-sized starwill eject its outer shell and then collapse for one finaltime becoming a white dwarf. White dwarves are whatremains of the star’s core and shell after it hasconcluded all of its nuclear burning. With no internalnuclear force to buoy the star outward or generateheat, the star’s mass collapses and begins to cool.
Stars whose masses differ greatly from that of the sunmay have much different fates. For example, smallstars do not go through the giant stage but simplycollapse into white dwarves after the main sequence.The death throes of large stars are the most dramatic ofall. After large stars finish their giant branch, theyexplode in what is called a supernova. This explosioncan utterly destroy the star or, for really massive stars,the aftermath of the supernova can leave behind anextremely massive and dense neutron star or even ablack hole.
For a more detailed description of stellar evolution,consult Iben (1967) and Sackmann et al. (1993).
Main Sequence Stage(star shines brightly)
Giant Stage(short-lived) Explosion and Aftermath
InterstellarHydrogen
Cloud
Protostar
Brown Dwarf(too small for
main sequence)
0.08 to 0.26 M
0.26 to 1.5 M
1.5 to 5 M
5 to 30 M
< 0.08 MWhite Dwarf
White Dwarf
Planetary Nebula
Star casts off its outer shell.Core collapses and begins to cool.
Star explodes and is utterly destroyed
Star explodes and leaves behind a massive core
Core collapses and begins to cool
Nebular Cloud
Neutron Staror
Black Hole
M represents the mass of a star measured in solar masses, where our sun has a solar mass of 1M .A star twice as massive as our sun would have a solar mass of 2M .
This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.stars based on their brightness (magnitude),temperature, radius, luminosity, mix of colors (spectralclass), and mass. For our purposes, we are going to usemass as our basis of stellar comparison.
The term, mass, is used by physicists to describe theproperty of an object that gives the object weight in agravitational field. On Earth, objects of great mass alsohave great weight, because gravity acts on mass.Objects of low mass have low weight. But out in space,where there is little or no gravity, the same objects willstill have their original mass although they may beweightless. Just watch the space shuttle astronauts tryto move a weightless satellite. It’s really hard becausethe satellite is very massive compared to the astronaut.So, mass can be used as a convenient way to describethe ‘bulk’ of an object anywhere.
The mass of a star (stellar mass) influences three maincircumstances that we are concerned about: 1) stellarluminosity and the position of circumstellar habitablezones; 2) the time that the star will stay in its mainsequence stage; 3) the orbital period for a planet at anygiven orbital radius.
5.3 Stellar Luminosity and Circumstellar Habitable Zones
Stars generate immense amounts of energy, which theyradiate to space in all directions. Astrophysicistsconsider a star’s total energy output as its luminosity.The star’s energy mostly is in the form ofelectromagnetic energy (usually as variable mixes ofinfrared, visible light, ultraviolet radiation).
Stars are a major source of heat in an otherwiseshockingly cold Universe. When a star’s radiationstrikes a planet, if it is absorbed and not reflected, itcan cause the planet’s surface to warm up. The closer aplanet is to a star, the more stellar radiation it will
receive each moment, so the warmer the planet will get(all other things being equal). This being the case,around all stars there is a distant zone in space wherethe star’s radiation levels would cause the surface of anEarth-like planet to be not too cold for liquid water, andnot too hot for liquid water. On Earth, water exists inthe liquid state at temperatures between 0° and 100° C(32°- 212° F) at sea level. If a water-rich, Earth-likeplanet orbits a star within this zone, liquid surfacewater would be possible (if we consider temperature asthe primary constraint). Since our unavoidable biasrecognizes that liquid water is essential for life, worldsthat possess it would be the most habitable – especiallyif we compare such moderate worlds with extremelycold or hot counterparts such as icy Pluto or bakingMercury.
Huang (1959) first described this region in space as thehabitable zone. The location of the habitable zonedepends on the magnitude of a star’s luminosity. Forstars with low luminosities, habitable zones will becloser to the star. Stars with great luminosities willhave more distant habitable zones.
Anything that influences stellar luminosity alsoinfluences the habitable zone. Mass matters. Forexample, the more massive the star, the more luminousit will be, and the farther out the habitable zone. Also,during the main sequence stage, stars graduallybecome more luminous as they get older. Habitablezones migrate outward. So the habitable zone at theend of the main sequence stage will be farther outcompared to its original location at the beginning of themain sequence stage. The speed of migration also willvary with star mass. Big stars brighten and die outfaster than do small stars. So habitable zones migrateslowly away from small stars and last a long time. Buthabitable zones move out quickly from big stars andare short-lived.
This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.These are the basic concepts that govern this limitedsuite of astronomical circumstances. But thinking ofthem only conceptually confuses me. I need a set ofquantitative benchmarks to help me bettercomprehend the consequences of differentcombinations of theses phenomena. That is thepurpose of the next section.
5.4 Preparations toward developing best case scenarios forbiological evolution on planets orbiting stars ofdifferent masses
I want to be able to compare the habitable zones forstars of different masses. Particularly, I want tocompare each hypothetical best case scenario forhabitable zone planets around stars of differentmasses. In order to accomplish this task, I must do thefollowing:
Construct historical profiles for habitable zonesaround stars of different masses.
I must develop a method for accomplishing thistask.
Develop a method for establishing the best casescenario conditions for biological evolution.
I will do this by identifying the orbit of greatestopportunity with regard to the habitable zone andby considering the influences of tidal braking.
I will start by developing a method for constructinghistorical profiles for habitable zones around stars ofdifferent masses. In order to do this, I must considerhow the habitable zone changes during the star’s mainsequence life.
5.5 Considering the historical profile of habitable zonesduring the star’s main sequence life
Because stars gradually become more luminous duringtheir main sequence lives, habitable zones mustmigrate outward. The phenomenon of migratinghabitable zones presents an interesting circumstanceto a star’s family of planets. After a time, planets thatwere once inside the habitable zone may findthemselves left behind to bake as the habitable zonemakes its way outward. And frozen outer planetsoriginally beyond the reach of the habitable zone mayeventually bask in ice-melting warmth as the habitablezone creeps out to them.
As there could be endless combinations of starproperties and planetary orbits, it becomes challengingto deal with these phenomena in a consistent waywithout getting confused. Therefore, I am proposing aquantitative method that considers these phenomenain terms of several benchmarks that can be used forquick comparisons.
For our purposes it isn’t necessary to be able to plotthe precise state of these astronomical phenomena atevery moment in time. I just want to be able to seetrends and I want to be able to make quickcomparisons between rough sets of circumstances. Inorder to get ‘on talking terms’ about the variety
Figure 3. View from a planet starward (closer towards its primarystar) to the habitable zone.
Figure 4. View from a planet in the habitable zone.
Figure 5. View from a planet beyond the habitable zone.
This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.astronomical circumstances possible for planets andstars, I am considering the placement of the habitablezone around a star at the following three milestonemoments in the star’s main sequence lifetime (fig. 2):
1) at Zero Age main Sequence (ZAMS). This is themoment when the star begins to ‘burn’ hydrogen ininternal nuclear fusion reactions. The mainsequence stage is the span of years in a star’s lifethat is dominated by this internal activity. It iswhen the star ‘shines’, like our sun for example.
2) At Habitable Zone Transit (HZT). This is themoment when the inner boundary of the habitablezone has migrated outward after many millions orbillions of years such that it now is the samedistance from the star as was the zero age mainsequence outer boundary of the habitable zone. Atthis moment, the habitable zone has transited itsentire zero age main sequence range such that itnow lies adjacent to and beyond its original range.
3) At Main Sequence End (MSE). This represents thefinal ‘moment’ of the main sequence stage. Thehabitable zone will continue to migrate outwarduntil the end of the star’s main sequence stage. Thehabitable zone at MSE is the farthest and finalposition of the habitable zone.
Habitable zone at end of main sequence stage
Habitable zone athabitable zone transit
Habitable zone atzero age main sequence
Orbit ofGreatest Opportunity
}}}
HotNon-Start
WarmStart Cold Start
Inner Outer Cold
Non-Start
Figure 6. Historical profile of a star's habitable zone.
5.6 Calculating the location of habitable zone at zero agemain sequence (ZAMS)
The placement of the habitable zone around a star is aconsequence of the star’s luminosity. If a star is veryluminous, its habitable zone will be far out. Stars withweak luminosity will have habitable zones close in. Inany case, in order for me to calculate the location of the
habitable zone around a star at zero age mainsequence, I must know its luminosity. If I do not knowits luminosity, then I can use its mass to estimate itsluminosity.
The science of astrophysics seeks to understand howstars work. Astrophysicists try to explain how starsform, why they shine the way they do, and why some ofthem eventually explode. Astrophysics involves anintricate combination of chemistry, conventionalphysics, nuclear physics and astronomy.Astrophysicists now understand stars to be verycomplex phenomena such that stars can haveelaborate internal structures and dynamic lives.Thankfully, it is not necessary for us to understandstars in such detail. In brief, there are two properties ofstars that concern us here: 1) The luminosity of a staris a consequence of the star’s mass; and 2) Howluminosity increases as the star ages.
First, let’s see how a star’s mass affects luminosity. Bigstars are very luminous and small stars are lessluminous. Remember that luminosity is a description ofthe star’s overall energy output per unit time. A big starwill be more luminous than a small star mainlybecause the bigger star simply is burning morehydrogen fuel at any given moment, like a bonfire isburning more wood at a given moment compared to asingle match.
Using the sun as our best-studied star, Martyn Fogg(1992) has developed a simple expression showing thisrelationship at the beginning of a star’s main sequencelife.
L
MML
n
ZAMS 71.0
L represents stellar luminosity normalized to our sun’spresent luminosity (L¤). For example, our sun’s presentabsolute energy output is about 3.86 x 1033 erg/second(Lang 1992, p. 103). Rather than using this complexenergy expression, astrophysicists invented a new unit,the L¤ where the sun’s present energy output simplyequals 1L¤. This makes it easier to compare theluminosities of different stars as simple fractions ormultiples of our sun’s luminosity. For example, thevery bright star, Sirius A, has a luminosity that is 23times greater than that of our sun. So we express itsluminosity as 23L¤. Meanwhile, Alpha Centauri B’sluminosity is only 45% of our sun’s luminosity. So weexpress its luminosity as 0.45L¤.
ZAMS means Zero Age Main Sequence. This is a timemarker that indicates we are considering the state ofthe star at the very beginning of its main sequencestage.
This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.M represents the mass of the star we are considering,normalized to our sun’s mass (M¤). As with luminosity,it is far easier and more meaningful to compare starsusing our sun as the known standard. For example,the star, Y Cygnus, is about 17 times more massivethan our sun. So we express its mass simply as 17 M¤ ,rather than 3.36 x 1034 g.
n represents a variable that quantifies the degree towhich luminosity will differ between stars of differentmagnitudes. The n variable is an outcome of stellarmodel calculations that include the star’s centraldensity, core temperature, core mean molecular weight,and interior opacity (Iben 1967). The value for n is notthe same for stars of different masses. Taking fromIben (1967), Fogg (1992) consolidated n values into thefollowing two categories:
n = 4.75 for stars 0.7 to 2 M¤
n = 3.75(M/ M¤) + 2.125, for stars 0.1 to 0.7 M¤
Once we have calculated the LZAMS, we can thendetermine the boundaries of the habitable zone at zeroage main sequence. Using the luminosity-habitablezone relationship presented by Whitmire and Reynolds(1996), we can calculate the inner and outerboundaries for the habitable zone at zero age mainsequence as
1.1ZAM S
ZAMSi
Lr and53.0
ZAMS
ZAMSo
Lr
where:
ZAMSir represents the distance from the star to the inner
radius of the habitable zone at zero age main sequence,in astronomical units (AU)
ZAMSor represents the distance from the star to the outer
radius of the habitable zone at zero age main sequence,in astronomical units (AU).
ZAMSL represents the luminosity of the star at zero age
main sequence, in solar luminosities (L¤)
The value 1.1 represents the stellar flux that definesthe inner boundary of the habitable zone [taken afterKasting et al. (1993)].
The value 0.53 represents the stellar flux that definesthe outer boundary of the habitable zone [taken afterKasting et al. (1993)].
The Astronomical Unit (AU) is a unit of distance that isequal to the average distance between the Earth andthe sun. So, a planet that is three times as far from thesun as Earth would have an orbit of 3 AU, which iseasier to work with than 4.485 x 1013 cm.
The model I am using estimates the location of thehabitable zone for an Earth-like planet as a function ofluminosity and stellar flux. The term, stellar flux,describes the quantity of stellar energy that strikes adistant object each second. Stellar flux is greatest nearthe star and smallest far from the star. You probablyhave experienced the energetic flux from a campfireand have adjusted your seating position accordingly.As energy radiates outward from the star (or campfire),it dissipates over larger and larger areas, becoming lessintense in the process. There is no material uncertaintyregarding this phenomenon. However there isuncertainty regarding the ranges of solar flux thatwould define a habitable zone around a star.
For example, Kasting, Whitmire and Reynolds (1993)studied the placement of habitable zones defined byseveral different combinations of solar fluxes. Based ontheir work, if habitable zones are defined too broadlyseveral things can happen. Planets near the innerboundary of the broadly defined habitable zone arelikely to lose all of their surface water in a short periodof time due to a runaway greenhouse effect. And the iceon planets near the outer boundary of the broadlydefined habitable zone may not melt. This is because,unlike a black body, ice and carbon dioxide cloudstend to reflect incoming stellar energy. So, higher fluxeswould be necessary to overcome these reflectivetendencies.
Because of these uncertainties, I am using the mostconservative stellar flux values considered by Kasting etal. (1993). I hope this will increase the reliability of themodel such that any Earth-like planet within thehabitable zone would have liquid surface water for theduration of its residency in the habitable zone (all otherthings being equal).
The luminosity of the star gradually increases duringthe main sequence. This happens generally as aconsequence of the two following things happening inthe star’s core:1) There is a gradual increase in the mean molecular
weight of the core. The mean molecular weight inthe core increases as nuclear fusion reactionssteadily convert hydrogen into helium. As thisreaction continues for millions or billions of years,the proportion of helium in the core increases, andthe proportion of hydrogen in the core decreases.This shift increases the core’s mean molecularweight. Böhm-Vitense (1992, p. 119) reported therelationship between Luminosity and meanmolecular weight to be
This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.
6Lµ represents the mean molecular weight of the star.
According to this relationship, luminosity isproportional to the mean molecular weight raisedto the sixth power. This means that even a smallincrease in the core’s mean molecular weight willresult in much larger increases in stellar output.
2) As a consequence of the increase in the core’smean molecular weight, there is an accompanyingreduction in outward pressure. This is because theconversion of hydrogen to helium reduces the totalnumber of particles in the core and, for a giventemperature, this reduces the outward pressure.So, the core contracts a bit, converting gravitationalenergy into electromagnetic energy and heat. As aresult of this contraction, the core gets hotter, andthe star becomes slightly more luminous. Thiscontraction process proceeds gradually during thecourse of the star’s main sequence life.
In other words, the star gets more luminous as it getsolder. As the star gets more luminous, the habitablezone moves outward from the star. Eventually, thehabitable zone will migrate outward completely beyondthe original, zero age main sequence habitable zone.When this happens, this is the moment of habitablezone transit.
Calculating the location of habitable zone at habitablezone transit (HZT)
Determining the location of the inner boundary of thehabitable zone at the moment of habitable zone transit(HZT) is simple. It is at the same place as the outerboundary of the original, zero age main sequencehabitable zone. So,
ZAMSo
HZTi rr
HZTir represents the inner boundary of the habitable
zone at habitable zone transit.
Before I can determine the outer boundary of thehabitable zone at HZT, I must know the luminosity ofthe star at the moment of HZT.
Since the inner boundary of the habitable zone at HZTequals the outer boundary at ZAMS, we have
1.1HZT
ZAMSo
HZTi
Lrr
where 1.1 represents the magnitude of stellar flux atthe inner boundary of the habitable zone.
Substituting and rearranging, we get an expressionthat allows us to estimate the luminosity of the star athabitable zone transit.
2
1.1
ZAMSoHZT rL
Now that we have a way of estimating the luminosity atHZT, we can calculate the outer boundary of thehabitable zone at HZT.
Since
53.0HZT
HZTo
Lr
using the above expression for LHZT, we get
53.0
1.12
ZAMSo
HZTo
rr
Calculating the location of habitable zone at main sequence end(MSE)
Before we can calculate the location of the habitablezone at main sequence end (MSE), we need todetermine the star’s luminosity at MSE. First I have todetermine how long the star will be in main sequencestage. Fogg (1992) supplies the expression
n
M S MMT
1
10
see above for the values for n.
Once I know how long the star will be in mainsequence, I can calculate its luminosity at MSE usingFogg’s (1992) expression adapted from Gough (1981).
This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.I can now calculate the boundaries of the habitablezone at MSE.
1.1MSE
MSEi
Lr
and
53.0M SE
M SEo
Lr
5.7 Determining the orbit of greatest opportunity (OGO
)relative to the habitable zone
Now that I have a way for developing an historicalprofile for a star’s habitable zone, I can find out thebest case scenario for the star. By ‘best case scenario’, Imean the situation in which a hypothetical planetwould have the greatest opportunity for life to start andevolve. This has a lot to do with the planet’s orbitalposition.
Around each star, there is an orbit of greatestopportunity in which the conditions are best for thestart and evolution of life. It serves as a benchmark forperforming comparisons. Once I know how to identifythe orbit of greatest opportunity around any star, I cancompare stars of different masses in terms of their bestcase potential to support life. This section examines mymethod for identifying the orbit of greatest opportunity.
One of my ultimate goals in this chapter is to developtools that will help me assess the maximum time thatwill be available on a planet for biological evolution.That is, for any particular kind of star, how much timewould life have available to evolve? I am assuming thatthe best conditions for life would occur on the habitableplanetary surface environment which is capable ofsustaining liquid water. Two main factors influence thelongevity of a planet’s habitable surface environment.They are:1. The amount of time that the planet resides within
the circumstellar habitable zone2. The vigor and longevity of the planet’s internal heat
engine
A planet could reside maximally within the habitablezone if its orbit was anywhere between the habitable
zone’s outer radius at zero age main sequence (ZAMSor ),
and the habitable zone’s inner radius at main sequence
end (MSEir ). Any planet orbiting within this range will
reside in the habitable zone for approximately the samespan of years. The difference is timing. Planets orbiting
closer in at ZAMSor will start their residency as soon as
they form. Planets farther out will have to wait theirturn as the habitable zone slowly makes its way out tothem. The question is, although their habitable zoneresidencies will be about the same amount of time, dothese orbital circumstances represent equivalentopportunities for the maintenance of a habitablesurface environment? I argue probably not. Thereasons have to do with the vigor and longevity of theplanet’s internal heat engine, and the problems withcold-starting planets.
I reason here that a terrestrial planet’s innate potentialto produce and sustain liquid surface water is at itsprime at the beginning of its life and becomes lesseffective later on.
While residing in the habitable zone, liquid surfacewater is possible on a planet in part because of thefollowing:1. First you need a source of water. Volcanic
outgassing releases water vapor into theatmosphere, which condenses and falls as rain.The rate and duration at which this happensdepends on how vigorous the planet’s internal heatengine is. The more vigorous and long-lived theplanet’s internal heat engine, the more and longer-lived volcanic outgassing will occur.
2. Then you need to recycle and re-emit removedgases. Plate tectonics and associated volcanismrecycles atmospheric gases that unavoidablybecome solidified on the ocean floors. Recyclingputs them back into the atmosphere which restoresthe greenhouse effect and maintains atmosphericpressure. These recycling activities also depend onthe vigor and endurance of the planet’s internalheat engine.
So, the planet’s internal heat engine can contribute inmany ways to the persistence of a habitable surfaceenvironment. But the internal heat engine has a finitelife. Like people, it is most vigorous in its youth andless so as it ages. This being the case, it is reasonableto consider that the greatest opportunity for thedevelopment and maintenance of a habitable surfaceenvironment probably occurs when the onset ofhabitable zone residency coincides with the start of theplanet’s internal heat engine.
Another issue along these lines has to do with theproblems posed by cold-starting a planet. A warm starthappens when the planet starts out in the habitablezone at zero age main sequence. A cold start is whenthe planet starts out beyond the original habitable zoneand is later overtaken by it as the habitable zonemigrates outward.
This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.Kasting, Whitmire and Reynolds (1993), Kasting (1996),and Whitmire and Reynolds (1996) explored thisquestion in detail. Their models suggested that despitethe encroachment of the habitable zone, cold startplanets might resist the star’s increased warmingefforts. This is because of the presence of surface iceand carbon dioxide clouds that increase the reflectance(albedo) of the planet. A cold start planet could resistwarming far into its habitable zone residency. And thiswould reduce the total amount of time possible for thedevelopment and maintenance of a habitable surfaceenvironment.
Based on these considerations, it seems reasonable toplace the Orbit of Greatest Opportunity (OGO) at thehabitable zone’s outer boundary at ZAMS.
ZAMSoGO rOrbit
The whole point about habitable zones is that theyrepresent a window of opportunity for the existence ofliquid surface water and maybe life. But thisopportunity must coincide with the planet’s innatestate of preparedness. Although warmth from the sundoes play a critical role in opening that window, thereare other processes on the planet itself that influencethe habitability of the planetary surface environment.That is, just because a planet resides in the habitablezone, there is no guarantee that it will be ‘habitable’ forany moment of its residency, much less the duration ofits residency. All habitable zone residency means isthat if the planet has any endowed potential to produceand support liquid surface water and life, now is thetime to do it. But if the planet is unable to marshal theresources and processes that are necessary to supportthis effort, then no amount of stellar accommodationwill make it happen. In any case, it seems that a planethas its greatest chances for habitable success at thebeginning of its career.
See chapter ??? (not in this edition) for moreinformation regarding the vigor and longevity ofplanetary internal heat engines.
5.8 Calculating the maximum amount of time in thehabitable zone for a planet at the orbit of greatestopportunity
Assuming that
ZAMSoGO rOrbit
I need to know how much time it will take for the innerboundary of the habitable zone to migrate to the pointin space originally occupied by the outer boundary ofthe habitable zone at ZAMS. Remember that since
ZAMSo
HZTi rr and since
1.1HZT
HZTi
Lr
and
53.0ZAM S
ZA M So
Lr
then53.01.1
ZAMSHZT LL
reorganizing, we get
ZAMSZAMS
HZT LLL
075.253.0
1.1
Now by substituting ZAMSL075.2 for L
T in
33.110045.0
MSTT
ZAMST eLL
where T represents the time for habitable zone transit,we get
33.110045.0
075.2 MS
HZT
TT
ZAMSZAMS eLL
Solving for THZT, we get
102.1633.1
MSHZT
TT
which works out to be roughly
81.0 MSHZTHZ TTT
According to this method, a planet that orbits at theouter boundary of the habitable zone at ZAMS (orbit ofgreatest opportunity), will reside in the habitable zonefor about 81% of the star’s main sequence life. Afterthis time the planet will lie starward (toward theplanet’s primary star) of the migrating habitable zoneand will face the consequences of overheating.
This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.And so, we have an extremely simple expression forcomparing the best case scenarios for stars of differentmasses.
The time that a planet spends in the habitable zone canconstrain the amount of time available for biologicalevolution. This constraint becomes more limiting forplanets around large stars because large stars burnout much faster. Planets orbiting smaller stars willhave much longer residencies. But all is not so rosy forplanets orbiting small stars. Remember that habitablezones around small stars are closer in. This closenessexposes the residing planets to stronger gravitationaltugs from the star. And this can slow down the rotationof the planet. So, we have to consider this tidal brakingand its effect on planets at the orbit of greatestopportunity.
5.9 Considering the influence of tidal braking
Tidal braking is the phenomenon in which a planet’sspin is slowed down because of gravitational forcesbetween itself and the parent body. By parent body Imean the dominant body of an orbital system. Forexample, the Earth is the parent body in the Earth-Moon orbital system. And the sun is the parent body tothe planets of the solar system.
As a planet orbits a star, gravitational forces from thestar pull on the planet causing the planet to bulgeslightly. If the planet has oceans, the effect is moreobvious. Tidal forces stretch the water in the directionof the gravitational attraction. This causes the sea levelto rise. The change of tides on Earth is a consequenceof the Earth rotating through this tidal bulge.
Rotating through a tidal bulge drains, or dissipates aplanet’s rotational momentum (spin angularmomentum). In other words, tidal forces act as a brakethat can slow down a planet’s spin. And given enoughtime, tidal braking can slow planetary spin until theplanet’s rotational period matches its orbital period.When this happens, the planet is tidally locked, and isin a synchronous orbit.
[Note: The spin angular momentum reduced by tidalbraking is converted into orbital angular momentum.So, although the planet may be rotating more slowly, itis moving through space at a faster pace. By movingfaster in its orbit, the planet’s orbit will stretchoutward. And so it goes. For now, we are going toignore this complication because it probably makeslittle difference in the overall orbital circumstances ofplanets in the habitable zone.]
When a heavenly body is in a synchronous orbit, italways shows the same face to the parent body. Forexample, our Moon always presents the same face toEarth. Standing on Earth, we never get to see the otherside. This is because the Moon is tidally locked and itsrate of rotation is synchronized with its rate of orbit. It
orbits the Earth every 27.3 days and it rotates on itsaxis every 27.3 days. Other planets in our solar systemalso have moons, and over half of them are insynchronous orbits. But let’s get back to planets andstars.
If a planet is in a synchronous orbit with its star, thiscould have grave consequences on the planetarysurface environment. Dole (1964) suggested that aplanet in synchronous orbit with a star might not behabitable because of extremely high temperatures onthe light side and extremely cold temperatures on thedark side. In an extreme case, the coldness of the darkside would create a gas sink that would deplete theatmosphere of its gases. It could do this by freezing theplanet’s atmospheric gases and depositing them asliquids or solids on the planetary surface.
The situation is not so tidy. Haberle et al. (1996)suggest that if the planet’s atmosphere is thick enough,it could transport heat to the planet’s dark side. Thiscould raise temperatures enough to preventatmospheric freezing. And air masses emerging fromthe dark side could cool the hot light side.
And there are other circumstances that mightcounteract the effects of tidal braking. Whitmire andReynolds (1996) present the case of Mercury as anexample. Mercury is so close to the sun that if weconsidered Mercury-Sun tidal forces alone, we wouldexpect it to be locked in a synchronous orbit. But it’snot. Mercury completes about 1.5 rotations for everyorbit. This surprising situation could be because ofMercury’s orbital resonance with Venus (I am notgoing to explain orbital resonance here). And speakingof Venus, Dobrovolskis (1980) argues that tides in itsthick atmosphere are sufficient to inhibit tidal brakinginfluences from the sun.
Despite the presence of complicating factors, we canmodel a simple situation for planets occupying the orbitof greatest opportunity. Remember, I have chosen thisorbital location to serve as the point of reference in thisdiscussion. This section has demonstrated that weneed to consider a star’s tidal braking action if we areto assess the maximum time for biological evolution.Below, I present a method for determining how long aplanet has before it becomes tidally locked. Althoughothers have pointed out that tidal locking might not besuch a bad thing, or that a planet could avoid from thisfate, I am assuming that tidal forces overall will prevail,and that synchronous orbits will severely impact thehabitability of the planetary surface environment.
So how much time is available? Peale (1977) examinedthis problem from the point of view of orbital positionand derived the expression
This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.
3/16/1
027.0 MQTPr SOo
T
where
rT is the orbital distance from the star, in centimeters
Po is the original rotational period of the planet, inseconds
TSO is the amount if time it would take for the star’stidal braking to slow the planet’s rotational period tothe point where it is synchronized with the planet’sorbital period, in seconds
Q is the solid body energy dissipation function
M is the mass of the star, in grams
Rearranging to solve for TSO, we get
o
T
SO PM
rQT
6
3/1027.0
Since we are going to fix the orbital distance of ourhypothetical planet at the orbit of greatest opportunity,then
GOT Orbitr
then we customize the equation to become
o
GO
SO PM
OrbitQT
6
3/1027.0
Now we need to establish some assumptions in order towork this equation. All values must be expressed inCGS units so we will assume that
Orbit GO is calculated elsewhere based on the location ofthe habitable zone’s outer radius at zero age mainsequence. To express it in centimeters multiply thedistance in astronomical units by 1.5 x 108 (this is thenumber of centimeters in one astronomical unit).
Po is 48,600 seconds (this is 13.5 hours). I selected thisvalue to be consistent with Whitmire and Reynolds(1986).
t will emerge as a number expressed in seconds. Toconvert to years, divide this value by 3.15 x 107 (This isthe number of seconds in one year).
Q is 100. Based upon Burns (1986) and concurrenceby Whitmire and Reynolds (1986).
M is the variable I am testing here. It must be enteredinto the equation in gram units. To convert solarmasses to grams, multiply the mass in solar masses by2 x 1033 (This is how many grams are in one solar massunit).
5.10 Determining the planet’s orbital period
I want to understand how the mass of the starinfluences the orbital period of a planet at differentorbital distances. The ‘orbital period’ is the time it takesa planet to complete one orbit around a star. Orbitalperiod has the potential to profoundly influence thepace of evolution on a world. On Earth for example,reproductive events for most plants and animals arelinked to the changes in the seasons (which occurbecause of the tilted planet orbiting the sun). As we willsee later in chapter 10, the reproductive eventrepresents the moment in time when living thingsintroduce innovations. Robust and directional evolutiondepends on frequent and fresh supplies of innovations.Using the evolution of life on Earth as a model,innovations might appear more frequently on worldswhere seasons cycle rapidly (worlds with short orbitalperiods). According to this thinking, we should considerthat (all other things being equal) evolution couldhappen faster on planets with short orbital periods andmore slowly on planets with long orbital periods.
Let’s deal with the idea of star mass and planetaryorbits. A star’s mass will influence the way otherheavenly bodies move around it. Back in the year1667, and while a young 25 years of age, Isaac Newton(1642-1727) theorized that an object’s mass isresponsible for the existence of gravitational attraction,or gravitational force that holds orbiting bodies inplace. The inspiration for his thinking was his curiosityabout the orbit of Earth’s moon. What was it that keptit from flying off into space? Newton reconciled histheory of gravitational attraction with the empiricalobservations of astronomer Tycho Brahe and JohannesKepler ingenious geometric method for predicting themotions of worlds.
There are two things that can affect the strength of thegravitational force between two heavenly bodies: 1)closeness; and 2) the amount of mass. Newton’s theoryexplains that as two heavenly bodies get closer to eachother, the gravitational force between them getsstronger. He also explained that as the mass of eitheror both heavenly bodies increases, so does thegravitational force between them.
This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.Here is the problem: If gravity is pulling a star and aplanet together, what keeps them from colliding? Theanswer is that the planet’s motion generates a forcethat opposes gravity. It’s called centrifugal force. Thefaster a planet moves in its orbit, the more centrifugalforce it generates. So, the weapon against gravity isspeed.
This understanding helps us make a generalizedstatement regarding orbital position and orbital period.Planets orbiting close to the star are able to do sodespite stronger gravitational influences because theytravel quickly. They have a rapid tangential velocity.And since their orbital paths are short anyway, it takesless time to complete a loop. Planets farther out, movemore slowly. They have lower tangential velocities. Andbecause their orbital paths are longer, so are theirorbital periods.
5.11 Calculating the orbital period for a planet at the orbitof greatest opportunity
The orbital period represents the time it takes for aplanet to complete one orbit around the star. Theorbital period gets longer as we encounter planetsfarther from the star. For simplicity, I am assuming acircular orbit. Using Johannes Kepler’s third law (asderived from Newton’s laws), the basic equation for theradius of a circular orbit is
23 PMMa PlanetStar
a represents the orbital radius, or semi-major axis ofthe planet in circular orbit (in astronomical units)
MStar represents the mass of the star (in solar masses)
MPlanet represents the mass of the planet (in solarmasses)
P represents the orbital period (in sidereal years, orjust plain Earth years)
Solving for P we get
PlanetStar MMaP
3
To find the orbital period for a planet at the orbit ofgreatest opportunity, we set a in the above equation tobe the radius of the orbit of greatest opportunity.
5.12 Combining the above calculations into an integratedmethod for predicting best case scenarios forbiological evolution on planets orbiting stars ofdifferent masses
Panel 1 shows how we can combine the previouslydiscussed individual steps into an integrated methodfor predicting best case scenarios for biologicalevolution near stars of different masses. Knowing onlythe mass of the star, we can develop an historicalprofile of the habitable zone that shows the originalposition of the habitable zone and how the habitablezone will migrate away from the star during the star’smain sequence life. Panel 1 does this for stars rangingin size between 0.1 and 2 solar masses. Using theconcept of the orbit of greatest opportunity we canquickly compare the potential viability of stars ofdifferent masses.
Based on this method, stars of 0.8 solar masses havethe longest time available for biological evolution. Aplanet at the orbit of greatest opportunity for such astar would reside in the habitable zone for about 20billion years. During this time, it would suffer onlymildly from its star’s tidal braking influences. A planetin such an orbit would have an orbital period of just229 sidereal days. If the planet’s orbital propertiespermit seasons, then the planet would have 16 springsin the time Earth would have 10 springs. The morerapid cycle of seasons could mean more frequentreproductive events for organisms tied to seasonalcycles. This would mean faster introductions ofinnovations and perhaps a swifter pace of evolution.
Panel 1 shows that the best case scenarios for planetsaround smaller stars become constrained by tidallocking influences. A planet at the orbit of greatestopportunity for a star of 0.1 solar mass would reside inthe habitable zone for as long as 252 billion years (ifthe Universe lasts that long). But it must orbit so closeto its star that it will become locked into a synchronousorbit in about 4 million years.
On the other side, stars moderately larger than ourown sun present their planets with very limitedopportunities. For example, a planet at the orbit ofgreatest opportunity around a star of 2 solar masseswill only reside in the habitable zone for about 600million years. Although there are stars much largerthan 2 solar masses, we are ignoring them because ofthe increasingly limited time constraints they impose.
I hope you can see the usefulness of this method. Withit, we can quickly estimate best case scenarios andthen use them as meaningful benchmarks forcomparing the potential for life around stars of differentmasses.
This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.So, where does Earth fit in this story? Well, if you lookat Panel 1 you can see that the orbit of greatestopportunity for a 1 solar mass star (like our sun) is at1.157 AU. This is slightly farther out than Earth’s orbit,which is at 1 AU. And what about Mars? Its orbitalradius (semi-major axis, actually) is about 1.5 AU.Panel 1 shows that the outer edge of the habitable zonewill be beyond Mars at the moment of habitable zonetransit. So, how long will Mars remain in the habitablezone? And finally, what about those extrasolar planetsthat astronomers have been discovering lately?
The historical profile for a star’s habitable zone that wehave just developed helps us compare best casescenarios for different stars. If we want to develop anhistorical profile for a specific planet in relation to thehabitable zone, then we can modify the currentmethod. Which is what I will show you how to do next.
5.13 Developing an historical profile for individual planets,in relation to the habitable zone
If we want to understand the historical profile of anindividual planet in relation to its star’s habitable zone,we must perform the following steps:1) Determine the historical profile of the habitable
zone for the star (like we did above).2) Determine what start region the planet occupies in
relation to the historical profile of the habitablezone (explained below).
3) Determine how much time the planet will spend inthe habitable zone (explained below). Thecalculations necessary for making thisdetermination will depend on which start region theplanet occupies.
Once we have developed the historical profile for thehabitable zone around a star, we can then overlayinformation for specific planets orbiting the star. Theplanets can be real or imagined. The method I willdescribe below requires us to use the habitable zone’shistorical profile in a slightly different way.
See figure 2 above. For this method, we need toconsider the orbital location of the planet in relation toone of the star’s following start regions: 1)Hot non-start;2) warm start; 3) Inner cold start; 4) outer cold start;and 5) cold non-start.
The hot non-start region is the region in space that liesstarward to the habitable zone at zero age mainsequence. It will always be too hot for the presence ofliquid surface water. So, if we are considering a planetwithin this region, then it will be a very poor candidatefor liquid surface water and life.
The warm start region lies between the habitable zone’s
inner radius (ZAMSir ) and outer radius (
ZAMSor ) at zero
age main sequence. The warm start region is the samespace spanned by the habitable zone at zero age mainsequence. Planets in this region will start out in thehabitable zone. I have already discussed the reasonswhy warm start planets are better candidates for thedevelopment of life than cold start planets.
I have broken the cold start region into two sub regionsfor reasons that will become clear later on.
The inner cold start region lies between the habitable
zone’s outer radius at zero age main sequence (ZAMSor
)and the habitable zone’s inner radius at main
sequence end (MSEir .). The inner cold start region is a
narrow band just beyond the warm start region.Planets residing in this region will start out cold. Later,the habitable zone will overtake them. And later still,the habitable zone will move on beyond them, leavingthem to overheat.
The outer cold start region lies beyond the inner coldstart region. The outer cold start region is the samespace occupied by the habitable zone at main sequenceend. It lies between the habitable zone’s inner radius
(MSEir ) and outer radius (
MSEor ) at main sequence end.
Planets in this region also start out cold, then are laterwarmed up by the migrating habitable zone. But thestar’s main sequence stage will end before the innerboundary of the habitable zone reaches any planets inthis region.
The cold non-start region lies beyond the habitable zoneat main sequence end. Planets in this region will neverbe touched by enough stellar warmth to melt the frozenwater they might possess.
Once we know in what start region the planet resides,we can estimate the length of time it will spend in thehabitable zone.
Estimating habitable zone residency for planets in thewarm start region
Planets in the warm start region begin their ‘lives’ inthe habitable zone. They will remain in the habitablezone until the habitable zone’s inner boundary (ri )migrates past them. We will designate this amount oftime as Ti . So for warm start planets, the time in thehabitable zone is equal to the time it takes thehabitable zone inner boundary to reach them, or
This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.
i
startwarmHZ TT
We will modify Fogg’s (1992) expression adapted fromGough (1981) to help us find Ti .
33.110045.0
MSTT
ZAMST eLL
If we solve for T (which represents time, in billions ofyears), then we get
33.1
045.0
ln
10
ZAMS
T
MS LL
TT
We should have calculated TMS (time in main sequence)and LZAMS (luminosity at zero age main sequence) earlierwhen developing the historical profile for the star’shabitable zone. What we don’t know right now is thestar’s luminosity at the point in time when the innerboundary of the habitable zone reaches the planet. Butwe can find out by using
1.1T
iLr
We use 1.1 as the flux factor marking the innerboundary of the habitable zone. Solving for LT , we get
1.12iT rL
Now we use this expression for LT to modify ourprevious equation to get the following equation fordetermining the warm start planet’s time of residencein the habitable zone.
33.1
2
045.0
1.1ln
10
ZAMS
i
MSi
Lr
TT
We then use the planet’s orbital radius, or semi-majoraxis (a) for ri .
Estimating habitable zone residency for planets inthe inner cold start region
Planets in the inner cold start region begin theirhabitable zone residency once the habitable zone’souter boundary (ro ) reaches out to them. They remainin the habitable zone until the habitable zone’s inner
boundary (ro ) passes them by. So their time in thehabitable zone is the time it takes for the habitablezone inner boundary to reach them minus the time ittook the outer boundary to reach them, or
oi
startcoldinnerHZ TTT
We will use the same expression already derived for Ti ,shown again as
33.1
2
045.0
1.1ln
10
ZAMS
i
MSi
Lr
TT
We can modify our equation for Ti to get an equationfor To by simply changing the flux constant to reflectconditions at the outer boundary of the habitable zone.So, instead of an inner boundary flux of 1.1, we use anouter boundary (ro ) flux of 0.53 and our expression forTo becomes
33.1
2
045.0
53.0ln
10
ZAMS
o
MSo
Lr
TT
We then substitute the planet’s orbital radius, or semi-major axis (a) for ri and ro and subtract To from Ti toget our estimate of the amount of time a planet in theinner cold start region will spend in the habitable zone.
Estimating habitable zone residency for planets inthe outer cold start region
Planets in the outer cold start region eventually willreside in the habitable zone as the habitable zone’souter boundary migrates out to them. But since thiswill happen so late in the game, the habitable zone’sinner boundary will never reach them. The star’s mainsequence stage will end first. So, the time that outercold start planets will reside in the habitable zone isequal to the time the star will be in main sequencestage minus the time it takes for the habitable zone’souter boundary to reach them, or
oMS
startcoldouterHZ TTT
Again, we use the following equation to estimate howmuch time will pass before the habitable zone’s outerboundary reaches the planet.
This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.
33.1
2
045.0
53.0ln
10
ZAMS
o
MSo
Lr
TT
For the planet in question, we use its orbital radius, orsemi-major axis (a) for ro .
5.14 Combining the above calculations into an integratedmethod for predicting the historical profile of a planetin relation to its star’s habitable zone
Panel 2 shows how we can combine the individualcalculations presented above into an integrated methodfor predicting a planet’s chances for habitable zoneresidency. Notice that the first step is to establish thestar’s habitable zone profile. These are the same initialsteps we took in the method for determining best casescenarios for stars of different masses.
Once the star’s habitable zone profile is set, then wecan overlay onto that profile information regarding aplanet’s orbital radius (or semi-major axis). The planet’sorbital radius will tell us in which start region theplanet belongs. After calculating the star’s time in mainsequence and luminosity at zero age main sequence,we can then determine how long it will take for thehabitable zone’s inner and/or outer boundaries tointersect the planet. Remember that the finalcalculations for predicting the amount of time that aplanet spends in the habitable zone is different for eachstart region.
In order to demonstrate this method, I have calculatedand presented in Panel 2 the possible habitable zoneresidencies for all of the planets in our solar system. Inaddition, I wanted to know about the prospects fornewly discovered extrasolar planets including: 51Pegasi (Mayor and Queloz, 1995), rho Coronae Borealis(Noyes et al., 1997), Tau Bootis A (Butler et al., 1997),70 Virginis (Marcy and Butler, 1996), and 47 UrsaeMajoris (Marcy and Butler, 1996). Finally, I included animaginary planet of my own mainly because I needed away to demonstrate how the method works withplanets in the inner cold start region.
Looking at the planets of the solar system, the onlyplanets that will have a chance at the habitable zoneare Earth and Mars. Earth started out in it and willstay in the habitable zone for about 5.5 billion years.This means that in about one billion years thehabitable zone will move outward beyond Earth.Meanwhile Mars resides in the outer cold start regionand awaits its moment in the sun. And what a wait itis. The habitable zone’s outer boundary will reach Mars
about 6.5 billion years after the start of the solarsystem. Since the solar system is about 4.5 billionyears old, Mars will not get its chance for about twobillion more years.
But other planets in system will get no chance at all.For example, Mercury and Venus occupy the hot non-start region. And Jupiter, Saturn, Uranus, Neptuneand Pluto sweep through the cold non-start region. Forthese planets, it always will be either too hot or too coldfor liquid water on their surfaces.
Turning to extrasolar planets, the only candidate (of theones I considered) for habitable zone residency is thecompanion to 47 Ursae Majoris (designated as 47Ursae Majoris b). Its orbit is in the outer cold startregion. According to this method, the planet started outin the cold space beyond the habitable zone. But theouter boundary of the habitable zone will reach thisplanet after about 5.8 billion years of waiting. But sinceits star’s main sequence stage will end after aboutseven billion years, this planet will occupy thehabitable zone for a little more than one billion years.
Despite the planet 47 Ursae Majoris b’s somewhatfavorable positioning relative to the oncoming habitablezone, this planet probably would be a poor candidatefor life. This is because it is probably a gas giant nearlythree times the size of Jupiter. Gas giants, regardless oftheir relationship to the habitable zone, don’t have theequitable mix of environmental surface conditions thatwe might find on terrestrial-type planets like Earth orMars. In other words, it is hard to imagine theemergence of a thriving biota on such a world.However, such a planet could have many terrestrial-type moons, and sizeable ones at that. Like their parentplanet, the moons eventually will be engulfed by thehabitable zone of 47 Ursae Majoris. Obviously, there isgreat interest in the potential of such a happening.
Regarding the other extrasolar planets I consideredhere in my demonstration, they all are residents oftheir own star’s hot non-start region. And they all aregiant planets, probably gas giants like Jupiter –meaning they are poor candidates as harbors for life.
This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.They could have moons but the prospect of life on suchmoons seems small since the moons are starting out inthe hot non-start region along with their parentplanets.
In conclusion, the value of this method is that itformalizes and systematizes a way to readily predict aplanet’s historical relationship with its star’s habitablezone. This method will help us to quickly and uniformlyassess a planet’s opportunity to support life andbiological evolution – on a first pass basis. The ability todo this is becoming increasingly more meaningful asastronomers discover more and more extrasolarplanets.
5.15 A method for assessing a planet’s risk of impact fromcomets and asteroids
The purpose of this section is to try to find out what setof circumstances increases or decreases a planet’s riskof collisions with comets or asteroids.
It is important to consider the risk from comets andasteroids because impacts can utterly disruptplanetary surface environments. Since life overall is abuilding phenomenon that benefits from a conservativeand stable environment, frequent asteroid or cometimpacts could slow or eliminate biological evolution.Even if the chance of collision is small, comet andasteroid collisions remain low probability high impactevents that can have important consequences to aplanet’s surface environment and everything alive in it.Chapter ?? discusses in greater detail how collisionscan harm the planetary surface environment.
Figure 7. Drawing of an imaginary asteroid strike. By Don Davis.
I want to foreshadow a bit here on the role of impactsand biological evolution on Earth. It may be thatoccasional impacts by comets and asteroids haveprofoundly influenced the pace and direction ofbiological evolution on Earth. The most famousexample is the widening acceptance that the greatdinosaur extinction was triggered by a large impact 65million years ago. Although mass extinctions certainlyare not a good thing for the species that go extinct,afterwards they result in ecological opportunity for thesurviving species. Mass extinctions on Earth often werefollowed by a flourish of new species and adaptations.As a result, a moderate amount of impacts actuallycould expedite life’s long-term colonization of theplanet, further elevating and cementing life’s stature asa planet-changing force. See chapter ??? for moredetails.
Threat sources and regional threat zones in our solarsystem
The planets in our solar system are threatened byplanet-crossing comets and planet-crossing asteroids.Since we have little or no information regarding thestate of these two kinds of threats for other stellarsystems, I will use our solar system as the definitivemodel.
In our solar system, comets are primarily small icybodies ranging in size from about 1 to 20 kilometers(0.6 to 12 miles) in diameter. Current theory holds thatthese icy bodies originally formed in the outer solarsystem at the same time as the outer planets (Jupiter,Saturn, Uranus, Neptune, and Pluto) and in the samevicinity. Then, close encounters with the massiveplanets of the outer system summarily ejected theseminor ice bodies out of planetary space.
Astronomer Jan Oort theorized that there is a vastreservoir of comets way beyond the orbit of Pluto. Thecomets surround the solar system forming the so-called Oort cloud. The Oort cloud idea suggests thatcomets could reside in a spherical region stretchingfrom 10,000 to 100,000 astronomical units from theSun. This is far, considering that the outermost planet,Pluto, orbits only as far as 39 AU. The Oort cloudtheory suggests that there could be as many as 200billion comets out there. Please note that astronomersare not in total agreement regarding thereasonableness of the Oort cloud theory. A competingidea suggests that the main comet reservoir is justbeyond the orbit of Neptune. Regardless of the locationof the reservoir, as long as the comets stay in thereservoir, they present no threat. But occasionallycomets leave the reservoir and head for the sun.Astronomers have identified about 800 comets andcontinue to discover about 10 new comets each year.
This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.When comets leave home and enter planetary space,they follow an extremely large elliptical orbital path.This orbital path often takes the comet near the sun(some get closer than Mercury’s orbit while others getno closer than Mars) then sends the comet back outagain. Comets can approach the Sun from a variety ofangles that may differ sharply from the planetaryorbital plane. Comets that come in at steep angles, aremore likely to avoid the gravitational influences of thelarge outer planets. For example, the Great comet of1861 approached at an angle of 85 degrees (Lang,1992). That’s nearly perpendicular to the orbital plain.Such comets may threaten only the inner planets andnot the outer planets. However most short periodcomets have more modest angles of attack, withaverage inclinations relative to the orbital plain of about18°. When comets enter planetary space at suchshallow angles they risk colliding with any one of allnine planets. The point is that it seems that mostcomets present a general threat to all planets in thesolar system.
In addition to comets, asteroids also threaten planetswith collisions. The asteroids of the solar system orbitthe sun in a more-or-less circular orbit between Marsand Jupiter – the asteroid belt (fig. 4). Unlike the icycomets, the asteroids are rocky objects. They range insize from small rock fragments to large rocky bodiesone thousand kilometers (620 miles) in diameter. Thereare about 20,000 asteroids a half mile to a mile indiameter in the asteroid belt.
Astronomers reason that the asteroid belt is all thatremains of a would-be planet. The asteroid belt orbitsat just about the place we would expect to find aplanet, according to the principle of planetary spacingfirst proposed in 1766 by Johann Tittius, then later byJohann Bode in 1772. The principle, now known asBode’s law, observes that the planets of the solarsystem are spaced in a predictable pattern. So, whydoes this orbital channel have a circling rock gardeninstead of a planet?
Remember that Jupiter is the next planet beyond theasteroid belt. It is the most massive planet in the solarsystem, and exerts tremendous gravitational influenceon anything that gets near it. Astronomers argue thatwhile other planets were forming in the solar system,this would-be planet could not coalesce into a discrete,single object because it was being constantly rippedapart by Jupiter’s strong gravity. As a result, no planet,just orbiting rocks.
Astronomers know the orbits of about 3500 sizableasteroids. Of these, we know of about 120 asteroidsthat are in Earth-crossing orbits. That is, their earlierorbits later became modified such that they no longerrun with the rest of the pack in the asteroid belt. Theremay be as many as 2000 Earth crossing asteroids,most of which are yet to be discovered. Why don’t theasteroids stay in the belt?
Despite their great distance from the asteroid belt, theplanets Jupiter and Saturn can still project theirgravitational influence on the asteroids. For someasteroids, repeated and regular gravitational pushesand pulls sets up a sort of long-term vibration called aresonance. After a time, the resonant asteroid’s orbitdeparts from the circular pattern of the belt andbecomes more and more eccentric and elliptical.
The evolving orbital path can take the asteroid eitherout toward Jupiter and beyond, or in toward Mars andthe other inner planets (generally). Therefore, we canimagine two separate planet crossing asteroidpopulations: 1) the outer population that threatens theouter planets but not the inner planets; and 2) theinner population that threatens the inner planets butnot the outer planets.
Given this situation, the outer planets must cope withtheir own and entirely separate regional asteroid threat.And the inner planets must cope with their own andseparate regional asteroid threat. This is different fromthe comet threat which is not so neatly partitioned andgenerally affects all of the planets in the system. Sohow do planets cope with the comets and asteroids thatbear down on them?
Let’s think about the comet threat first. A comet willcontinue to orbit through planetary space until one oftwo things happens: 1) it collides with a planet and isdestroyed; or 2) it has a close encounter with a planetand is ejected out of the system for good by agravitational slingshot. In both cases, the comet iseliminated as a threat. In the case of the collision, thereis a price to pay. But that price is only paid by oneplanet. The other eight planets no longer are threatenedby that particular comet. But the next comet mightstrike a different planet. A planet that avoids a collisionthis time might get hit next. So the system-widecollision risk is spread out among the whole populationof planets. The more planets, the less the individualplanet’s collision risk for any given threat. Schools offish use this same technique when swimming throughbarracuda territory.
This same general principle applies to ejectingencounters. A greater number of planets provide moreopportunities for ejecting encounters. So we can makethe general observation that if a star system is plaguedwith the threat of comet impacts, individual planets willsuffer less if they are part of a large population ofplanets. They will suffer more if the number of planetsin the system is small.
This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.
Figure 9. Comet Shoemaker-Levy 9 strikes Jupiter.
So far we have not considered planetary size and massas a factor. But we should. It turns out that asplanetary size and mass increase, the chances forcollision also increase. So, for a given threat, Jupiter ismore at risk than Earth simply because it’s a muchbigger target. In addition, Jupiter’s strong gravity cansteer more comets its way converting a near miss into adirect hit. But a large planet’s gravity also can ejectmore comets over a larger range of encounterdistances. So, although Earth’s gravity might onlyslightly shift the orbit of a nearby comet passing by,Jupiter could eject that comet. We can now modify ourobservation that for any given collision threat, giantplanets like Jupiter and Saturn provide additionalprotections to smaller planets by taking more thantheir share of hits and by ejecting more than theirshare of comets.
These same rules apply for planets under the gun fromasteroids, with a few modifications. Mainly, the cometthreat is a system-wide threat, and the asteroid threatis a regional threat. So, a planet will receive asteroidprotection only by the other planets within theirregional threat zone. The planets outside the region canprovide no protection from asteroids.
For example, let’s imagine a star system with 10planets and an asteroid belt. But the asteroid belt isplaced such that there are only two planets in the innerthreat zone, and eight planets in the outer threat zone.Under these circumstances, the two inner planets musttogether deal with all of the planet crossing asteroids inthe inner threat zone, with no help from the outerplanets. If the regional impact threat is about the samein both the inner and outer regions (that is, the planetcrossing asteroid populations are equal), then eachindividual planet in the outer region will be struck byasteroids only a fourth as much as will individualplanets in the inner region (not taking size and massinto consideration). So, where does this leave us?
The picture I have presented regarding impact threatsfrom comets and asteroids is very simplified. Objectsmove around a star in a complex furball thatchallenges our desire to understand things in neatpackages. Still, we have to find some purchase if we areto see the essence of the problem. Otherwise we willslip back into the storm of infinite individualcircumstance, examining the history and dreary detailof each raindrop while ignoring the reality andexcitement of the storm itself. In short, I have tried toreveal a comprehensible way for non-astronomers toconsider the otherwise overwhelming complexityinherent in this field. It will serve our purposes.
The above discussion leads to several main points thatI want to present below as a series of statements.1. Planet crossing bodies that originate outside the
outermost planet generally will present a system-wide collision threat in which the entering bodiescould collide with any one of the planets in thesystem. Comets do this in our system. As a result,all the planets of the solar system share a commoncomet collision risk.
2. Planet crossing bodies that originate betweenplanets in the fashion of the solar system’s asteroidbelt may have a tendency to accumulate as twodistinct populations, an inner and an outerpopulation. This will result in an inner threat zoneand an outer threat zone. As a result the planets inthe inner threat zone will share a common collisionrisk amongst themselves for this particular threatsource. And planets in the outer threat zone alsowill share a common risk amongst themselves, butone that is separate from that experienced by theinner planets. Reductions or increases in the threatlevel in one threat zone will not affect the threatlevel in the other threat zone.
3. Planets burdened by a common collision threat canreduce their individual risk if:
· The planet in question shares the risk with manyother planets that will take their share of hits.
· The planet in question shares the risk with giantplanets who will take more than their share of hitsand will eject more of their share of planet crossingbodies.
4. Giant planets provide a mixture of problems andbenefits. On the benefit side, giant planets sweepup or eject planet-crossing bodies in greaternumbers than do small planets like Earth. As aresult, giant planets provide a great measure ofprotection for the smaller planets. Of course thisservice only happens amongst planets thatexperience a common threat. In the eventualitythat regional threat zones exist, giant planets mayprovide no benefit to planets in another regionalthreat zone. Jupiter provides Earth with little or noprotection from asteroid collisions, since theasteroids menacing Earth do not cross Jupiter’spath. But what of the asteroids in the first place?
This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.5. Giant planets actually may be the primary cause of
an increased collision threat. They could do this bycreating an asteroid belt. Then if that wasn’t badenough, their continual gravitational influence willperturb the orbits of some asteroids, sending theminto planet crossing orbits. So although giantplanets may provide a great measure of protectionagainst system wide collision threat, they may alsocause asteroid threat in the first place. Earthenjoys some of Jupiter’s comet protection servicesbut at the expense of increased exposure to fallingrocks.
There is great uncertainty regarding the nature andconfiguration of comets, asteroids and otherthreatening bodies in other star systems. We haveabsolutely no empirical information on them. Cometscould be rare or they could be extremely numerous.Asteroids may or may not exist. Instead of just oneasteroid belt, there could be several. So, until we findout more, it is reasonable to assume that planets inother star systems also must cope with impacts ofsimilar kinds of threatening bodies.
5.16 A note on uncertainties and inaccuracies in my method
Please note that the numerical constants andmathematical expressions for the models I havepresented in this chapter are not Universally agreedupon by astrophysicists and planetary scientists. Theycontinue to change and refine. Nevertheless, the pointof this chapter is not to present a model of extremeaccuracy. Instead, the mission of the chapter is topresent systematic methods and a set of benchmarksby which biologists can begin the otherwiseoverwhelming task of considering a planet’sastronomical circumstances. They are relevant to theprospects of planetary life and they can and should beintegrated into the biologist’s cadre of concerns. Whileon my soap box let me emphasize that biologists have agrowing responsibility to contribute to discussions inthe expanding field of extrasolar planetology – and notjust as biochemical or cellular consultants. Biologistsneed to be able to sit down with astronomers andastrophysicists as full participants in the wideningdiscussion about worlds with life.
So, what have I left out? A more realistic attempt wouldshow that there are many, many other factors thatchallenge our understanding of the nearly infinitecombination of astronomical circumstances that arepossible for planets. For example, the methods I havepresented here only are useful for stellar systems witha single star. But about half of the star systems in ourgalaxy are binary star systems – two stars instead ofone. My approach doesn’t apply to them although I amnot ruling out the possibility of life on worlds in binarysystems.
The habitable zone limits that I used were based on acondensed and simplified approach presented byWhitmire and Reynolds (1996) after much deliberationby Kasting, Whitmire and Reynolds (1993) and manyothers before them. These are generic habitable zonelimits for an Earth-like planet. But the actual habitablezone for a different kind of planet needs to take intoaccount the characteristics of the planet in question,such as planetary albedo, the mix of atmosphericgases, the atmospheric pressure, the thickness of theatmosphere, the presence and distribution of oceans,the planet’s orbital properties and much more. We willexplore many of these phenomena in later chapters.
I used Martyn Fogg’s graceful and simplified approachfor the prediction of stellar luminosities and forestimating a star’s main sequence life span. Althoughhis expressions are derived from such respectedastrophysical authorities as Iben and Gough, mynumerical results differ somewhat from other reportedestimates. For example, my method predicts that thesun will achieve a final luminosity of about 1.9L¤ atmain sequence end. This differs from the more widelyaccepted and more rigorously calculated values of 2.2L¤reported by Sackman et al (1993), or 1.8 L¤ from themodel used by Turck-Chiéze et al. (1988). The mainreason for this discrepancy is because of theassumption that a star of 1 solar mass will burn inmain sequence for 10 billion years. Others estimateslightly longer, and maybe as long as 12 billion years. Ido not ignore such discrepancies but I am notparticularly bothered by them either. They are aninevitable consequence of the simplification andcondensation process that was necessary during theproduction of a generic method. The result, perhapscrippled by oversimplification, is sufficient for mypurpose, which is to get you into the ballpark whileavoiding the steep learning curve taken by professionalastrophysicists and astronomers.
5.17 Summary and conclusions
The main thing we want to know from all this is howmuch time is available for life to evolve on a habitableplanet. This is an interesting question in its own right,but it also helps us assess life’s chances for achievingplanet-changing stature. Life is more likely to achieveplanet-changing stature if it has more time to evolve.According to the methods here, we must consider thetradeoff between star life and tidal braking forces.Small stars burn longer but since they also are cooler,habitable planets must orbit closer to them to receivethe benefit of their heat. The closer a planet orbits itsstar, the quicker tidal braking forces will lock theplanet into a synchronous orbit, and this could be abad thing. On the other hand, habitable planetsorbiting large stars do so at greater distances because
This chapter is an excerpt from Principles of Planetary Biology, by Tom E. Morris.large stars burn hotter. This places the habitableplanets beyond the star’s tidal braking influence. Butthe benefit is short-lived since large stars burn outquickly.
We had to do some preliminary figuring in order tobuild up to these answers. This chapter presents twomathematical and systematic methods for doing this,inputting only the star mass, the orbital radius of theplanet and the initial rotational period of the planet.These methods are shown in panels 1 and 2. I willsummarize the essential results below.
5.17.1 Life would have the most time for evolutionon medium-sized planets
Planets at the orbit of greatest opportunity (OGO)around stars of 0.8 solar mass experience the optimummix of stellar longevity and freedom from tidal braking.There could be as much as 20 billion years forbiological evolution on such a planet. Biologicalevolution on planets at the OGO for 0.1 M¤ stars isconstrained by tidal braking which locks the planetafter only about 4 million years. And planets at theOGO around stars of 2 M¤ will only have a quickglimpse of pleasantness since their star’s will burn outafter about 600 million years. For comparison, Earthorbits somewhat inside the OGO of the sun. Ourestimated habitable zone residency will be about 6billion years or so. That gives us 1.5 billion more years.
5.17.2 A system of many planets could reduceeach planet’s impact risk
It is probably a good thing if a life-bearing world hasmany other planets in its stellar system. This is mainlybecause the other planets can help protect thehabitable world from comet and asteroid impacts.Large planets can be a good thing because they takemore than their share of impacts and eject more thantheir share of threatening bodies. But the tidal forces ofa large planet could also create an asteroid belt (or two)in the first place. And a giant’s continued tidalinfluence on the resulting belt could create planet-crossing asteroids. Too much of this would be a badthing. In the case of a single asteroid belt, the asteroidbelt would divide the stellar system into two regionalthreat zones, occupied by two distinct populations ofasteroids. The planets in each threat zone must dealwith the asteroid threat in there zone and will receiveno help from planets outside their threat zone.
This chapter presents a great deal of astronomy for abiologist to digest. And this is the short version. Iencourage biologists to adopt the notion that life and itsbiological processes occur in the context of theextended environment. By extended environment, Imean beyond the confines of the planet alone, not justin a canyon ecosystem or the Amazon Rain Forest. If weare to fully comprehend the state of our presentbiological and ecological circumstances, we must knowthe nature of the environment in which this dramaoccurs. I believe that it is less than satisfactory forbiologists at large to ignore the recent breathtakingaccomplishments of astronomy and planetary science.But I also deeply sympathize with biologists who, iftheir experience is like mine, are overwhelmed by theastronomical literature. Out of this growing mountainof knowledge and differential equations, what isimportant to me, the biologist? That is my job – to makethis stuff more accessible and usable to biologists whoare curious about life from a planetary andastronomical perspective.
5.18 References
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