NSCI 314
LIFE IN THE COSMOS
12 - WHERE TO SEARCH FOR LIFE OUTSIDE OUR SOLAR SYSTEM:
SUITABLE STARS AND PLANETS
Dr. Karen KolehmainenDepartment of Physics, CSUSB
http://physics.csusb.edu/~karen/
SEARCHING FOR LIFE IN OTHER SOLAR SYSTEMSWE WILL TAKE A CONSERVATIVE APPROACH:
-WE WILL CONSIDER ONLY LIFE THAT IS BASED ON THE ELECTROMAGNETIC INTERACTION, i.e., ATOMS AND MOLECULES.
- WE WILL ONLY CONSIDER LIFE THAT USES CARBON-BASED CHEMISTRY
- WE WILL ONLY CONSIDER LIFE THAT USES WATER AS ITS LIQUID SOLVENT.
- IGNORE POSSIBILITY OF “EXOTIC LIFE,” SUCH AS LIFE THAT USES A LIQUID SOLVENT OTHER THAN WATER, SILICON-BASED LIFE, OR LIFE BASED ON THE STRONG FORCE.
– THEREFORE CONSIDER ONLY EARTH-LIKE PLANETS (OR LARGE MOONS) WITH LIQUID WATER.
– IF EXOTIC LIFE CAN EXIST, THEN LIFE MAY EXIST IN A LARGER RANGE OF LOCATIONS AND BE MORE COMMON THAN WHAT WE WILL ESTIMATE.
PROPERTIES OF A PLANET THAT IS SUITABLE FOR LIFE
1. RELATIVELY LARGE ABUNDANCES OF CARBON, NITROGEN, AND OXYGEN (PLUS TRACE AMOUNTS OF HEAVIER ELEMENTS)
2. NOT NEAR A SITE OF COSMIC VIOLENCE3. ROCKY PLANET - SOLID SURFACE4. MASSIVE ENOUGH TO RETAIN A
REASONABLY DENSE ATMOSPHERE 5. CORRECT TEMPERATURE RANGE FOR LIQUID
WATER (SOLVENT)6. STABLE ENVIRONMENT FOR SEVERAL
BILLION YEARS (TIME NEEDED ON EARTH FOR “ADVANCED” LIFE FORMS TO EVOLVE)
ABUNDANCES OF ELEMENTS HYDROGEN AND HELIUM (BUT NO HEAVIER
ELEMENTS) ARE PRODUCED SHORTLY AFTER THE BIG BANG (DURING THE FIRST FEW MINUTES OF THE UNIVERSE).
HEAVIER ELEMENTS ARE PRODUCED VIA FUSION IN STARS (AND IN SUPERNOVA EXPLOSIONS ), AND RETURNED TO THE INTERSTELLAR MEDIUM VIA PLANETARY NEBULAE AND SUPERNOVAE.
HEAVIER ELEMENTS ARE INCORPORATED IN NEW STARS (AND THEIR PLANETARY SYSTEMS) FORMED FROM THE ENRICHED INTERSTELLAR MEDIUM.
ABUNDANCES OF ELEMENTS THEREFORE, PLANETARY SYSTEMS AROUND
YOUNGER STARS CONTAIN LARGER ABUNDANCES OF HEAVY ELEMENTS (ANYTHING HEAVIER THAN H AND He) THAN PLANETARY SYSTEMS OF OLDER STARS.
THUS PLANETS THAT CONTAIN ELEMENTS NECESSARY FOR LIFE ARE MORE LIKELY TO BE FOUND ORBITING YOUNGER STARS THAN OLDER STARS.
PLANETS MAY NOT EVEN BE ABLE TO FORM AROUND VERY OLD STARS WITH VERY LOW ABUNDANCES OF HEAVY ELEMENTS.
ABUNDANCES OF ELEMENTS POPULATION II STARS: OLD STARS, LOW
ABUNDANCES OF HEAVY ELEMENTS (0.1 – 0.5%)
POPULATION I STARS: YOUNGER STARS, HIGHER ABUNDANCES OF HEAVY ELEMENTS (1 - 2%)
PLANETS ORBITING POPULATON I STARS ARE MORE SUITABLE FOR LIFE – MORE HEAVY ELEMENTS
MOST STARS EXISTING TODAY (INCLUDING THE SUN) ARE POPULATION I STARS.
WHERE ARE POP I AND POP II STARS?
POPULATION I (PLANETS MAY BE SUITABLE CANDIDATES FOR LIFE):– DISKS OF SPIRAL GALAXIES, ESPECIALLY
IN SPIRAL ARMS– MOST IRREGULAR GALAXIES
POPULATION II (PLANETS ARE PROBABLY NOT SUITABLE CANDIDATES FOR LIFE):– NUCLEUS AND OUTER HALO OF SPIRAL
GALAXIES, PLUS GLOBULAR CLUSTERS– ELLIPTICAL GALAXIES
COSMIC VIOLENCE SOME ASTRONOMICAL ENVIRONMENTS WOULD
PROBABLY BE FATAL TO LIFE:– HIGH INCIDENCE OF GAMMA RAYS OR X-RAYS (HIGH
ENERGY PHOTONS) – FREQUENT STELLAR COLLISIONS OR NEAR-
COLLISIONS LOTS OF COMETARY IMPACTS LOCATIONS WHERE THIS IS A PROBLEM:
– NEAR GALACTIC CENTER (RADIATION FROM VICINITY OF CENTRAL BLACK HOLE, PLUS FREQUENT STELLAR COLLISIONS)
– NEAR SUPERNOVAE – NEAR ANY OTHER X-RAY AND GAMMA RAY SOURCE
ROCKY PLANETS
PROBABLY OCCUR MOST OF THE TIME AS LONG AS:
– THE ABUNDANCE OF HEAVY ELEMENTS IS HIGH ENOUGH TO MAKE ROCK, i.e., IN A SOLAR SYSTEM AROUND A POPULATION I STAR.
– THE PLANET IS IN THE INNER PART OF ITS SOLAR SYSTEM. (THEREFORE LIGHT GAS MOLECULES LIKE HYDROGEN AND HELIUM ARE MOVING FAST ENOUGH TO ESCAPE FROM ITS ATMOSPHERE, PREVENTING IT FROM BECOMING A GAS GIANT.)
ATMOSPHERE NEEDED TO KEEP WATER (OR OTHER LIQUID
SOLVENT) FROM BOILING AWAY INTO SPACE HELPS TO SPREAD HEAT AROUND, AVOIDING
TEMPERATURE EXTREMES MAY ALSO SUPPLY GASES FOR ORGANISMS TO
BREATHE MAY ALSO SUPPLY GASES NEEDED FOR CHEMICAL
REACTIONS INVOLVED WITH THE ORIGIN OF LIFE PLANET MUST BE MASSIVE ENOUGH TO RETAIN AN
ATMOSPHERE, i.e., ITS GRAVITY MUST BE SUFFICIENT TO PREVENT MOST GAS MOLECULES FROM ESCAPING. – RECALL THAT IN OUR SOLAR SYSTEM, MARS IS
TOO LIGHT.
RIGHT TEMPERATURE RANGE MAINTAINED VIA A NEARLY CIRCULAR ORBIT
AT THE RIGHT DISTANCE FROM ITS STAR FOR A TEMPERATURE ALLOWING LIQUID WATER (i.e., IN THE “HABITABLE ZONE”)
OR
PROVIDED BY A SOURCE OF INTERNAL HEAT (E.G., RADIOACTIVE DECAY, OR TIDAL FORCES AS WITH JUPITER’S MOONS IO AND EUROPA). THIS CAN HAPPEN EVEN IF THE PLANET OR MOON IS LOCATED FARTHER FROM ITS STAR.
HABITABLE ZONE SPHERICAL SHELL SURROUNDING STAR IN
WHICH ANY ORBITING PLANETS WILL HAVE THE RIGHT TEMPERATURE FOR LIQUID WATER
INNER EDGE OF ZONE IS WHERE AVERAGE TEMPERATURE = 100oC
OUTER EDGE OF ZONE IS WHERE AVERAGE TEMPERATURE = 0oC
WHERE IS THE HABITABLE ZONE FOR FOR OUR SUN?
OUR SUN’S HABITABLE ZONE INNER EDGE: ABOUT 0.85 - 0.95 AU FROM SUN OUTER EDGE: ROUGHLY 1.4 - 1.7 AU FROM SUN
– EXACT VALUES DEPENDS ON DETAILS OF GREENHOUSE EFFECT
HABITABLE ZONE MOVES OUTWARD WITH TIME, AS SUN BRIGHTENS SLIGHTLY– WHEN SOLAR SYSTEM FIRST FORMED, INNER EDGE
AT 0.65 – 0.80 AU AND OUTER EDGE AT 1.1 – 1.5 AU CONTINUOUSLY HABITABLE ZONE (FOR FIRST
FEW BILLION YEARS AFTER SOLAR SYSTEM FORMED)– INNER EDGE AT 0.85 TO 0.95 AU– OUTER EDGE AT 1.1 TO 1.5 AU
OUR SUN’S HABITABLE ZONE WHERE ARE PLANETS RELATIVE TO SUN’S
HABITABLE ZONE? EARTH IS AT 1 AU – WITHIN CONTINUOUSLY
HABITABLE ZONE (OBVIOUSLY!) VENUS IS AT 0.72 AU – TOO CLOSE NOW, BUT
MAY HAVE BEEN BARELY HABITABLE VERY EARLY IN ITS HISTORY.
MARS IS AT 1.52 AU – PROBABLY WITHIN HABITABLE ZONE NOW, BUT POSSIBLY NOT EARLIER IN ITS HISTORY.– BUT THE REAL PROBLEM IS THAT MARS IS TOO
LIGHT, SO IT LOST MOST OF ITS ATMOSPHERE.
PROPERTIES OF MAIN SEQUENCE STARS
SPECT. TYPE
BRIGHTNESS SUN=1
NUMBER OF
STARS(IN MW)
PERCENT OF TOTAL
O
B
A
F
G
K
M
100,000
500
10
2
0.9
0.2
0.005
80,000
360 MILLION
2.4 BILLION
12 BILLION
28 BILLION
60 BILLION
290 BILLION
0.00002%
0.09%
0.6%
3%
7%
15%
73%
COLOR
BLUE
BLUE
WHITE
YELLOW
YELLOW
ORANGE
RED
HABITABLE ZONES AROUND OTHER STARS
FOR BRIGHTER STARS:– HABITABLE ZONE IS FARTHER FROM STAR AND
LARGER IN EXTENT (E.G., 5 TO 20 AU FOR AN A-TYPE MAIN SEQUENCE STAR)
FOR FAINTER STARS:– HABITABLE ZONE IS CLOSER TO STAR AND SMALLER
IN EXTENT (E.G., 0.02 TO 0.06 AU FOR AN M-TYPE MAIN SEQUENCE STAR)
– HABITABLE ZONE MAY BE SO SMALL THAT IT IS UNLIKELY THAT ANY PLANETS ARE FOUND WITHIN IT
– IF PLANET IS TOO CLOSE TO STAR, OTHER POSSIBLE PROBLEMS INCLUDE:
• SOLAR FLARES • PLANET’S ROTATION MAY BE TIDALLY LOCKED (MIGHT BE
OK IF ATMOSPHERE CAN SPREAD HEAT AROUND ENOUGH)
STABLE ENVIRONMENT STAR MUST NOT CHANGE TOO MUCH IN
TEMPERATURE OR BRIGHTNESS FOR SEVERAL BILLION YEARS
THIS REQUIRES A MAIN SEQUENCE STAR THAT IS COOLER/REDDER/FAINTER THAN MID-F SPECTRAL TYPE – MAIN SEQUENCE LIFETIME IS TOO SHORT FOR
HOTTER/BLUER/BRIGHTER STARS– THIS REQUIREMENT ELIMINATES ONLY A FEW
PERCENT OF ALL MAIN SEQUENCE STARS
PROPERTIES OF MAIN SEQUENCE STARS
SPECT. TYPE
BRIGHTNESS SUN=1
LIFETIME (YEARS)
# OF STARS (IN MW)
PERCENT OF TOTAL
O
B
A
F
G
K
M
100,000
500
10
2
0.9
0.2
0.005
5 MILLION
10 MILLION
500 MILLION
1 BILLION
10 BILLION
100 BILLION
1 TRILLION
80,000
360 MILLION
2.4 BILLION
12 BILLION
28 BILLION
60 BILLION
290 BILLION
0.00002%
0.09%
0.6%
3%
7%
15%
73%
COLOR
BLUE
BLUE
WHITE
YELLOW
YELLOW
ORANGE
RED
SUITABLE STARSDEFINED AS STARS AROUND WHICH
PLANETS THAT ARE SUITABLE FOR LIFE MIGHT BE ORBITING.
REMEMBER THAT WE ARE RESTRICTING OUR ATTENTION TO LIFE SIMILAR TO THAT ON THE EARTH (e.g., CARBON-BASED, USES WATER AS LIQUID SOLVENT)
WHAT PROPERTIES MUST A STAR POSSESS IN ORDER TO BE A SUITABLE STAR?
PROPERTIES OF SUITABLE STARS
MAIN SEQUENCE - MOST NON-MAIN SEQUENCE STARS (RED
GIANTS & WHITE DWARFS) AREN’T STABLE ENOUGH IN BRIGHTNESS AND TEMPERATURE OVER A LONG ENOUGH TIME
- 90% OF STARS ARE MAIN SEQUENCE
SUFFICIENTLY LONG MAIN SEQUENCE LIFETIME- AT LEAST SEVERAL BILLION YEARS
- SPECTRAL TYPES M, K, G, AND SOME F - 97% OF MAIN SEQUENCE STARS ARE OF
THESE SPECTRAL TYPES
PROPERTIES OF SUITABLE STARS
SUFFICIENTLY LARGE LUMINOSITY - REASONABLY LARGE HABITABLE ZONE THAT ISN’T TOO CLOSE TO THE STAR
- SPECTRAL TYPE M TOO FAINT, HABITABLE ZONE TOO SMALL AND TOO CLOSE TO STAR
LOCATION IN SPIRAL ARMS OR DISK OF A SPIRAL GALAXY, OR IN AN IRREGULAR GALAXY- STARS HERE ARE YOUNGER (POPULATION I) AND THUS HAVE SUFFICIENT ABUNDANCE OF HEAVY ELEMENTS
- NOT TOO NEAR BLACK HOLE IN GALACTIC CENTER (AVOIDS COSMIC VIOLENCE)
SUITABLE STARSSOME STARS IN BINARY OR MULTIPLE STAR
SYSTEMS ARE EXCLUDED - 50% OF STARS ARE BINARY OR MULTIPLE- SOME PLANETS IN BINARY SYSTEMS WILL
NOT HAVE STABLE ORBITS- PLANETARY ORBITS IN DOUBLE OR MULTIPLE
STAR SYSTEMS CAN BE STABLE IF:THE STARS ARE FAR APART, AND THE PLANET IS MUCH CLOSER TO ONE STAR (THE ONE IT ORBITS) THAN TO THE OTHER STAR
ORTHE TWO STARS ARE CLOSE TOGETHER, AND THE PLANET ORBITS BOTH STARS AT A DISTANCE THAT IS LARGE COMPARED TO THEIR SEPARATION
SUITABLE STARS DEFINED AS STARS AROUND WHICH A PLANET (OR
PLANETS) SUITABLE FOR LIFE COULD BE ORBITING.
THIS RESTRICTS US TO:1. MAIN SEQUENCE STARS2. SPECTRAL TYPES K, G, AND SOME F3. POPULATION I (ENOUGH HEAVY ELEMENTS)4. NOT TOO NEAR GALACTIC CENTER5. SINGLE STARS (NOT BINARY OR MULTIPLE)?
IN OUR MILKY WAY GALAXY (OR A SIMILAR SPIRAL GALAXY), THIS LIMITS US TO ABOUT 5 TO 10% OF STARS (MORE IF WE ALLOW SOME BINARIES).
LET’S LOOK AT NEARBY SUITABLE STARS.
THE NEAREST 40 STARS (WITHIN 14 LY)
NameLuminosity (Sun = 1)
Spectral Type
Sun
Proxima Centauri
Alpha Centauri A
Alpha Centauri B
Barnard’s Star
Wolf 359
BD+36°2147
Sirius A
Sirius B
Luyten 726-8 A
Luyten 726-8 B
Ross 154
1
0.00005
1.58
0.44
0.0003
0.00002
0.006
23.0
0.003
0.00006
0.00004
0.0005
G2
M5
G2
K1
M5
M6
M2
A1
A2 (WD)
M6
M6
M4
Name LuminositySpectral
Type
Ross 248
Epsilon Eridani
CD-36°15693
Ross 128
Luyten 796-6
61 Cygani A
61 Cygni B
Procyon A
Procyon B
BD+59°1915 A
BD+59°1915 B
BD+43°44 A
BD+43°44 B
0.0001
0.30
0.012
0.0003
0.0003
0.08
0.04
7.6
0.0005
0.002
0.0015
0.006
0.0004
M6
K2
M2
M5
M5
K5
K7
F5
F5 (WD)
M4
M4
M2
M4
Name LuminositySpectral
Type
G51-15
Epsilon Indi
Luyten 372-58
Luyten 725-32
Tau Ceti
BD+5 1668
Kapteyn’s Star
CD-39 14192
Kruger 60 A
Kruger 60 B
Ross 614 A
Ross 614 B
CD-25 10553A
0.000016
0.14
0.0003
0.0001
0.47
0.0005
0.004
0.03
0.0002
0.0003
0.0003
0.00005
0.00005
M7
K4
M5
M6
G8
M4
M1
M0
M4
M4
M4
M
M
NameLuminosity
Spectral Type
BD-12 4523
CD-37 15492
0.0004
0.0002
M4
M4
OF THE 40 CLOSEST STARS (THOUGHT TO BE TYPICAL), 4 STARS (10% OF TOTAL), ARE “SUITABLE STARS” – SUN, EPSILON ERIDANI, EPSILON INDI, AND TAU CETI. (THESE ARE MARKED WITH ARROWS ABOVE.)
NUMBER OF SUITABLE STARS
LIMITATIONS ON SEARCH TIMES(WE'LL TALK ABOUT HOW TO SEARCH LATER)
OUT TO 100 LY: OUT TO 1,000 LY:
20,000 SUITABLE STARS 20 MILLION SUITABLE STARS
200,000 STARS 200 MILLION STARS
SEARCH RATE:
1 PER DAY
1 PER HOUR
1 PER MINUTE
1 PER SECOND
OUT TO 100 LY:
55 YEARS
2.3 YEARS
14 DAYS
6 HOURS
OUT TO 1000 LY:
55,000 YEARS
2,300 YEARS
38 YEARS
230 DAYS
PROPERTIES OF A SUITABLE PLANET A SUITABLE PLANET IS A PLANET ON WHICH LIFE
COULD BE FOUND PROPERTIES:
– MUST ORBIT A SUITABLE STAR
– TEMPERATURE MUST BE IN THE CORRECT RANGE FOR LIQUID WATER VIA EITHER:
• MUST ORBIT ITS STAR WITHIN THE HABITABLE ZONE, PLUS ORBIT MUST BE NEARLY CIRCULAR SO THAT IT IS ENTIRELY WITHIN THE HABITABLE ZONE, OR
• MUST HAVE A SIGNIFICANT SOURCE OF INTERNAL HEAT (PERHAPS ON A LARGE MOON EXPERIENCING SIGNIFICANT TIDAL FORCES FROM THE PLANET IT ORBITS)
– MUST BE ROCKY, HAVE A SOLID SURFACE
– MUST BE MASSIVE ENOUGH TO RETAIN A REASONABLY THICK ATMOSPHERE
HELPFUL (BUT MAYBE NOT CRUCIAL) FEATURES FOR A SUITABLE PLANET PRESENCE OF A LARGE MOON
– PRODUCES TIDES (TIDEPOOLS ARE A POSSIBLE PLACE FOR THE ORIGIN OF LIFE)
– STABILIZES ROTATION AXIS (PREVENTS SEASONAL CHANGES FROM VARYING DRAMATICALLY WITH TIME)
PLATE TECTONICS – PROVIDES A VARIETY OF ENVIRONMENTS– PLAYS A ROLE IN CO2 CYCLE ON EARTH, WHICH
HELPS REGULATE CLIMATE– MAY BE INEVITABLE FOR A SUFFICIENTLY
MASSIVE ROCKY PLANET (STILL HOT INSIDE)
HELPFUL (BUT MAYBE NOT CRUCIAL) FEATURES FOR A SUITABLE PLANET IMPACTS NOT TOO FREQUENT
– A SUFFICIENTLY LARGE IMPACT COULD WIPE OUT ALL LIFE
– THIS MAY DEPEND ON POSITIONS OF LARGE PLANETS WITHIN SOLAR SYSTEM
• THEIR GRAVITATIONAL EFFECTS EXPEL COMETS TO OORT CLOUD
• JUPITER MAY PROTECT EARTH IN THIS RESPECT
BUT OCCASIONAL IMPACTS MAY BE HELPFUL FOR EVOLUTION OF ADVANCED LIFE FORMS– IMPACTS CAUSE MASS EXTINCTIONS (E.G., DINOSAURS)– MASS EXTINCTIONS OPEN UP ECOLOGICAL NICHES FOR
NEW SPECIES
HELPFUL (BUT MAYBE NOT CRUCIAL) FEATURES FOR A SUITABLE PLANET
OCCASIONAL MAJOR CLIMATE CHANGES MAY BE HELPFUL FOR THE EVOLUTION OF “ADVANCED” LIFE FORMS
- THESE CAN “STIMULATE” EVOLUTION BY OPENING UP NEW ECOLOGICAL NICHES FOR A VARIETY OF SPECIES.
- THE END OF THE “SNOWBALL EARTH” STAGE IN THE EARTH'S HISTORY (A VERY SEVERE GLOBAL ICE AGE) HAPPENED ABOUT THE SAME TIME AS THE “CAMBRIAN EXPLOSION,” A MAJOR INCREASE IN THE DIVERSITY OF LIFE FORMS ON EARTH, AND THE ORIGIN OF “ADVANCED” ORGANISMS.
- SIMILAR, BUT LESS SEVERE, CLIMATE CHANGES MAY HAVE PROMPTED THE EVOLUTION OF HUMANS.