3677 Life in the Universe: Extra-solar planets

DEPARTMENT OF PHYSICS AND ASTRONOMY

3677 Life in the Universe:3677 Life in the Universe:Extra-solar planetsExtra-solar planets

Dr. Matt BurleighDr. Matt Burleighwww.star.le.ac.uk/mrb1/lectures.htmlwww.star.le.ac.uk/mrb1/lectures.html

Dr. Matt Burleigh 3677: Life in the Universe


Course outlineCourse outline

• Lecture 1Lecture 1– Definition of a planetDefinition of a planet– A little historyA little history– Pulsar planetsPulsar planets– Doppler “wobble” (radial velocity) techniqueDoppler “wobble” (radial velocity) technique

• Lecture 2Lecture 2– Transiting planetsTransiting planets– Transit search projects Transit search projects – Detecting the atmospheres of transiting planets: Detecting the atmospheres of transiting planets:

secondary eclipses & transmission spectroscopysecondary eclipses & transmission spectroscopy– Transit timing variationsTransit timing variations



• Lecture 3Lecture 3– MicrolensingMicrolensing– Direct ImagingDirect Imaging– Other methods: astrometry, eclipse timingOther methods: astrometry, eclipse timing– Planets around evolved starsPlanets around evolved stars

• Lecture 4Lecture 4– Statistics: mass and orbital distributions, incidence of solar Statistics: mass and orbital distributions, incidence of solar

systems, etc.systems, etc.– Hot JupitersHot Jupiters– Super-EarthsSuper-Earths– Planetary formationPlanetary formation– Planetary atmospheresPlanetary atmospheres– The host starsThe host stars



• Lecture 5Lecture 5– The quest for an Earth-like planetThe quest for an Earth-like planet– Habitable zonesHabitable zones– Results from the Kepler missionResults from the Kepler mission

• How common are rocky planets?How common are rocky planets?• Amazing solar systemsAmazing solar systems

– BiomarkersBiomarkers– Future telescopes and space missionsFuture telescopes and space missions


Useful web sitesUseful web sites

• Extra-solar planets encyclopaedia: exoplanets.eu• Exoplanet Data Explorer (California Planet Survey):

exoplanets.org• NASA exoplanet archive: exoplanetarchive.ipac.calt

ech.edu• Planet hunters (Zooniverse): www.planethunters.org• Kepler mission: kepler.nasa.govKepler mission: kepler.nasa.gov• Next Generation Transit Survey: www.ngtransits.orgNext Generation Transit Survey: www.ngtransits.org


Useful booksUseful books

• Extrasolar planets & Astrobiology: Extrasolar planets & Astrobiology: Caleb A. Caleb A. ScharfScharf

• Extrasolar planets: the search for new worlds: Extrasolar planets: the search for new worlds: Stuart ClarkStuart Clark

• Transiting Exoplanets: Transiting Exoplanets: Carole A. HaswellCarole A. Haswell• The Exoplanet Handbook: The Exoplanet Handbook: Michael PerrymanMichael Perryman• An Introduction to Astrobiology: An Introduction to Astrobiology: Iain Gilmore & Iain Gilmore &

Mark SephtonMark Sephton• Life in the Universe: Life in the Universe: Bennett & ShostakBennett & Shostak


Useful numbersUseful numbers• RRSunSun = 6.995x10 = 6.995x1088m m

• RRjupjup = 6.9961x10 = 6.9961x1077m ~ 0.1Rm ~ 0.1RSunSun

• RRnep nep = 2.4622x10= 2.4622x1077m ~ 4Rm ~ 4Rearthearth

• RRearth earth = 6.371x10= 6.371x1066m ~ 0.1Rm ~ 0.1Rjup jup ~ 0.01R~ 0.01RSunSun

• MMSunSun= 1.989x10= 1.989x103030kgkg

• MMjupjup= 1.898x10= 1.898x102727kg ~ 0.001Mkg ~ 0.001MSunSun = 317.8M = 317.8Mearthearth

• MMnepnep= 1.02x10= 1.02x102626kg ~ 5x10kg ~ 5x10-5-5MMSunSun ~ 0.05M ~ 0.05Mjupjup = 17.15M = 17.15Mearthearth

• MMearthearth= 5.97x10= 5.97x102424kg = 3x10kg = 3x10-6-6MMSunSun = 3.14x10 = 3.14x10-3-3MMjupjup

• 1AU = 1.496x101AU = 1.496x101111mm• 1 day = 86400s1 day = 86400s


What is a planet?What is a planet? • International Astronomical Union International Astronomical Union

definition –definition –– An object orbiting a star An object orbiting a star – Too small for dueterium fusion to Too small for dueterium fusion to

occuroccur• Less than 13 times the mass of JupiterLess than 13 times the mass of Jupiter

– Formation mechanism?Formation mechanism?• Forms from a circumstellar disk of dust Forms from a circumstellar disk of dust

and gas around a young starand gas around a young star

– Lower mass limit – IAU decided Lower mass limit – IAU decided that Pluto should be downgraded!that Pluto should be downgraded!


What is a planet?What is a planet?

Above, left to right: limb of Sun, late M (red) dwarf, L brown dwarf, T brown dwarf, Jupiter.

The coolest stars, old brown dwarfs and gas giant planets have the same radii!


A brief history of extra-solar planetsA brief history of extra-solar planets• 16th century: 16th century: the Italian philosopher Giordano the Italian philosopher Giordano

Bruno said that the fixed stars are really suns like Bruno said that the fixed stars are really suns like our own, with planets going round themour own, with planets going round them

• 1919thth Century: Century: astronomers believed orbital astronomers believed orbital anomalies in the binary star 70 Oph could be anomalies in the binary star 70 Oph could be explained by an unseen planet, but later explained by an unseen planet, but later disproveddisproved

• 1950s & 60s: 1950s & 60s: Peter van de Kamp concluded that Peter van de Kamp concluded that irregularities in the high proper motion of nearby irregularities in the high proper motion of nearby Barnard’s Star were caused by a planet. Sadly, Barnard’s Star were caused by a planet. Sadly, this too turned out to be erroneous.this too turned out to be erroneous.

• late 1980s: late 1980s: Canadian Gordon Walker found Canadian Gordon Walker found tentative evidence for exoplanets using radial tentative evidence for exoplanets using radial velocity method: but not confirmed until 2000s!velocity method: but not confirmed until 2000s!

• 1991: 1991: Andrew Lyne & Setnam Shemar at Jodrell Andrew Lyne & Setnam Shemar at Jodrell Bank Bank claimed to have discovered a pulsar planet claimed to have discovered a pulsar planet in orbit around PSR 1829-10, using pulsar timing in orbit around PSR 1829-10, using pulsar timing variations. They withdrew the claim later that variations. They withdrew the claim later that year due to an error in their calculations.year due to an error in their calculations. New York Times

16th April 1963


A brief history of extra-solar planetsA brief history of extra-solar planets

• 1991 Radio astronomers Alex Wolszczan & Dale 1991 Radio astronomers Alex Wolszczan & Dale Frail discovered planets around a pulsar Frail discovered planets around a pulsar PSR1257+12PSR1257+12– Variations in arrival times of pulses suggests Variations in arrival times of pulses suggests

presence of three or more planetspresence of three or more planets

– Planets probably formed from debris left after Planets probably formed from debris left after supernova explosionsupernova explosion

• 1995 Planet found around nearby Sun-like star 51 1995 Planet found around nearby Sun-like star 51 Peg by Swiss astronomers Michel Mayor & Didier Peg by Swiss astronomers Michel Mayor & Didier Queloz using the Queloz using the ““Doppler WobbleDoppler Wobble”” method method– Most successful detection method by far, but other Most successful detection method by far, but other

methods like transits are now very successfulmethods like transits are now very successful

• >1000 exoplanets >1000 exoplanets confirmedconfirmed to date by all to date by all methodsmethods– >100 found since I gave this lecture last year>100 found since I gave this lecture last year

– Kepler has several thousand more Kepler has several thousand more candidatescandidates


Blue: radial velocity, Green: transiting, Red: microlensing, Orange: direct imaging, Yellow: pulsar timing




Pulsar planetsPulsar planets

• Pulsars are neutron stars that emit radio pulses every ~second as they Pulsars are neutron stars that emit radio pulses every ~second as they spinspin– More stable and accurate than an atomic clockMore stable and accurate than an atomic clock

• If a planet accompanies the pulsar, then the pulsar will orbit the centre If a planet accompanies the pulsar, then the pulsar will orbit the centre of mass of the systemof mass of the system– The pulses will then arrive earlier or later than expectedThe pulses will then arrive earlier or later than expected

• Radio observations have found a dozen or so such “pulsar planets”Radio observations have found a dozen or so such “pulsar planets”– Wolszczan & Frail’s discovery of Wolszczan & Frail’s discovery of PSR1257+12’s planets in 1991 at Arecibo PSR1257+12’s planets in 1991 at Arecibo

in Puerto Rico were the first confirmed exoplanetsin Puerto Rico were the first confirmed exoplanets– PSR1257+12PSR1257+12’s planets are all ~Earth mass or smaller’s planets are all ~Earth mass or smaller


Pulsar planetsPulsar planets

• Pulsars are created when Pulsars are created when a massive star (>8Ma massive star (>8Msunsun) ) explodes as a supernovaexplodes as a supernova– Their original planetary Their original planetary

systems will not survivesystems will not survive– Radio-detected planets Radio-detected planets

thought to have formed thought to have formed from supernova debrisfrom supernova debris

– Planets will be bathed in Planets will be bathed in high energy radiation from high energy radiation from pulsar – no chance of life!pulsar – no chance of life!


Planet Hunting: The Radial Velocity TechniquePlanet Hunting: The Radial Velocity Technique((““Doppler WobbleDoppler Wobble””))

• Star + planet orbit Star + planet orbit common centre of gravitycommon centre of gravity

• As star moves towards As star moves towards observer, wavelength of observer, wavelength of light shortens (blue-shifted)light shortens (blue-shifted)

• Light red-shifted as star Light red-shifted as star moves awaymoves away

873 planets detected by Doppler 873 planets detected by Doppler Wobble inc. 142 multiple systemsWobble inc. 142 multiple systems


Measuring Stellar Doppler shiftsMeasuring Stellar Doppler shifts• Method:Method:

– Observe starObserve star’’s spectrum through a cell of iodine gass spectrum through a cell of iodine gas– Iodine superimposes many lines on starIodine superimposes many lines on star’’s spectrums spectrum– Measure wavelength (or velocity) of starMeasure wavelength (or velocity) of star’’s lines relative to the s lines relative to the

iodineiodine

• Measure:Measure: ee = ( = (ee) / ) / ee = v = vrr / c / c observed wavelength, observed wavelength, ee=emitted wavelength=emitted wavelength


M* from spectral type


Doppler Wobble MethodDoppler Wobble Method

• Since measure K (= vSince measure K (= v* * sin i), not vsin i), not v** directly, only know directly, only know

mass in terms of the orbital inclination imass in terms of the orbital inclination i• Therefore only know the planetTherefore only know the planet’’s s minimum minimum mass, mass, MM sin sin ii

– If i=90If i=90oo (eclipsing or (eclipsing or transitingtransiting) then know mass exactly) then know mass exactly

i=90i=9000

Orbital Orbital planeplane

ii00

Orbital Orbital planeplane


Example: 51 PegExample: 51 Peg

• P= 4.15days = P= 4.15days = 4.15x86400s = 4.15x86400s = 3.5856x103.5856x1055ss

• G5V star, MG5V star, M**=1.11M=1.11Msunsun = =

1.11x1.989x101.11x1.989x103030kg = kg = 2.21x102.21x103030kgkg

• Find r = 0.052AU,Find r = 0.052AU,vvplpl=1.37x10=1.37x1055msms-1-1 M Mpl pl sin sin i = i =

0.45M0.45Mjupjup


Above: eccentric orbit (e=0.93)

Top right: 55 Cancri multiple-planet system (4, maybe 5 planets)

Bottom right: 3 planet HD37124 system


HARPS radial velocity spectrographHARPS radial velocity spectrograph• Built by Geneva Observatory• First installed on ESO 3.6m at La

Silla, Chile in 2002/3• Has found over 130 planets• Precision 30cm/s – 1m/s• Simultaneously observes star and a

reference Thorium lamp through two separate fibres

• Highly stable optical bench, housed in sealed, thermally stable room

• Second HARPS installed on Italain Galileo telescope on la Palma 2012


Doppler Wobble MethodDoppler Wobble Method• Precision of current surveys routinely <1m/sPrecision of current surveys routinely <1m/s

– Jupiter causes SunJupiter causes Sun’’s velocity to vary by 12.5m/ss velocity to vary by 12.5m/s– All nearby, bright Sun-like stars are good targetsAll nearby, bright Sun-like stars are good targets

• Lots of lines in spectra, relatively inactiveLots of lines in spectra, relatively inactive

– Smallest planet found by this method is Smallest planet found by this method is ~1~1MMearthearth:: Alpha Cen Bb – nearest star Alpha Cen Bb – nearest star

system to us!system to us!– Most are Neptune size and largerMost are Neptune size and larger

• Length of surveys limits distances planets have been found from starsLength of surveys limits distances planets have been found from stars– Earliest surveys started 1988Earliest surveys started 1988– Jupiter (5AU from Sun) takes 12 yrs to orbit SunJupiter (5AU from Sun) takes 12 yrs to orbit Sun– Saturn takes 30 yearsSaturn takes 30 years

• Would be strongly hinted at but not yet completed one orbit since surveys beganWould be strongly hinted at but not yet completed one orbit since surveys began

– Do not see planet Do not see planet directlydirectly


Alpha Cen BbAlpha Cen Bb

• Alpha Cen system is the nearest star system to us

• Alpha Cen B has been monitored by radial velocity method

• Very recent discovery of a rocky planet:• Minimum mass 1.1xEarth• Period 3.2 days• Dumusque et al. 2012, Nature

• Thought: if Alpha Cen B has a rocky planet, do most stars have rocky planets?

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3677 Life in the Universe: Extra-solar planets

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