1. DEPARTMENT OF PHYSICS AND ASTRONOMY Life in the Universe: Extra-solar planets Dr. Matt Burleigh www.star.le.ac.uk/~mbu

2. Dr. Matt Burleigh 3677: Life in the Universe 3677 Timetable Today and Friday 10am: MB Extrasolar planetsToday and Friday 10am: MB Extrasolar planets Then Derek Raine (Origins of Life)Then Derek Raine (Origins of Life) Then Mark Sims (Life in the solar system)Then Mark Sims (Life in the solar system)

3. Dr. Matt Burleigh 3677: Life in the Universe Contents Methods for detectionMethods for detection –Doppler “wobble” –Transits –Direct Imaging CharacterisationCharacterisation –Statistics –Implications for formation scenarios

4. Dr. Matt Burleigh 3677: Life in the Universe Useful reading / web sites Extra-solar planets encyclopaediaExtra-solar planets encyclopaediaExtra-solar planets encyclopaediaExtra-solar planets encyclopaedia California & Carnegie Planets SearchCalifornia & Carnegie Planets SearchCalifornia & Carnegie Planets SearchCalifornia & Carnegie Planets Search How stuff works planet-hunting pageHow stuff works planet-hunting pageHow stuff works planet-hunting pageHow stuff works planet-hunting page –Includes lots of animations & graphics JPL planet finding pageJPL planet finding pageJPL planet finding pageJPL planet finding page –Look at the science & multimedia gallery pages

5. Dr. Matt Burleigh 3677: Life in the Universe What is a planet? International Astronomical Union definition –International Astronomical Union definition – –An object orbiting a star But see later this lecture…But see later this lecture… –Too small for dueterium fusion to occur Less than 13 times the mass of JupiterLess than 13 times the mass of Jupiter –Formation mechanism? Forms from a circumstellar diskForms from a circumstellar disk –Lower mass limit – IAU decided that Pluto should be downgraded!

6. Dr. Matt Burleigh 3677: Life in the Universe A brief history of extra-solar planets In the 16th century the Italian philosopher Giordano Bruno said that the fixed stars are really suns like our own, with planets going round themIn the 16th century the Italian philosopher Giordano Bruno said that the fixed stars are really suns like our own, with planets going round them 1991 Radio astronomers Alex Wolszczan & Dale Frail discovered planets around a pulsar PSR1257+121991 Radio astronomers Alex Wolszczan & Dale Frail discovered planets around a pulsar PSR1257+12 –Variations in arrival times of pulses suggests presence of three or more planets –Planets probably formed from debris left after supernova explosion 1995 Planet found around nearby Sun-like star 51 Peg by Swiss astronomers Michel Mayor & Didier Queloz using the “Doppler Wobble” method1995 Planet found around nearby Sun-like star 51 Peg by Swiss astronomers Michel Mayor & Didier Queloz using the “Doppler Wobble” method –Most successful detection method by far, but other methods like transits are now very successful –502 exoplanets in total found to date by all methods –~100 found since I gave this lecture last year

7. Dr. Matt Burleigh 3677: Life in the Universe Planet Hunting: The Radial Velocity Technique (“Doppler Wobble”) (“Doppler Wobble”) (“Doppler Wobble”) Star + planet orbit common centre of gravity Star + planet orbit common centre of gravity As star moves towards observer, wavelength of light shortens (is blue- shifted) As star moves towards observer, wavelength of light shortens (is blue- shifted) Light red-shifted as star moves away Light red-shifted as star moves away Measure: Measure:  e = (   e ) / e = v r / c   observed wavelength, e =emitted wavelength   observed wavelength, e =emitted wavelength 468 planets detected by Doppler Wobble inc. 47 multiple systems

8. Dr. Matt Burleigh 3677: Life in the Universe M * from spectral type

9. Dr. Matt Burleigh 3677: Life in the Universe Doppler Wobble Method: Summary Precision of current surveys is now 1m/s:Precision of current surveys is now 1m/s: –Jupiter causes Sun’s velocity to vary by 12.5m/s –All 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 ~2M earth Length of surveys limits distances planets have been found from starsLength of surveys limits distances planets have been found from stars –Earliest surveys started 1989 –Jupiter (5AU from Sun) takes 12 yrs to orbit Sun –Saturn takes 30 years Would remain undetectedWould remain undetected Do not see planet directlyDo not see planet directly

10. Dr. Matt Burleigh 3677: Life in the Universe Doppler Wobble Method: Summary Since measure K (= v * sin i), not v * directly, only know mass in terms of the orbital inclination iSince measure K (= v * sin i), not v * directly, only know mass in terms of the orbital inclination i Therefore only know the planet’s minimum mass, M sin iTherefore only know the planet’s minimum mass, M sin i –If i=90 o (eclipsing or transiting) then know mass exactly i=90 0 Orbital plane i0i0i0i0

11. Dr. Matt Burleigh 3677: Life in the Universe Transits Planets observed at inclinations near 90 o will transit their host stars Planets observed at inclinations near 90 o will transit their host stars

12. Dr. Matt Burleigh 3677: Life in the Universe Transits AssumingAssuming –The whole planet passes in front of the star –And ignoring limb darkening as negligible Then the depth of the eclipse is simply the ratio of the planetary and stellar disk areas:Then the depth of the eclipse is simply the ratio of the planetary and stellar disk areas: –i.e.  f / f * =  R p 2 /  R * 2 = (R p / R * ) 2 We measure the change in magnitude  m, and obtain the stellar radius from the spectral typeWe measure the change in magnitude  m, and obtain the stellar radius from the spectral type –Hence by converting to flux we can measure the planet’s radius –Rem.  m = m transit – m * = 2.5 log (f * / f transit ) (smaller number means brighter)(smaller number means brighter)

13. Dr. Matt Burleigh 3677: Life in the Universe Transits Example: first known transiting planet HD209458b –  m = 0.017 mags –So (f * / f transit ) = 1.0158, i.e.  f=1.58% –From the spectral type (G0) R=1.15R sun –So using  f / f * = (R p / R * ) 2 and setting f * =100% –Find R p =0.145R sun –Since R sun =9.73R J then –R p = 1.41R J

14. Dr. Matt Burleigh 3677: Life in the Universe Transits HD209458b more:HD209458b more: –From Doppler wobble method know M sin i = 0.62M J –Transiting, hence assume i=90 o, so M=0.62M J –Density = 0.29 g/cm 3 c.f. Saturn 0.69 g/cm 3c.f. Saturn 0.69 g/cm 3 –HD209458b is a gas giant!

15. Dr. Matt Burleigh 3677: Life in the Universe Transits For an edge-on orbit, transit duration is given by:For an edge-on orbit, transit duration is given by: –  t = (PR * ) / (  a) Where P=period in days, a=semi-major axis of orbitWhere P=period in days, a=semi-major axis of orbit Probability of transit (for random orbit)Probability of transit (for random orbit) –P transit = R * / a –For Earth (P=1yr, a=1AU), P transit =0.5% –But for close, “hot” Jupiters, P transit =10% –Of course, relative probability of detecting Earths is lower since would have to observe for up to 1 year

16. Dr. Matt Burleigh 3677: Life in the Universe Transits AdvantagesAdvantages –Easy. Can be done with small, cheap telescopes E.g. WASPE.g. WASP –Possible to detect low mass planets, including “Earths”, especially from space (Kepler mission, 2008) DisadvantagesDisadvantages –Probability of seeing a transit is low Need to observe many stars simultaneouslyNeed to observe many stars simultaneously –Easy to confuse with starspots, binary/triple systems –Needs radial velocity measurements for confirmation, masses

17. Dr. Matt Burleigh 3677: Life in the Universe Super WASP Wide Angle Search for Planets (by transit method)Wide Angle Search for Planets (by transit method) First telescope located in La Palma, second in South AfricaFirst telescope located in La Palma, second in South Africa Operations started May ‘04Operations started May ‘04 Data stored and processed at LeicesterData stored and processed at Leicester >40 new planets detected!>40 new planets detected! www.superwasp.orgwww.superwasp.orgwww.superwasp.org www.wasp.le.ac.ukwww.wasp.le.ac.ukwww.wasp.le.ac.uk

18. Dr. Matt Burleigh 3677: Life in the Universe Super WASP SuperWASP monitors about 1/4 of the sky from each siteSuperWASP monitors about 1/4 of the sky from each site That means millions of stars, every night!That means millions of stars, every night!

19. Dr. Matt Burleigh 3677: Life in the Universe Direct detection Imaging = spectroscopy = physics: composition & structureImaging = spectroscopy = physics: composition & structure DifficultDifficult Why?Why? –Stars like the Sun are billions of times brighter than planets –Planets and stars lie very close together on the sky At 10pc Jupiter and the Sun are separated by 0.5”At 10pc Jupiter and the Sun are separated by 0.5”

20. Dr. Matt Burleigh 3677: Life in the Universe Direct detection Problem 1:Problem 1: –Stars bright, planets faint Solution:Solution: –Block starlight with a coronagraph Problem 2:Problem 2: –Earth’s atmosphere distorts starlight, reduces resolution Solution:Solution: –Adaptive optics, Interferometry – difficult, expensive –Or look around very young and/or intrinsically faint stars (not Sun-like)

21. Dr. Matt Burleigh 3677: Life in the Universe First directly imaged planet? 2M1207 in TW Hya association2M1207 in TW Hya association Discovered at ESO VLT in ChileDiscovered at ESO VLT in Chile 25M jup Brown dwarf + 5M jup “planet”25M jup Brown dwarf + 5M jup “planet” Distance ~55pcDistance ~55pc Very young cluster ~10M yearsVery young cluster ~10M years Physical separation ~55AUPhysical separation ~55AU A brown dwarf is a failed starA brown dwarf is a failed star –Can this really be called a planet? –Formation mechanism may be crucial!

22. Dr. Matt Burleigh 3677: Life in the Universe First directly imaged planetary system Last year 3 planets imaged around the star HR8799Last year 3 planets imaged around the star HR8799 130 light years away (40pc)130 light years away (40pc) Three planets at 24, 38 and 68AU separationThree planets at 24, 38 and 68AU separation –In comparison, Jupiter is at 5AU and Neptune at 30AU Masses of 7Mjup, 10Mjup and 10MjupMasses of 7Mjup, 10Mjup and 10Mjup Young: 60MyrYoung: 60Myr –Earth is ~4.5Gyr

23. Dr. Matt Burleigh 3677: Life in the Universe Fomalhaut (alpha Piscis Austrini) One of the brightest stars in the southern sky One of the brightest stars in the southern sky Long known to have a dusty debris disk Long known to have a dusty debris disk Shape of disk suggested presence of planet Shape of disk suggested presence of planet 2M jup planet imaged by HST inside disk 2M jup planet imaged by HST inside disk 200Myr old 200Myr old Like early solar system Like early solar system

24. Dr. Matt Burleigh 3677: Life in the Universe Direct detection: White Dwarfs White dwarfs are the end state of stars like the SunWhite dwarfs are the end state of stars like the Sun –What will happen to the solar system in the future? WDs are 1,000-10,000 times fainter than Sun-like starsWDs are 1,000-10,000 times fainter than Sun-like stars – contrast problem reduced Outer planets should survive evolution of Sun to white dwarf stage, and migrate outwards Outer planets should survive evolution of Sun to white dwarf stage, and migrate outwards – more easily resolved Over 100 WD within 20pc Over 100 WD within 20pc – At 10pc a separation of 100AU = 10” on sky At Leicester we are searching for planets around nearby WD with 8m telescopes and the Spitzer space telescope At Leicester we are searching for planets around nearby WD with 8m telescopes and the Spitzer space telescope

25. Dr. Matt Burleigh 3677: Life in the Universe Direct detection: White Dwarfs No planets yet, just brown dwarfsNo planets yet, just brown dwarfs –Currently limited to finding planets >5M jup –Not very common –May have to wait for JWST But have found some WDs are surrounded by dust and gas disksBut have found some WDs are surrounded by dust and gas disks –Remains of small rocky planets and asteroids that strayed too close to WD –Ripped apart by tidal forces –Can study composition of extra-solar terrestrial bodies!

26. Dr. Matt Burleigh 3677: Life in the Universe What we know about extra-solar planets 502 planets now found502 planets now found 52 multiple systems52 multiple systems 106 transiting planets106 transiting planets Unexpected population with periods of 2-4 days: “hot Jupiters”Unexpected population with periods of 2-4 days: “hot Jupiters” Planets with orbits like Jupiter discovered (eg 55 Cancri d)Planets with orbits like Jupiter discovered (eg 55 Cancri d) Smallest planet: CoRoT-7b - 1.7R earthSmallest planet: CoRoT-7b - 1.7R earth

27. Dr. Matt Burleigh 3677: Life in the Universe Extra-solar planet period distribution Notice the “pile-up” at periods of 2-4 days / 0.04-0.05AUNotice the “pile-up” at periods of 2-4 days / 0.04-0.05AU The most distant planets discovered by radial velocities so far are at 5-6AUThe most distant planets discovered by radial velocities so far are at 5-6AU Imaging surveys finding very wide orbit planetsImaging surveys finding very wide orbit planets

28. Dr. Matt Burleigh 3677: Life in the Universe “Hot Jupiter” planets Doppler Wobble and transit surveys find many gas giants in orbits of 2-4 daysDoppler Wobble and transit surveys find many gas giants in orbits of 2-4 days –cf Mercury’s orbit is 80 days Surveys are biased towards finding themSurveys are biased towards finding them –Larger Doppler Wobble signal –Greater probability of transit These planets are heated to >1000 o F on “day” sideThese planets are heated to >1000 o F on “day” side –And are “tidally locked” like the Moon –Causes extreme weather conditions

29. Dr. Matt Burleigh 3677: Life in the Universe Extra-solar planet mass distribution Mass distribution peaks at 1- 2 x mass of JupiterMass distribution peaks at 1- 2 x mass of Jupiter Lowest mass planet so far: 5.5xM EarthLowest mass planet so far: 5.5xM Earth Super-Jupiters (>few M Jup ) are not commonSuper-Jupiters (>few M Jup ) are not common –Implications for planet formation theories? –Or only exist in number at large separation? –Or exist around massive stars?

30. Dr. Matt Burleigh 3677: Life in the Universe Eccentricity vs semi-major axis observational bias extra-solar planets solar system planets : - large distribution of e (same as close binary stars) - - but some planets in circular orbits do exist far away from star - the planets in our own system have small eccentricities ie STABLE - planets close to the star are tidally circularized What we know about extra-solar planets most extra-solar planets are in much more eccentric orbits than the giant planets in the solar system

31. Dr. Matt Burleigh 3677: Life in the Universe Results of the Planet Hunting surveys Of 2000 stars surveyedOf 2000 stars surveyed –5% have gas giants between 0.02AU and 5AU –10% may have gas giants in wider orbits –<1% have Hot Jupiters How many have Earths…..?How many have Earths…..?

32. Dr. Matt Burleigh 3677: Life in the Universe What about the stars themselves? Surveys began by targeting sun-like stars (spectral types F, G and K)Surveys began by targeting sun-like stars (spectral types F, G and K) Now extended to M dwarfs ( 1.5M sun )Now extended to M dwarfs ( 1.5M sun ) –Subgiants are the descendants of A stars Incidence of planets is greatest for late F starsIncidence of planets is greatest for late F stars –F7-9V > GV > KV > MV Stars that host planets appear to be on average more metal-richStars that host planets appear to be on average more metal-rich More massive stars tend to have more massive planetsMore massive stars tend to have more massive planets

33. Dr. Matt Burleigh 3677: Life in the Universe Metallicity Metallicity The abundance of elements heavier than He relative to the Sun Overall, ~5% of solar-like stars have radial velocity –detected JupitersOverall, ~5% of solar-like stars have radial velocity –detected Jupiters But if we take metallicity into account:But if we take metallicity into account: –>20% of stars with 3x the metal content of the Sun have planets –~3% of stars with 1/3 rd of the Sun’s metallicity have planets Does this result imply that planets more easily form in metal-rich environments?Does this result imply that planets more easily form in metal-rich environments? –If so, then maybe planet hunters should be targeting metal-rich stars –Especially if we are looking for rocky planets

34. Dr. Matt Burleigh 3677: Life in the Universe Planet formation scenarios There are two main models which have been proposed to describe the formation of the extra-solar planets: – –(I) Planets form from dust which agglomerates into cores which then accrete gas from a disc. – –(II) A gravitational instability in a protostellar disc creates a number of giant planets. Both models have trouble reproducing both the observed distribution of extra-solar planets and the solar-system.

35. Dr. Matt Burleigh 3677: Life in the Universe Accretion onto cores Planetary cores form through the agglomeration of dust into grains, pebbles, rocks and planetesimals within a gaseous disc At the smallest scale (<1 cm) cohesion occurs by non-gravitational forces e.g. chemical processes. On the largest scale (>1 km) gravitational attraction will dominate. On intermediate scales the process is poorly understood. These planetesimals coalesce to form planetary cores The most massive cores accrete gas to form the giant planets Planet formation occurs over 10 7 yrs.

36. Dr. Matt Burleigh 3677: Life in the Universe Gravitational instability A gravitational instability requires a sudden change in disc properties on a timescale less than the dynamical timescale of the disc. Planet formation occurs on a timescale of 1000 yrs. A number of planets in eccentric orbits may be formed. Sudden change in disc properties could be achieved by cooling or by a dynamical interaction. Simulations show a large number of planets form from a single disc. Only produces gaseous planets – rocky (terrestrial) planets are not formed. Is not applicable to the solar system.

37. Dr. Matt Burleigh 3677: Life in the Universe Where do the hot Jupiters come from? No element will condense within ~0.1AU of a star since T>1000KNo element will condense within ~0.1AU of a star since T>1000K Planets most likely form beyond the “ice-line”, the distance at which ice formsPlanets most likely form beyond the “ice-line”, the distance at which ice forms –More solids available for building planets –Distance depends on mass and conditions of proto- planetary disk, but generally >1AU Hot Jupiters currently at ~0.03-0.04AU cannot have formed thereHot Jupiters currently at ~0.03-0.04AU cannot have formed there –Migration: Planets migrate inwards and stop when disk is finally cleared If migration time < disk lifetimeIf migration time < disk lifetime –Planets fall into star –Excess of planets at 0.03-0.04AU is evidence of a stopping mechanism tides? magnetic cavities? mass transfer?tides? magnetic cavities? mass transfer? Large planets will migrate more slowlyLarge planets will migrate more slowly –Explanation for lack of super-Jupiters in close orbits

38. Dr. Matt Burleigh 3677: Life in the Universe Hunting for Earth-like planets Pace of planet discoveries will continue to increase in next few yearsPace of planet discoveries will continue to increase in next few years Radial velocity and direct imaging surveys will reveal outer giant planets with long periods like our own Solar SystemRadial velocity and direct imaging surveys will reveal outer giant planets with long periods like our own Solar System Transit surveys will reveal small planets in close orbits to their sunsTransit surveys will reveal small planets in close orbits to their suns But the greatest goal is the detection of other EarthsBut the greatest goal is the detection of other Earths

39. Dr. Matt Burleigh 3677: Life in the Universe Towards other Earths TelescopeMethodDate Corot (Fr) Transits2007 Kepler (NASA) Transits2008 GAIA (ESA) Astrometry2012 SIM (NASA) Astrometry 2015 (?) Plato (ESA) Transits2017 Darwin (ESA) Imaging 2025+ (?) 42m E-ELT Imaging2018

40. Dr. Matt Burleigh 3677: Life in the UniverseKepler Searching for Earths by transit methodSearching for Earths by transit method Launched last year by NASALaunched last year by NASA Aims to find an Earth around a Sun- like star in a one year orbitAims to find an Earth around a Sun- like star in a one year orbit Need three transits to confirmNeed three transits to confirm So mission lasts at least three years…So mission lasts at least three years…

41. Dr. Matt Burleigh 3677: Life in the Universe Towards Other Earths: Habitable Zones Habitable zone defined as where liquid water existsHabitable zone defined as where liquid water exists Changes in extent and distance from star according to star’s spectral type (ie temperature)Changes in extent and distance from star according to star’s spectral type (ie temperature)

42. Dr. Matt Burleigh 3677: Life in the Universe Towards Other Earths: Biomarkers So we find a planet with the same mass as Earth, and in the habitable zone:So we find a planet with the same mass as Earth, and in the habitable zone: –How can we tell it harbours life? Search for biomarkersSearch for biomarkers –Water –Ozone –Albedo