Life in the Universe: Extra-solar planets

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

2. 3677 Timetable • Today, and next Friday: MB Extrasolar planets • Then Derek Raine • After Xmas: Mark Sims (Life in the solar system)

4. Contents • Methods for detection • Doppler “wobble” • Transits • Microlensing • Direct Imaging • Characterisation • Statistics • Implications for formation scenarios

5. Useful reading / web sites • Nature, Vol. 419, p. 355 (26 September 2002) • Extra-solar planets encyclopaedia • California & Carnegie Planets Search • How stuff works planet-hunting page • Includes lots of animations & graphics • JPL planet finding page • Look at the science & multimedia gallery pages

6. What is a planet? • International Astronomical Union definition – • An object orbiting a star • But see later this lecture… • Too small for dueterium fusion to occur • Less than 13 times the mass of Jupiter • Formation mechanism? • Forms from a circumstellar disk • Lower mass limit – IAU decided last year that Pluto should be downgraded!

7. A brief history of exoplanets • 1991 Wolszczan & 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 Planets found around nearby Sun-like star 51 Peg by Doppler “wobble” method • Most successful detection method by far • 265 exoplanets found to date

8. Radial Velocity Technique(Doppler “Wobble”) • Star + planet orbit common centre of gravity • As star moves towards observer, wavelength of light shortens (is blue-shifted) • Light red-shifted as star moves away

9. Measuring Stellar Doppler shifts • Method: • Observe star’s spectrum through a cell of iodine gas • Iodine superimposes many lines on star’s spectrum • Measure wavelength (or velocity) of star’s lines relative to the iodine

10. Measuring Stellar Doppler shifts • Method: • Measure wavelength (or velocity) of star’s lines relative to the iodine • Dl / le = (l0-le) / le = vr / c lo=observed wavelength, le=emitted wavelength

11. N.B. M* comes from the spectraltype

12. Doppler Wobble Method: Summary • 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 inactive • Limited to gas planets and larger • Note recently discovered “hot Neptunes” (>14MEarth) • Not yet suitable for Earth-like planets • Length 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 undetected • Do not see planet directly

13. Doppler Wobble Method: Summary • Since 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 • If i=90o (eclipsing or transiting) then know mass exactly Orbital plane i=900 Orbital plane i0

14. Transits • Planets observed at inclinations near 90o will transit their host stars

15. Transits • Assuming • 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: • i.e. Df / f* = pRp2 / pR*2 = (Rp / R*)2 • We measure the change in magnitude Dm, and obtain the stellar radius from the spectral type • Hence by converting to flux we can measure the planetary radius • Rem. Dm = mtransit – m* = 2.5 log (f* / ftransit) • (smaller number means brighter)

16. Transits Example: first known transiting planet HD209458b • Dm = 0.017 mags • So (f* / ftransit) = 1.0158, i.e. Df=1.58% • From the spectral type (G0) R=1.15Rsun • So using Df / f* = (Rp / R*)2 and setting f*=100% • Find Rp=0.145Rsun • Since Rsun=9.73RJ then • Rp = 1.41RJ

17. Transits • HD209458b more: • From Doppler wobble method know M sin i = 0.62MJ • Transiting, hence assume i=90o, so M=0.62MJ • Density = 0.29 g/cm3 • c.f. Saturn 0.69 g/cm3 • HD209458b is a gas giant!

18. Transits • For an edge-on orbit, transit duration is given by: • Dt = (PR*) / (pa) • Where P=period in days, a=semi-major axis of orbit • Probability of transit (for random orbit) • Ptransit= R* / a • For Earth (P=1yr, a=1AU), Ptransit=0.5% • But for close, “hot” Jupiters, Ptransit=10% • Of course, relative probability of detecting Earths is lower since would have to observe for up to 1 year

19. Transits • Advantages • Easy. Can be done with small, cheap telescopes • E.g. WASP, • Possible to detect low mass planets, including “Earths”, especially from space (Kepler mission, 2008) • Disadvantages • Probability of seeing a transit is low • Need to observe many stars simultaneously • Easy to confuse with starspots, binary/triple systems • Needs radial velocity measurements for confirmation, masses

20. Super WASP • Wide Angle Search for Planets (by transit method) • First telescope located in La Palma • Operations started May ‘04 • Data stored at Leicester • Three new planets detected! • www.superwasp.org • www.wasp.le.ac.uk

21. Gravitational Microlensing • A consequence of general relativity • The grav. field of a relatively nearby star can bend the light of a more distant object as it passes in front of it, as seen from Earth • The star doing the lensing brightens as a result • We record this brightening, which can last for days • If the lensed star has a planetary companion, the characteristic lensing light curve is modified • Signals from an Earth-like planet would be strong (>5%) but brief (few hours) • 4 planets found so far, including one at 5.5 Earth masses!

22. Direct detection • Imaging = spectroscopy = physics: composition & structure • No planet in orbit around another star has been directly imaged • 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”

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

24. First directly imaged planet? • 2M1207 in TW Hya association • Discovered at ESO VLT in Chile • 25Mjup Brown dwarf + 5Mjup “planet” • Distance ~55pc • Very young cluster ~10M years • Physical separation ~55AU • A brown dwarf is a failed star • Can this really be called a planet? • Formation mechanism may be crucial!

25. Direct detection: White Dwarfs • White dwarfs are the end state of stars like the Sun • 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 • more easily resolved • Over 100 WD within 20pc • At 10pc a separation of 100AU = 10” on sky • I have a programme to search for planets around nearby WD with the Gemini 8m telescopes • We call it “DODO” – Degenerate Objects around Degenerate Objects or Dead Objects etc

26. Direct Detection: White Dwarfs Proper motions Two images taken one year apart • The faint objects in this field could be massive planets in wide orbits around this nearby white dwarf • The white dwarf moves relatively quickly compared to background stars in the field (see movie) • If a faint object moves with the WD, then I would get excited • But in this case, there is nothing, but we could have detected something as small as ~5MJup! • www.le.ac.uk/~mbu

27. What we know about extra-solar planets • 265 planets now found • 25 multiple systems • 33 transiting planets – can directly measure radii • Unexpected population with periods of 3-4 days: “hot Jupiters” • New population from transit surveys at ~2 days • First planet with an orbit like Jupiter discovered (55 Cancri d) • Is our solar system typical?

28. Extra-solar planet period distribution • Notice the “pile-up” at periods of 3-4 days / 0.04-0.05AU • The most distant planets discovered so far are at 5-6AU • New discovery of transiting planets at ~2 days

29. Extra-solar planet mass distribution • Mass distribution peaks at 1-2 x mass of Jupiter • Lowest mass planet so far: 5.5xMEarth • Super-Jupiters (>few MJup) are not common • Implications for planet formation theories? • Or only exist in number at large separation? • Or exist around massive stars?

30. Selection effects • Astronomical surveys tend to have built in biases • These “selection effects” must be understood before we can interpret results • The Doppler Wobble method is most sensitive to massive, close-in planets • It is not yet sensitive to planets as small as Earth, even close-in • As orbital period increases, the method becomes insensitive to planets less massive than Jupiter • The length of time that the surveys have been active (since 1989) sets the upper orbital period limit • Only now are analogues of Jupiter in our own Solar System going to be found

31. What we know about extra-solar planets:Mass versus semi-major axis • Blue – exoplanets • Red – solar system • Many of the known solar systems have ~Jupiter-mass planets in small orbits, <0.1AU • Selection effect of Doppler surveys • But almost no super-Jupiters are found in close orbits • Real, not a selection effect

32. : - large distribution of e (same as close binaries) What we know about extra-solar planets Eccentricity vs semi-major axis - most extra-solar planets are on orbits much more eccentric than the giant planets in the solar system: bad news for survivability of terrestrial planets - planets close to the star are tidally circularized observational bias extra-solar planets - planets on circular orbits do exist far away from star - the planets in our own system have small eccentricities ie STABLE solar system planets

33. Statistics of the Doppler Wobble surveys: Summary • Of 2000 stars surveyed • ~5% have gas giants between 0.02AU and 5AU • Trends suggest ~10% of stars have planets in orbits 5-7AU • 0.85% have hot Jupiters • Real effect • Hot Jupiters are not massive • Almost all have Msini~1Mjup or less • No close-in, “super-Jupiters” • Mass distribution strongly peaks at 1Mjup and falls as dN/dM~M-0.7 • But surveys currently biased towards hot Jupiters • Expect mass distribution to flatten somewhat as long periods, super-Jupiters are discovered

34. What about the stars themselves? • Surveys began by targeting sun-like stars (spectral types F, G and K) • Now extended to M dwarfs • Incidence of planets is greatest for late F stars • F7-9V > GV > KV > MV • Few low mass M dwarfs known to have a planets despite ease of detectability • Stars that host planets appear to be on average more metal-rich

35. MetallicityThe abundance of elements heavier than He relative to the Sun • Overall, ~5% of solar-like stars have radial velocity –detected Jupiters • But if we take metallicity into account: • >20% of stars with 3x the metal content of the Sun have planets • ~3% of stars with 1/3rd of the Sun’s metallicity have planets

36. Metallicity • 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 • This result also implies that chances of very old lifeforms (> few billion years) in the Universe are slim • With less heavy elements available terrestrial planets may be smaller and lower in mass than in our solar system • Is there a threshold metallicity for life to start (e.g. ½ solar)? • BUT Sigurdsson et al. (2003, Science, 301, 193) claim that a milli-second pulsar in globular M4 has a Jupiter size companion • Claim based on timing anomalies • If true, then planets may have been forming 12 billion years ago in a very metal-poor environment (<0.1 x solar) • Alternatively, planet may have formed from debris of supernova explosion that created the pulsar • Or planet does not exist, timing anomalies have another cause

37. Planet formation scenarios • There are two main models which have been proposed to • describe the formation of the extra-solar planets: • Planets form from dust which agglomerates into cores which then accrete gas from a disc. • 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.

38. Gas 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 and for the most massive cores these accrete gas to form the giant planets. • Planet formation occurs over 107 yrs.

39. 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.

40. Where do the hot Jupiters come from? • No 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 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 there • Migration!

41. Planetary migration • Planets migrate inwards and stop when disk is finally cleared • If migration time < disk lifetime • Planets fall into star • Excess of planets at 0.03-0.04AU is evidence of a stopping mechanism in some cases • Nature of stopping mechanism unclear: tides? magnetic cavities? mass transfer? • Large planets will migrate more slowly • Explanation for lack of super-Jupiters in close orbits

42. Planetary migration & terrestrial planets • Migrating giant planets will be detrimental to terrestrial planet survivability, if they both form at same time • Planets interior to a migrating giant planet will be disrupted and lost • Of course, these small planets may also migrate into star! • If terrestrial planets can only survive when migration doesn’t take place through their formation zone (few AU), • then 3%-20% of planet forming systems will possess them • Alternatively, terrestrial planet formation may occur after dissipation of gas in proto-planetary disk (after 107 years) • Disruption by a migrating giant planet unlikely • Almost all planet-forming stars will have terrestrial planets

43. The future: towards other Earths • Pace of planet discoveries will increase in next few years • Radial velocity surveys will reveal outer giant planets with long periods like our own Solar System • Transit surveys will reveal planets smaller than Saturn in close orbits • First direct images will be obtained • But the greatest goal is the detection of other Earths

44. Towards other Earths

45. Towards Other Earths: Habitable Zones • Habitable zone defined as where liquid water exists • Changes in extent and distance from star according to star’s spectral type (ie temperature) • It is possible for rocky planets to exist in stable orbits of habitable zones of known hot Jupiter systems • If they were not previously cleared out by migration Left: courtesy Prof. Keith Horne, St.Andrews Right: courtesy Prof. Barry Jones, Open

46. Towards Other Earths: Biomarkers • 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 biomarkers • Water • Ozone • Albedo

48. Direct detection: proto-planetary disks • Dust disk around Fomalhaut • Sub-mm image taken from James Clarke Maxwell telescope on Hawaii • Disk has a hole in centre like a doughnut • Part of disk appears to be perturbed

49. Direct detection: proto-planetary disks • Disk is 200 million years old • Like the early Solar System • Is a planet perturbing the disk & forming the hole?

50. Direct detection: Epsilon Eridani • Young, Sun-like star only 3pc away • Dust disk is clumpy • Clumps seen to rotate • Requires presence of at least one Jovian planet • 1.55MJup companion confirmed by HST measurements