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 Tuesday 11am: MB Extrasolar planets • Then Mark Sims (Life in the solar system)

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

4. Useful reading / web sites • 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

5. 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 that Pluto should be downgraded!

6. 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 them • 1991 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” method • Most successful detection method by far, but other methods like transits are now very successful • 405 exoplanets in total found to date by all methods

7. Planet Hunting: The 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 • Measure: • Dl / le = (l0-le) / le = vr / c • lo=observed wavelength, le=emitted wavelength 377 planets detected by Doppler Wobble inc. 38 multiple systems

8. M* from spectral type

9. 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 • Smallest planet found by this method is ~2Mearth • 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

10. 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, M sin i • If i=90o (eclipsing or transiting) then know mass exactly Orbital plane i=900 Orbital plane i0

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

12. 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 planet’s radius • Rem. Dm = mtransit – m* = 2.5 log (f* / ftransit) • (smaller number means brighter)

13. 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

14. 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!

15. 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

16. 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

17. Super WASP • Wide Angle Search for Planets (by transit method) • First telescope located in La Palma, second in South Africa • Operations started May ‘04 • Data stored and processed at Leicester • >30 new planets detected! • www.superwasp.org • www.wasp.le.ac.uk

18. Super WASP • SuperWASP monitors about 1/4 of the sky from each site • That means millions of stars, every night!

19. Direct detection • Imaging = spectroscopy = physics: composition & structure • Difficult • 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”

20. 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)

21. 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!

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

23. Fomalhaut (alpha Piscis Austrini) • One of the brightest stars in the southern sky • Long known to have a dusty debris disk • Shape of disk suggested presence of planet • 2Mjup planet imaged by HST inside disk • 200Myr old • Like early solar system

24. Direct detection: White Dwarfs • White 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 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 • At Leicester we are searching for planets around nearby WD with 8m telescopes and the Spitzer space telescope

25. Direct detection: White Dwarfs • No planets yet, just brown dwarfs • Currently limited to finding planets >5Mjup • Not very common • May have to wait for JWST • But 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. What we know about extra-solar planets • 405 planets now found • 38 multiple systems • 62 transiting planets • Unexpected population with periods of 2-4 days: “hot Jupiters” • Planets with orbits like Jupiter discovered (eg 55 Cancri d) • Smallest planet: CoRoT-7b - 1.7Rearth

27. Extra-solar planet period distribution • Notice 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 5-6AU • Imaging surveys finding very wide orbit planets

28. “Hot Jupiter” planets • Doppler 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 them • Larger Doppler Wobble signal • Greater probability of transit • These planets are heated to >1000oF on “day” side • And are “tidally locked” like the Moon • Causes extreme weather conditions

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

31. Results of the Planet Hunting surveys • Of 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…..?

32. What about the stars themselves? • Surveys began by targeting sun-like stars (spectral types F, G and K) • Now extended to M dwarfs (<1 Msun) and subgiants (>1.5Msun) • Subgiants are the descendants of A stars • Incidence of planets is greatest for late F stars • F7-9V > GV > KV > MV • Stars that host planets appear to be on average more metal-rich • More massive stars tend to have more massive planets

33. 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 • 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. 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. 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 107 yrs.

36. 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. 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: 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 • tides? magnetic cavities? mass transfer? • Large planets will migrate more slowly • Explanation for lack of super-Jupiters in close orbits

38. Hunting for Earth-like planets • Pace 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 System • Transit surveys will reveal small planets in close orbits to their suns • But the greatest goal is the detection of other Earths

39. Towards other Earths

40. Kepler • Searching for Earths by transit method • Launched last year by NASA • Aims to find an Earth around a Sun-like star in a one year orbit • Need three transits to confirm • So mission lasts at least three years…

41. 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)

42. 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