DEPARTMENT OF PHYSICS AND ASTRONOMY Life in the Universe: Extra-solar planets Dr. Matt Burleigh

Dr. Matt Burleigh 3677: Life in the Universe 3677 Timetable Today, and next Friday: MB Extrasolar planetsToday, and next Friday: MB Extrasolar planets Then Derek RaineThen Derek Raine After Xmas: Mark Sims (Life in the solar system)After Xmas: Mark Sims (Life in the solar system)

Dr. Matt Burleigh 3677: Life in the Universe

Contents Methods for detectionMethods for detection –Doppler “wobble” –Transits –Microlensing –Direct Imaging CharacterisationCharacterisation –Statistics –Implications for formation scenarios

Dr. Matt Burleigh 3677: Life in the Universe Useful reading / web sites Nature, Vol. 419, p. 355 (26 September 2002)Nature, Vol. 419, p. 355 (26 September 2002) 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

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 last year that Pluto should be downgraded!

Dr. Matt Burleigh 3677: Life in the Universe A brief history of exoplanets 1991 Wolszczan & Frail discovered planets around a pulsar PSR Wolszczan & Frail discovered planets around a pulsar PSR –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” method1995 Planets found around nearby Sun-like star 51 Peg by Doppler “wobble” method –Most successful detection method by far –265 exoplanets found to date

Dr. Matt Burleigh 3677: Life in the Universe Radial Velocity Technique (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

Dr. Matt Burleigh 3677: Life in the Universe Measuring Stellar Doppler shifts Method: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

Dr. Matt Burleigh 3677: Life in the Universe Measuring Stellar Doppler shifts Method:Method: –Measure wavelength (or velocity) of star’s lines relative to the iodine –  e = (   e ) / e = v r / c   observed wavelength, e =emitted wavelength   observed wavelength, e =emitted wavelength

Dr. Matt Burleigh 3677: Life in the Universe N.B. M * comes from the spectral typeN.B. M * comes from the spectral type

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 Limited to gas planets and largerLimited to gas planets and larger –Note recently discovered “hot Neptunes” (>14M Earth ) –Not yet suitable for Earth-like planets 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

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 massTherefore only know the planet’s minimum mass –If i=90 o (eclipsing or transiting) then know mass exactly i=90 0 Orbital plane i0i0i0i0

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

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 planetary radius –Rem.  m = m transit – m * = 2.5 log (f * / f transit ) (smaller number means brighter)(smaller number means brighter)

Dr. Matt Burleigh 3677: Life in the Universe Transits Example: first known transiting planet HD209458b –  m = mags –So (f * / f transit ) = , 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

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!

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

Dr. Matt Burleigh 3677: Life in the Universe Transits AdvantagesAdvantages –Easy. Can be done with small, cheap telescopes E.g. WASP,E.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

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 PalmaFirst telescope located in La Palma Operations started May ‘04Operations started May ‘04 Data stored at LeicesterData stored at Leicester Three new planets detected!Three new planets detected!

Dr. Matt Burleigh 3677: Life in the Universe Gravitational Microlensing A consequence of general relativityA 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 EarthThe 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 resultThe star doing the lensing brightens as a result We record this brightening, which can last for daysWe record this brightening, which can last for days If the lensed star has a planetary companion, the characteristic lensing light curve is modifiedIf 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)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! 4 planets found so far, including one at 5.5 Earth masses!

Dr. Matt Burleigh 3677: Life in the Universe Direct detection Imaging = spectroscopy = physics: composition & structureImaging = spectroscopy = physics: composition & structure No planet in orbit around another star has been directly imagedNo planet in orbit around another star has been directly imaged 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”

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)

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!

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 1,000-10,000 times fainter than Sun-like stars1,000-10,000 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 I have a programme to search for planets around nearby WD with the Gemini 8m telescopes 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 etcWe call it “DODO” – Degenerate Objects around Degenerate Objects or Dead Objects etc

Dr. Matt Burleigh 3677: Life in the Universe Direct Detection: White Dwarfs 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 ~5M Jup ! Proper motions Two images taken one year apart

Dr. Matt Burleigh 3677: Life in the Universe What we know about extra-solar planets 265 planets now found265 planets now found 25 multiple systems25 multiple systems 33 transiting planets – can directly measure radii33 transiting planets – can directly measure radii Unexpected population with periods of 3-4 days: “hot Jupiters”Unexpected population with periods of 3-4 days: “hot Jupiters” New population from transit surveys at ~2 daysNew population from transit surveys at ~2 days First planet with an orbit like Jupiter discovered (55 Cancri d)First planet with an orbit like Jupiter discovered (55 Cancri d) Is our solar system typical?Is our solar system typical?

Dr. Matt Burleigh 3677: Life in the Universe Extra-solar planet period distribution Notice the “pile- up” at periods of 3-4 days / AUNotice the “pile- up” at periods of 3-4 days / AU The most distant planets discovered so far are at 5-6AUThe most distant planets discovered so far are at 5-6AU New discovery of transiting planets at ~2 daysNew discovery of transiting planets at ~2 days

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?

Dr. Matt Burleigh 3677: Life in the Universe Selection effects Astronomical surveys tend to have built in biasesAstronomical surveys tend to have built in biases These “selection effects” must be understood before we can interpret resultsThese “selection effects” must be understood before we can interpret results The Doppler Wobble method is most sensitive to massive, close-in planetsThe Doppler Wobble method is most sensitive to massive, close-in planets It is not yet sensitive to planets as small as Earth, even close-inIt 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 JupiterAs 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 limitThe 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

Dr. Matt Burleigh 3677: Life in the Universe What we know about extra-solar planets: Mass versus semi-major axis Blue – exoplanetsBlue – exoplanets Red – solar systemRed – solar system Many of the known solar systems have ~Jupiter- mass planets in small orbits, <0.1AUMany 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 orbitsBut almost no super- Jupiters are found in close orbits –Real, not a selection effect

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 binaries) - 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 on 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

Dr. Matt Burleigh 3677: Life in the Universe Statistics of the Doppler Wobble surveys: Summary Of 2000 stars surveyedOf 2000 stars surveyed –~5% have gas giants between 0.02AU and 5AU Trends suggest ~10% of stars have planets in orbits 5-7AUTrends suggest ~10% of stars have planets in orbits 5-7AU –0.85% have hot Jupiters Real effectReal effect –Hot Jupiters are not massive Almost all have Msini~1M jup or lessAlmost all have Msini~1M jup or less No close-in, “super-Jupiters”No close-in, “super-Jupiters” –Mass distribution strongly peaks at 1M jup and falls as dN/dM~M -0.7 But surveys currently biased towards hot JupitersBut surveys currently biased towards hot Jupiters Expect mass distribution to flatten somewhat as long periods, super-Jupiters are discoveredExpect mass distribution to flatten somewhat as long periods, super-Jupiters are discovered

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 dwarfsNow extended to M dwarfs Incidence of planets is greatest for late F starsIncidence 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-richStars that host planets appear to be on average more metal-rich

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

Dr. Matt Burleigh 3677: Life in the Universe Metallicity 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 This result also implies that chances of very old lifeforms (> few billion years) in the Universe are slimThis 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 companionBUT 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

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: 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.

Dr. Matt Burleigh 3677: Life in the Universe 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 10 7 yrs.

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.

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 ~ AU cannot have formed thereHot Jupiters currently at ~ AU cannot have formed there Migration!Migration!

Dr. Matt Burleigh 3677: Life in the Universe Planetary migration Planets migrate inwards and stop when disk is finally clearedPlanets 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 AU is evidence of a stopping mechanism in some cases –Nature of stopping mechanism unclear: 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

Dr. Matt Burleigh 3677: Life in the Universe Planetary migration & terrestrial planets Migrating giant planets will be detrimental to terrestrial planet survivability, if they both form at same timeMigrating 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),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 10 7 years)Alternatively, terrestrial planet formation may occur after dissipation of gas in proto-planetary disk (after 10 7 years) –Disruption by a migrating giant planet unlikely –Almost all planet-forming stars will have terrestrial planets

Dr. Matt Burleigh 3677: Life in the Universe The future: towards other Earths Pace of planet discoveries will increase in next few yearsPace of planet discoveries will increase in next few years Radial velocity surveys will reveal outer giant planets with long periods like our own Solar SystemRadial 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 orbitsTransit surveys will reveal planets smaller than Saturn in close orbits First direct images will be obtainedFirst direct images will be obtained But the greatest goal is the detection of other EarthsBut the greatest goal is the detection of other Earths

Dr. Matt Burleigh 3677: Life in the Universe Towards other Earths TelescopeMethodDate CorotTransits2007 KeplerTransits2008 GAIAAstrometry2012 SIMInterferometry (?) Darwin/TPFInterferometry (?) 50m ELT Imaging2019

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) It is possible for rocky planets to exist in stable orbits of habitable zones of known hot Jupiter systemsIt 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

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

Dr. Matt Burleigh 3677: Life in the Universe

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

Dr. Matt Burleigh 3677: Life in the Universe Direct detection: proto-planetary disks Disk is 200 million years oldDisk is 200 million years old Like the early Solar SystemLike the early Solar System Is a planet perturbing the disk & forming the hole?Is a planet perturbing the disk & forming the hole?

Dr. Matt Burleigh 3677: Life in the Universe Direct detection: Epsilon Eridani Young, Sun-like star only 3pc awayYoung, Sun-like star only 3pc away Dust disk is clumpyDust disk is clumpy Clumps seen to rotateClumps seen to rotate Requires presence of at least one Jovian planetRequires presence of at least one Jovian planet 1.55M Jup companion confirmed by HST measurements1.55M Jup companion confirmed by HST measurements