Observational constraints from the Solar System and from Extrasolar Planets

Transcription

1 Lecture 1 Part II Observational constraints from the Solar System and from Extrasolar Planets Lecture Universität Heidelberg WS 11/12 Dr. Christoph Mordasini mordasini@mpia.de Mentor Prof. T. Henning

2 Lecture overview 1. Introduction 2. Planet formation paradigm 3. Structure of the Solar System 4. The surprise: 51 Peg b 5. Detection techniques: radial velocity, transits, direct imaging, (microlensing, timing, astrometry) 6. Properties of extrasolar planets: mass, distance, eccentricity distributions, metallicity effect, massradius diagram,...

3 6. Properties of extrasolar planets

4 Current status 692 planets Candidates detected by radial velocity or astrometry 524 planetary systems 640 planets 76 multiple planet systems Transiting planets 171 planetary systems 184 planets 14 multiple planet systems Candidates detected by microlensing 12 planetary systems 13 planets 1 multiple planet systems planet candidates from the KEPLER satellite (transit) Candidates detected by imaging 22 planetary systems 25 planets 1 multiple planet systems Candidates detected by timing 9 planetary systems 14 planets 4 multiple planet systems Extra-solar planet encyclopedia (

5 Different techniques - different constraints direct imaging

6 A quickly progressing field Jupiter Saturn Uranus Neptune Earth Venus Mars Incredible wealth of data provided by already flying space mission (e.g. CoRoT and Kepler). More to come. Field observationally driven. Formation theory struggles to keep up... Observation and theory often don t match well. Common characteristic: provide observations of a large number of exoplanets. This data should therefore be treated as a statistical ensemble. This could help.

7 Overview on observed exoplanet properties Extrasolar planets exhibit a very large diversity (all techniques). Frequencies -Low mass close-in planets: approx. 50 % (radial velocity) -Jovian planets inside a few AU: approx % (rv) -Hot Jupiters: 0.5-1% (rv, transits) -Cold Neptunes are common (microlensing) -Overall (FGK stars): any mass, P<10 years: 75% (rv) The mass function is strongly rising towards small masses. There might be local minimum in the planetary mass function around Mearth (rv). The radius distribution is strongly increasing towards small radii (transits). The semimajor axis distribution of giant planets consists of a pile up at a period of about 3 days, a period valley, and an upturn at about 1 AU. (rv) Close-in low mass (or small radius) planets are found somewhat further out than Hot Jupiters (rv, transits). Hot Jupiters are lonely (rv, transits). Low mass close-in planets are in multiple systems (rv, transits). Massive giants planets at large distances are rare, at least around solar like stars (direct imaging). Giant planet frequency and host star [Fe/H] are positively correlated.

8 Today, formation theory cannot explain all these observed characteristics in one coherent picture. But at least for some observations, theory can give us ideas about possible mechanisms responsible for them.

9 6.1 a-m diagram

10 Diversity & structures in the a-m diagram Direct imaging Diversity - close-in giant planets - evaporating planets - eccentric planets - super Jupiters - Hot Neptunes & Super Earth - pile-ups and voids: planetary desert period valley - planets at large distances Microlensing Radial velocity & Transits J. Schneider s exoplanet.eu ~2010 (already outdated!) HARPS high precision sample 2011

11 Frequency of planet types Bias-corrected frequency (at least one planet per star) in the a-m (or equivalently period-mass) plane found by high precision RV around solar like FGK stars. Mayor et al Close in planets: P<50 d

12 6.2 Mass distribution

13 Mass distribution: old versions (giants) Udry et al Marcy et al Number of Planets Planet Mass Distribution dn/dm! M Planets Keck, Lick, AAT M sin i (M JUP )? HARPS mass distribution from RV observations. rising towards smaller masses. No obs. bias: smaller masses are more difficult to detect. beware of uncorrected (biased) distributions! frequency of Jovian planets falls as about M -1. maximum of giant planet masses at about 1 Jupiter mass. HARPS gave around 2007 the first hint of a second population of low mass planets.

14 Mass distribution II: new view (w. low masses) uncorrected for obs. bias corrected for obs. bias Mayor et al Mayor et al RV: thanks to 1 m/s precision observations, a new huge population of low mass planets has emerged in the last few years (mostly planets found by HARPS). bi (tri?) modal distribution: minimum at about 30 Earth masses. Imprint of formation? neptunian bump: strong increase between ME. overall maximum at small masses more than 50% of solar-type stars harbor at least one planet of any mass and with period up to 100 days (!)

15 Mass distribution III: upper boundary Grether & Lineweaver 2006 Sahlmann et al desert Segresan et al. less than 0.6 % of Sun-like stars have a brown-dwarf companion: so called Brown dwarf desert mass distribution function shows a lack of objects between MJ. Upper end of planet mass distribution? Nothing particular is seen at 13 MJ (Dburning limit). A distinction of BD vs. planets based on formation seems advisable, but difficult to realize in practice.

16 6.3 Semimajor axis distribution

17 Semimajor axis distribution I: giants Msini>0.75 MJ Msini<0.75 MJ Msini<21 Mearth Msini>50 Mearth uncorrected for obs. bias corrected for obs. bias N Udry & Santos log(period) (days) uncorrected for obs. bias Mayor et al The semimajor axis distribution of giant planets found by RV consists of a pile up at a period of about 3 days (0.04 AU). Stopping mechanism for migration? Tidal circularization? Magnetospheric cavity? maybe this pile up is only tiny, when properly correcting for obs. bias... a period valley (10 d < P < 100 d). Timescale effect? an upturn at about 1 AU. Reservoir at large distances. Original formation region?

18 Number of Planets per Star Transits P 0 = 1.7 days P 0 = 2.2 days Orbital Period (days) Howard et al Semimajor axis distribution II: P 0 = 7.0 days low mass/radius planets 2!4 R E 4!8 R E 8!32 R E Cut off below P0: -small radii 2-4 Re: P0 = 7 days -large radii >4 Re : P0 = 2 days. Neptunian and smaller sized further out than giant planets. No pile up at 3 days. Consistent with earlier results from high precision RV. Lovis et al estimated 10 days. Different stopping mechanism? RV Msini<50 Mearth uncorrected for obs. bias corrected for obs. bias The semimajor axis distribution of low mass planets found by RV consists of a continuous increase to about 40 d. then again a decrease. Why? affected by obs. bias? In principle corrected... such planets seem extremely abundant. Mayor et al. 2011

19 6.4 Eccentricity distribution

20 high and even very high eccentricities are common among exoplanets. This is very different than in the Solar System. mean eccentricity for giants: 0.28 > any planet of the Solar System. lower mass planets seem to have lower (but still quite high) e <0.5. planets very close to the star get tidally circularized. origin? formation - evolution? several possible explanations: Planet-planet interactions, influence of stellar or planet companion (Kozai effect), planet-disk interaction (M >10 MJ), dynamical interactions in a cluster Eccentricity distribution Mayor et al. 2011

21 6.5 Metallicity

22 Stellar metallicity Mordasini et al [Fe/H]: iron content of the star ([Fe/H]=0: solar composition, [Fe/H]=0.5: ~ 3 times more iron than the sun). Iron serves as a proxy for the overall metal content in the star (scaled solar composition). Stars in the the solar neighborhood have a distribution of metallicities which is roughly Gaussian around zero. Other parts in the galaxies can have completely different [Fe/H] distributions. These stars can also have a non-scaled solar composition (e.g. thick disk stars). There exists also a galactic metallicity gradient (higher [Fe/H] towards the center).

23 the detection probability for giant planets is a strongly increasing function of the host star metallicity. No hot Jupiters found in globular cluster 47 Tuc ([Fe/H]=-0.76). Expected for solar neighborhood frequency (~0.5%): seven discoveries. Best known star-planet correlation for exoplanets. Important constraint for formation. Explanation: planets form more readily in metal rich systems (primordial hypothesis). Likely. falling in planets have enriched the star (pollution hypothesis) Metallicity effect for giant planets N. Santos et al. (2005) search sample (all stars) stars with giant planets

24 No metallicity effect for low mass planets?? Mayor et al Mayor et al HARPS high precision sample: [Fe/H] for giant gaseous planets (black), for planets less massive than 30 ME (red), and for the global combined sample stars (blue). No metallicity effect for low mass planets. Even absence of low mass planets at high [Fe/H]? Natural outcome in the core accretion formation model.

25 Metallicity effect as function of mass Sousa et al The division between metalophile and not metalophile planets coincides with a minimum in the planetary mass function. (ca. 30 ME) Different populations: Giant planets (w. gas runaway accretion) vs. Neptunian planets. Correlation with other elemental abundances in the stars are less clear (maybe Lithium-planet anticorrelation).

26 6.6 Stellar mass

27 Influence of the host star mass Equal bin in log(mstar) M dwarfs solar stars intermediate masses Planetary system mass / star number => mass of planetary material scales with Mstar Planets around more massive stars are more massive and more frequent. RV bias underestimate the last bin. The Neptunian vs Jovian planet ratio is higher around M dwarfs. Consistent with a correlation of stellar mass, protoplanetary disk mass, and (giant) planet formation probability.

28 6.7 Multiplicity

29 Multiplicity Fraction of giant planets in multiple systems: ~25%. Incomplete... Fraction of low mass planets in multiple systems: ~70% Hot Jupiters seem to be lonely. Formation? Disk cleaning? Kozai? HD10180: up to 7 planets (RV) HD10180 a[au] Msini (b) c d e f g h Lovis et al Eccentricities Solar like star Fe/H=0.08, M=1.06 Msun Some period ratios are fairly close to integer or half-integer values, but no mean-motion resonances. Roughly regularly spaced on a logarithmic scale Kepler-11: six transiting planets Kepler-11 a[au] Msini b c d e f g 0.46 <300 Lissauer et al all within i 1.5 deg. Very complanar. Solar like star Fe/H=0, M=0.95 Msun b, c close to 5:4 resonance, but otherwise not in resonances. Low densities Dynamically packed

30 Packed systems numbers=distance in mutual Hill spheres. Lovis et al Low mass planets seem to follow a radius exclusion law: they cannot be too close together when measured in mutual Hill spheres. Many systems seem to be dynamically packed. Could not add another planet. Numerical simulations show that systems with 3 5 planets and masses between a few ME and a few MJ, separations between adjacent planets should be of at least 7 9 mutual Hill radii to ensure stability on a 10-Gyr timescale. Additional stability islands exist at resonances. Dynamical evolution of the systems. Ejection/collision of surplus planets.

31 Kepler multiple systems Lissauer et al N/N TOT < 5 Kepler adjacent pairings RV adjacent pairings Kepler has detected a lot of systems with multiple transiting planet candidates. The distribution of observed period ratios shows that the majority of candidate pairs are neither in nor near low-order mean motion resonances. Nonetheless, there is a small but statistically significant excesses of pairs both in resonance and spaced slightly further apart, particularly near 2:1. Resonant capture due during migration? Slope of Cumulative Period Ratio Period Ratio Kepler adjacent pairings RV adjacent pairings Period Ratio

32 6.8 Radius distribution

33 Kepler results Planet Radius, R p (R E ) (2) (4) (5) (6) (11) (6) (21) (17) (9) (39) (34) (9) (45) (20) (64) (85) Planet Occurrence! d 2 f/dlogp/dlogr p Planet Occurrence! f cell (69) (25) (73) (18) (104) (269) (262) (15) (15) (74) (60) (353) (521) (159) (28) (70) (31) (153) (607) (893) (375) (25) (154) (160) (208) (591) (1101) (410) (10) (10) (50) (59) (18) (85) (41) Orbital Period, P (days) 1 (15) (168) (52) (278) (198) (799) (749) (295) Howard et al Planet Radius (R E ) Only reliable KEPLER candidates around bright, main sequence GK stars. Correct for observational bias. Complete to p=50 d, and R > 2 RE. decrease with period decrease with size (S/N) Diagonal band of increasing planet frequency. Number of Planets per Star with P < 50 days Incompleteness Strong increase towards small radius. Reminiscent of RV results. But absolute fraction less than HARPS. Radius - mass relationship? Does HARPS detect high density planets that KEPLER cannot see?

34 6.9 Mass-Radius diagram

35 M-R: Giant planets During evolution on Gyrs, giant planets contract and cool. The more massive the core, the smaller the total radius. Many transiting Hot Jupiters are bloated planets: not explainable by standard internal structure modeling. Energy source must act deep in the interior. Several mechanism proposed for explanation. Some giant exoplanets seem to contain very large amounts of metals (>100 ME)

36 M-R: Low mass planets Kepler-11 b,c,d,e,f Kepler 10b Corot-7b Diversity: very different radii for a given M. Some are clearly rocky planets. Observational constraints on the internal composition. Migration? Problem: degeneracy. different composition can give the same M-R. e.g. Ice=rock + some H2/He. Spectra of atmospheres can help to distinguish: Water vapor atmosphere has a smaller scale height than a H2/He atmosphere. Close in planets! Evaporation (atmospheric escape) could play an important role on Gyrs. Must consider formation and evolution. Origin unknown... low mass planets from the beginning or boiled down giant planets?

37 Additional observations

38 Direct imaging: planets form hot Janson et al The two competing models for giant planet formation, core accretion and direct collapse, predict different initial conditions for planet evolution. For direct collapse, planets should initially be very hot. For core accretion, they can also be cold. Observations point to a Hot start. This could help to distinguish formation models.

39 Transits: correlation planetary core mass and stellar metallicity Guillot et al. Miller & Fortney 2010 Transit observations & RV mass measurements show that the core mass of giant planets, and the stellar metallicity are positively correlated. This is reproduced by core accretion models. Recent observations maybe even indicate that that all giant planets contain at least 10 ME of metals. For direct collapse, planets can result both enriched and depleted.

40 Questions?