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PH709 4 Extrasolar Planets Professor Michael Smith 1 The extrasolar planet population NEW STATISTICS. DO DWARFS HARBOUR PLANETS? Is the disc massive enough? Paper: Observed Properties of Exoplanets: Masses, Orbits, and Metallicities Marcy, Butler et al…….1987-2007 Large Stellar Sample 1330 Nearby FGKM Stars Star Selection Criteria: • Vmag < 10 mag • No Close Binaries Age > 2 Gyr (~2000 stars total with Mayor et al. ) PH709 Extrasolar Planets Professor Michael Smith 2 Ongoing 18-year survey of 1330 FGKM type stars at Lick, Keck, and the Anglo-Australian Telescopes that offers both uniform Doppler precision (3 m s-1) and long duration. The 104 planets detected in this survey have minimum masses (M sin i) as low as 6 M Earth, orbiting between 0.02 and 6 AU EXAMPLE. GJ 849 (M3.5 V) 8.8pc 0.36 M We report precise Doppler measurements of GJ 849 (M3.5 V) that reveal the presence of a planet with a minimum mass of 0.82 MJup in a 5.16 yr orbit. (Butler et al 1006, PASP , 118 , 1685). At a = 2.35 AU, GJ 849b is 1. the first Doppler-detected planet discovered around an M dwarf orbiting beyond 0.21 AU, and 2. is only the second Jupiter-mass planet discovered around a star less massive than 0.5 Msolar. This detection brings to 4 the number of M stars known to harbour planets. We find: Giant planets within 2.5 AU are ~3 times more common around GK stars than around M stars. PH709 Extrasolar Planets Professor Michael Smith 3 Due to GJ 849's proximity of 8.8 pc, the planet's angular separation is 0.27", making this system a prime target for highresolution imaging using adaptive optics and future space-borne missions such as the Space Interferometry Mission PlanetQuest. STATISTICS. DO GIANT STARS HARBOUR PLANETS? Yes, a few, but there are no planets orbiting closer than 0.6 AU. (Close-in, Jovian planets are relatively easy to detect using radial velocities because of the larger amplitude they induce and the increased number of orbit cycles per observing time baseline.) It appears that stellar mass has a dramatic effect on the semimajor axis distribution of planets. However, it is not clear whether this effect is a reflection of the process of planet formation and migration, or instead related to the effects of the evolution of the host stars. Different Classes of Object found: 1. Giant Planets or Brown Dwarfs in Unclosed, Long-Period Orbits: PH709 Extrasolar Planets Professor Michael Smith 4 2. Examples of Jupiter-mass/Saturn mass Planets Detected by RV: 3. Sub-Saturn mass: PH709 Extrasolar Planets Professor Michael Smith 5 Detectable mass increases with semi-major axis: a selection effect: STATISTICS: Eccentricity: Planet orbits have significant eccentricity. The median orbital eccentricity is <e >= 0.25, and even planets beyond 3 AU reside in eccentric orbits, suggesting that the circular orbits in our Solar System are unusual. The eccentricity of an orbit is how much it varies from a perfect circle. A stable orbit can have an eccentricity anywhere from a perfect circle with an eccentricity of 0, up to a highly elliptical orbit with an eccentricity up to (but not including) 1. If an orbit had an eccentricity of 1, it would be parabolic and escape from the system. If it were larger than 1, it would be hyperbolic and also escape from the system. PH709 Extrasolar Planets With k=1 for an ellipse: Professor Michael Smith e = (da – dp)/(da + dp) 6 PH709 Extrasolar Planets Professor Michael Smith 7 Most extrasolar planets reside in non-circular orbits. Of the 90 extrasolar planets that reside beyond 0.15 AU, their average orbital PH709 Extrasolar Planets Professor Michael Smith 8 eccentricity is 0.32. These planets have a wide scatter in e, including some planets with very large e. In contrast, planets orbiting within 0.1 AU of their host star all reside have close to circular orbits. All detected planets with semi-major axis < 0.07 AU have low e. This is similar to binary stars, and is likely due to tidal circularization. Earth's eccentricity is 0.017, while Jupiter's is 0.094. In our solar system, the planet with the largest eccentricity is Pluto at 0.244, and Mercury with 0.205. The planet with the lowest eccentricity is Venus with 0.007. Unless there is some gravitational tugging (such as with the Galilean Satellites) that keeps an orbit eccentric, orbits will usually circularize with time. About 10% of the planets found so far have an eccentricity of nearly 0. About 15% have an eccentricity smaller than Earth's, and over 25% have an eccentricity smaller than Jupiter's. 45% are smaller than Mercury's eccentricity, and 50% are lower than Pluto's. The other half have very eccentric orbits; this means that, throughout their years, they come very close to and very far from their parent star. This will create wide temperature swings, and for any life like Earth's, this would make survival quite difficult, if not impossible. Theories: explanations uncertain Most proposed mechanisms invoke gravitationally scattering or perturbations of planets by other planets, perhaps in resonances, or by interactions with the protoplanetary disk. Beyond 0.1 AU, the distribution of eccentricities appears essentially uniform between 0 and 0.8. For most Doppler surveys, sensitivity is not a strong function of eccentricity for 0 < e < 0.8 and a < 3 AU. PH709 Extrasolar Planets Professor Michael Smith 9 This plot represents results from many surveys, and so is drawn from an inhomogeneous sample. Distribution of Eccentricity: Eccentricity v. planet mass Figure: the 130 known nearby exoplanets with M sin i < 13 MJup, excluding those for which a < 0.1 AU, i.e., those planets which may have been tidally circularized. High-mass exoplanets (M sin i > 5MJup) have a slightly higher median eccentricity than lower-mass exoplanets. The completeness of Doppler surveys increases with M sin i and is generally insensitive to eccentricity. PH709 Extrasolar Planets Professor Michael Smith 10 Ignoring the hot Jupiters, no obvious correlation between planet mass and eccentricity. Planets within 0.1 AU are presumably tidally circularized. Note that the distance of closest approach is a(1-e), and that the effect of tidal torques scales as separation d-6. The very eccentric planet around HD80606 (a = 0.438 AU, e = 0.93, a(1-e) = 0.03 AU) may pose some problems for tidal circularization theory. STATISTICS: Minimum mass as a function of semimajor axis: Nothing very striking in these plots: Accessible region of mp - a space is fully occupied by detected planets Get rid of the log (Mj) : Minimum mass as a function of semimajor axis for the known nearby exoplanets. Doppler surveys are generally incomplete for exoplanets with a > 3 AU, low-mass planets (M sin i < 1MJup) beyond 1 AU, and very low-mass planets PH709 Extrasolar Planets Professor Michael Smith 11 (M sin i < 0.1MJup) everywhere. This plot represents results from many surveys, and so is drawn from an inhomogeneous sample. STATISTICS: THE HISTOGRAMS Results from radial velocity searches Massive planets exist at small orbital radii. Closest-in planet is at a = 0.035 AU (cf Mercury at ~ 0.4 AU). Less than 10 solar radii! EXAMPLES: Best-fit orbit to the radial velocities measured at Keck Observatory for HD 11964, with P = 5.8yr, e ~ 0, and M sin i = 0.6MJup. PH709 Extrasolar Planets Professor Michael Smith 12 Best-fit orbit to the radial velocities measured at Keck Observatory for HD 66428, with P = 5.4yr, e = 0.5, and M sin i = 3MJup. PH709 Extrasolar Planets Professor Michael Smith 13 Account for this by considering only planets with masses large enough to be detectable at any a < 2.7 AU. -> dN / dlog(a) rises steeply with orbital radius Orbital distance distribution of the 197 known nearby exoplanets with 0.03 <a < 10 in logarithmic distance bins. Planets with a > 3AU have periods comparable to or longer than the length of most Doppler surveys, so the distribution is incomplete beyond that distance. STATISTICS: Location Strong selection effect in favour of detecting planets at small orbital radii, arising from: - lower mass planets can be detected there - mass function rises to smaller masses The distribution of semi-major axes rises from 0.3 – 3.0 AU (dN/d log a) and extrapolation suggests that about 12% of the FGK stars harbour gas-giant exoplanets within 20 AU. The occurrence of “hot Jupiters” within 0.1 AU of FGK stars is 1.2 ± 0.2%. Among stars with one planet, 14% have at least one additional planet, occasionally locked in resonances. PH709 Extrasolar Planets Professor Michael Smith 14 Planet fraction among ~ solar-type stars exceeds 7%. Most are beyond 1 AU: STATISTICS. IS THERE A PERIOD VALLEY? A Pile-up: PH709 Extrasolar Planets Professor Michael Smith 15 Distribution of periods among the known nearby “hot Jupiters”. There is a clear “pile-up” of planets with orbital periods near 3 days. The "period valley" observed around Sun-like stars, marked by a deficit of planets with periods ranging from roughly 10 to 100 days, a sharp increase in the number of detected planets beyond 1 AU and a pile-up near P = 3 day. Implies that the currently detected planet fraction ~7% is likely to be a substantial underestimate of the actual fraction of stars with massive planets. STATISTICS: THE MASS DISTRIBUTION PH709 Extrasolar Planets Professor Michael Smith 16 15-25% of solar-type stars may have planets with masses 0.2 MJ < mp < 10 MJ. MJ sin i Observed mass function increases to smaller Mp: Note: the brown dwarf desert! The mass distribution shows a dramatic decrease in the number of planets at high masses, a decrease that is roughly characterized by a power law, dN/dM ~ M-1.25. STATISTICS: Metallicity distribution of stars with and without planets PH709 Extrasolar Planets Professor Michael Smith 17 Here NFe and NH is the number of iron and hydrogen atoms per unit of volume respectively Left plot: metallicity of stars with planets (shaded histogram) compared to a sample of stars with no evidence for planets (open histogram) (data from Santos, Israelian & Mayor, 2001) PH709 Extrasolar Planets Professor Michael Smith 18 Host star metallicity: Planets are preferentially found around stars with enhanced metal abundance. Cause or effect? High metal abundance could: (a) Reflect a higher abundance in the material which formed the star + protoplanetary disc, making planet formation more likely. (b) Result from the star swallowing planets or planetesimals subsequent to planets forming. If the convection zone is fairly shallow, this can apparently enrich the star with metals even if the primordial material had Solar abundance. DO THESE PLANETS FORM FROM DISCS? The core-accretion model of planet formation is supported by four observations: 1) The mass distribution rises toward the lowest detectable masses, dN/dM ~ M -1.0. 2) Stellar metallicity correlates strongly with the presence of planets. Sample exhibits a planet-metallicity correlation at all stellar masses; this argues that the high metallicities of stars with planets is not likely due to convective envelope "pollution." BUT: Neptune-mass planets form preferentially around metal-poor stars, in contrast to stars with giant planets PH709 Extrasolar Planets Professor Michael Smith 19 3) One planet (1.3 M Sat) has a massive rocky core, M Core ≈ 70 M Earth. 4) A super-Earth of about 7 M Earth has been discovered. Four very low mass planets have been detected (20 earth masses). Detailed pattern of abundances can distinguish these possibilities, but results currently still controversial. SUMMARY The core-accretion model of planet formation is supported by four observations: 1) The mass distribution rises toward the lowest detectable masses, dN/dM =M-1.0. 2) Stellar metallicity correlates strongly with the presence of planets. 3) One planet (1.3 MSat) has a massive rocky core, MCore ≈ 70 MEarth. 4) A super-Earth of ˜ 7 MEarth has been discovered. BUT HOW …. do Cluster issue: Lack of transits in 47 Tuc A long HST observation monitored ~34,000 stars in the globular cluster 47 Tuc looking for planetary transits. Locally: 1% of stars have hot Jupiters ~ 10% of those show transits Expect 0.001 x 34,000 ~ few tens of planets None were detected. Possible explanations: • Low metallicity in cluster prevented planet formation • Cluster environment destroyed discs before planets formed • Stellar fly-bys ejected planets from bound orbits All of these seem plausible - make different predictions for other clusters. PH709 Extrasolar Planets Professor Michael Smith 20 The lack of detections in the uncrowded outer regions of both clusters indicates that stellar metallicity is the dominant factor inhibiting Hot Jupiter formation in the cluster environment. Microlensing Statistics: Constraint from monitoring of 43 microlensing events. Typically, the lenses are low mass stars. At most 1/3 of 0.3 Solar mass stars have Jupiter mass planets between 1.5 AU and 4 AU. Currently consistent with the numbers seen in radial velocity searches. massive planets end up so close to their stars?