Survey
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PH709 Extrasolar Planets Professor Michael Smith 1 3 The extrasolar planet population review: http://arxiv.org/pdf/1501.05685v1.pdf http://adsabs.harvard.edu/abs/2014arXiv1410.4199W Early detections by Radial Velocity method: ALL HOT JUPITERS. What appears in the observed distribution to be a “pile-up” of hot Jupiters (a few hundred Earth masses at a few hundredths of an AU) has NOW vanished in volume-limited samples. 3.1 OCCURRENCE RATES DO DWARF STARS (M STARS) HARBOUR PLANETS? Is the disc massive enough? In particular, a much lower incidence of Jupiter mass planets is expected around M dwarfs in the core accretion scenario for planet formation. 1 PH709 Extrasolar Planets Professor Michael Smith 2 Small planets are more abundant around smaller stars M dwarfs have few Jovian planets, but a wealth of Neptune-mass planets Ongoin 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. We find: Giant planets within 2.5 AU are ~3 times more common around GK stars than around M stars. dN / ( d ln M d ln P ) p ∝ Mpα Pβ, with α = −0.31 ± 0.20, β = 0.26 ± 0.10. 3.2 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 by Doppler (RV) technique: 1. Giant Planets or Brown Dwarfs, Unclosed, LongPeriod Orbits: 2 PH709 Extrasolar Planets Professor Michael Smith 3 2. Jupiter-mass/Saturn mass Planets Detected by RV: What appears in the observed distribution to be a “pile-up” of hot Jupiters (a few hundred Earth masses at a few hundredths of an AU) has vanished in the volume-limited sample. 3 PH709 Extrasolar Planets Professor Michael Smith 4 3. Sub-Saturn mass: Detectable mass increases with semi-major axis: a selection effect 4 PH709 Extrasolar Planets Professor Michael Smith 5 STATISTICS: Eccentricity: Planet orbits have significant eccentricity. The median orbital eccentricity is <e >= 0.25. 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. Ellipse with semi-major and semi-minor axes: e2 = 1 – b2 /a2 or, for periastron and apastron: 5 PH709 Extrasolar Planets e = (da – dp)/(da + dp) 6 Professor Michael Smith 6 PH709 Extrasolar Planets Professor Michael Smith 7 Most extrasolar planets reside in non-circular orbits. Beyond 0.1 AU, the distribution of eccentricities appears essentially uniform between 0 and 0.8. Highest e = 0.93 belongs to HD 80606b . 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. Earth's eccentricity is 0.017, while Jupiter's is 0.094. Unless there is some gravitational tugging (such as with the Galilean Satellites) that keeps an orbit eccentric, orbits will usually circularize with time. In contrast, planets orbiting within 0.1 AU of their host star tend to reside in close to circular orbits. Many detected planets with semi7 PH709 Extrasolar Planets Professor Michael Smith 8 major axis < 0.07 AU have low e. This is similar to binary stars, and is likely due to tidal circularization. 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. 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. 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. . Distribution of Eccentricity: Eccentricity v. planet mass 8 PH709 Extrasolar Planets Professor Michael Smith 9 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. Ignoring the hot Jupiters, no obvious correlation between planet mass and eccentricity. STATISTICS: Minimum mass as a function of semimajor axis: 9 PH709 10 Extrasolar Planets Professor Michael Smith 10 PH709 Extrasolar Planets Professor Michael Smith 11 Nothing very striking in these plots: Accessible region of mp - a space is fully occupied by detected planets 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 (M sin i < 0.1MJup) everywhere. This plot represents results from many surveys, and so is drawn from an inhomogeneous sample. Account for this by considering only planets with masses large enough to be detectable at any a < 2.7 AU Get rid of the log (Mj) : 11 PH709 Extrasolar Planets Professor Michael Smith 12 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. 12 PH709 Extrasolar Planets Professor Michael Smith 13 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. . 13 PH709 Extrasolar Planets Professor Michael Smith 14 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. Planet fraction among ~ solar-type stars exceeds 7%. Most are beyond 1 AU: 14 PH709 Extrasolar Planets Professor Michael Smith 15 dN / dlog(a) rises steeply with orbital radius ? STATISTICS. IS THERE A PERIOD VALLEY? A Pile-up – linear : Distribution of periods among the known nearby “hot Jupiters”. There is a clear “pile-up” of planets with orbital periods near 3 days. 15 PH709 Extrasolar Planets Professor Michael Smith 16 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 15-25% of solar-type stars may have planets with masses 0.2 MJ < mp < 10 MJ. 16 PH709 Extrasolar Planets Professor Michael Smith 17 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. 17 PH709 Extrasolar Planets Professor Michael Smith 18 STATISTICS: Metallicity distribution of stars with and without planets • Metallicity is a measure of the proportion of elements with masses greater than those of hydrogen and helium in a star. • Often given in terms of [Fe/H], the logarithm of the ratio of the star’s iron abundance to its hydrogen abundance, with respect to that of the Sun. • • [Fe/H] = log[n(Fe)/n(H)]star – log[n(Fe)/n(H)]sun • • n(Fe) and n(H) are the number of Fe and H atoms in a given volume respectively. • Iron chosen as it is relatively easy to measure iron abundance through spectroscopic analysis, but other m < 56 u elements could be used. • Would you expect a star rich in heavier elements to be more or less likely to host a planetary system? Here NFe and NH is the number of iron and hydrogen atoms per unit of volume respectively 18 PH709 Extrasolar Planets Professor Michael Smith 19 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) THIS HOLDS FOR MASSIVE PLANETS ONLY: 19 PH709 Extrasolar Planets Professor Michael Smith 20 Host star metallicity: Planets are preferentially found around stars with enhanced metal abundance. Cause or effect? High metal abundance could: 20 PH709 Extrasolar Planets Professor Michael Smith 21 (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 associted with jupiter-sized planets 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 3) Some planet have a massive rocky core, e.g. M Core ≈ 70 M Earth. 4) Many super-Earths and Earth-twins of about 1 - 7 M Earth have been discovered. SUMMARY Smaller planets (1-4 R⊕) with P ∼< 1 yr are found around about half of Sun-like stars, often in closely-spaced multiplanet systems. The probability density is nearly constant in log P between ≈10–300 days. For P ∼< 10 days the occurrence rate declines more sharply with decreasing period. The core-accretion model of planet formation is supported by four observations: 1) The mass distribution rises toward the lowest -1.0 detectable masses, dN/dM =∝ . 2) Stellar metallicity 21 PH709 Extrasolar Planets Professor Michael Smith 22 correlates strongly with the presence of massive planets. 3) Some planets have massive rocky core. 22