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Register a new userOn The Relative Sizes of Planets Within Kepler Multiple Candidate Systems
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David R. Ciardi
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We present a study of the relative sizes of planets within the multiple candidate systems discovered with the $Kepler$ mission. We have compared the size of each planet to the size of every other planet within a given planetary system after correcting the sample for detection and geometric biases. We find that for planet-pairs for which one or both objects is approximately Neptune-sized or larger, the larger planet is most often the planet with the longer period. No such size--location correlation is seen for pairs of planets when both planets are smaller than Neptune. Specifically, if at least one planet in a planet-pair has a radius of $gtrsim 3R_oplus$, $68pm 6%$ of the planet pairs have the inner planet smaller than the outer planet, while no preferred sequential ordering of the planets is observed if both planets in a pair are smaller than $lesssim3 R_oplus$.
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We analyze the effect of companion stars on the bulk density of 29 planets orbiting 15 stars in the Kepler field. These stars have at least one stellar companion within 2, and the planets have measured masses and radii, allowing an estimate of their bulk density. The transit dilution by the companion star requires the planet radii to be revised upward, even if the planet orbits the primary star; as a consequence, the planetary bulk density decreases. We find that, if planets orbited a faint companion star, they would be more volatile-rich, and in several cases their densities would become unrealistically low, requiring large, inflated atmospheres or unusually large mass fractions in a H/He envelope. In addition, for planets detected in radial velocity data, the primary star has to be the host. We can exclude 14 planets from orbiting the companion star; the remaining 15 planets in seven planetary systems could orbit either the primary or the secondary star, and for five of these planets the decrease in density would be substantial even if they orbited the primary, since the companion is of almost equal brightness as the primary. Substantial follow-up work is required in order to accurately determine the radii of transiting planets. Of particular interest are small, rocky planets that may be habitable; a lower mean density might imply a more volatile-rich composition. Reliable radii, masses, and thus bulk densities will allow us to identify which small planets are truly Earth-like.
We present a test for spin-orbit alignment for the host stars of 25 candidate planetary systems detected by the {it Kepler} spacecraft. The inclination angle of each stars rotation axis was estimated from its rotation period, rotational line broadening, and radius. The rotation periods were determined using the {it Kepler} photometric time series. The rotational line broadening was determined from high-resolution optical spectra with Subaru/HDS. Those same spectra were used to determine the stars photospheric parameters (effective temperature, surface gravity, metallicity) which were then interpreted with stellar-evolutionary models to determine stellar radii. We combine the new sample with the 7 stars from our previous work on this subject, finding that the stars show a statistical tendency to have inclinations near 90$^circ$, in alignment with the planetary orbits. Possible spin-orbit misalignments are seen in several systems, including three multiple-planet systems (KOI-304, 988, 2261). Ideally these systems should be scrutinized with complementary techniques---such as the Rossiter-McLaughlin effect, starspot-crossing anomalies or asteroseismology---but the measurements will be difficult owing to the relatively faint apparent magnitudes and small transit signals in these systems.
Most Sun-like stars in the Galaxy reside in gravitationally-bound pairs of stars called binary stars. While long anticipated, the existence of a circumbinary planet orbiting such a pair of normal stars was not definitively established until the discovery of Kepler-16. Incontrovertible evidence was provided by the miniature eclipses (transits) of the stars by the planet. However, questions remain about the prevalence of circumbinary planets and their range of orbital and physical properties. Here we present two additional transiting circumbinary planets, Kepler-34 and Kepler-35. Each is a low-density gas giant planet on an orbit closely aligned with that of its parent stars. Kepler-34 orbits two Sun-like stars every 289 days, while Kepler-35 orbits a pair of smaller stars (89% and 81% of the Suns mass) every 131 days. Due to the orbital motion of the stars, the planets experience large multi-periodic variations in incident stellar radiation. The observed rate of circumbinary planets implies > ~1% of close binary stars have giant planets in nearly coplanar orbits, yielding a Galactic population of at least several million.
The impact of stellar multiplicity on the evolution of planet-forming disks is still the subject of debate. Here we present and analyze disk structures around ten multiple stellar systems that were included in an unbiased, high spatial resolution survey performed with ALMA of 32 protoplanetary disks in the Taurus star-forming region. At the unprecedented spatial resolution of ~0.12 we detect and spatially resolve the disks around all primary stars, and those around eight secondary and one tertiary star. The dust radii of disks around multiple stellar systems are smaller than those around single stars in the same stellar mass range and in the same region. The disks in multiple stellar systems also show a steeper decay of the millimeter continuum emission at the outer radius than disks around single stars, suggestive of the impact of tidal truncation on the shape of the disks in multiple systems. However, the observed ratio between the dust disk radii and the observed separation of the stars in the multiple systems is consistent with analytic predictions of the effect of tidal truncation only if the eccentricities of the binaries are rather high (typically >0.5), or if the observed dust radii are a factor of two smaller than the gas radii, as is typical for isolated systems. Similar high-resolution studies targeting the gaseous emission from disks in multiple stellar systems are required to resolve this question.
Using data from the California-Kepler-Survey (CKS) we study trends in planetary properties with host star metallicity for close-in planets. By incorporating knowledge of the properties of the planetary radius gap identified by the CKS survey, we are able to investigate the properties of planetary cores and their gaseous envelopes separately. Our primary findings are that the solid core masses of planets are higher around higher metallicity stars and that these more massive cores were able to accrete larger gas envelopes. Furthermore, investigating the recently reported result that planets with radii in the range (2-6Rearth) are more common at short periods around higher metallicity stars in detail, we find that the average host star metallicity of H/He atmosphere-hosting planets increases smoothly inside an orbital period of ~20 days. We interpret the location of the metallicity increase within the context of atmospheric photoevaporation: higher metallicity stars are likely to host planets with higher atmospheric metallicity, which increases the cooling in the photoevaporative outflow, lowering the mass-loss rates. Therefore, planets with higher metallicity atmospheres are able to resist photoevaporation at shorter orbital periods. Finally, we find evidence at 2.8 sigma that planets that do not host H/He atmospheres at long periods are more commonly found around lower metallicity stars. Such planets are difficult to explain by photoevaporative stripping of planets which originally accreted H/He atmospheres. Alternatively, this population of planets could be representative of planets that formed in a terrestrial-like fashion, after the gas disc dispersed.
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