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Title:Mass-Radius Relationships for Very Low Mass Gaseous Planets
Authors:Konstantin Batygin, David J. Stevenson

Recently, the Kepler spacecraft has detected a sizable aggregate of objects, characterized by giant-planet-like radii and modest levels of stellar irradiation. With the exception of a handful of objects, the physical nature, and specifically the average densities, of these bodies remain unknown. Here, we propose that the detected giant planet radii may partially belong to planets somewhat less massive than Uranus and Neptune. Accordingly, in this work, we seek to identify a physically sound upper limit to planetary radii at low masses and moderate equilibrium temperatures. As a guiding example, we analyse the interior structure of the Neptune-mass planet Kepler-30d and show that it is acutely deficient in heavy elements, especially compared with its solar system counterparts. Subsequently, we perform numerical simulations of planetary thermal evolution and in agreement with previous studies, show that generally, 10 - 20 Earth mass, multi-billion year old planets, composed of high density cores and extended H/He envelopes can have radii that firmly reside in the giant planet range. We subject our results to stability criteria based on extreme ultraviolet radiation, as well as Roche-lobe overflow driven mass-loss and construct mass-radius relationships for the considered objects. We conclude by discussing observational avenues that may be used to confirm or repudiate the existence of putative low mass, gas-dominated planets.

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Title: Fast Rise of "Neptune-Size" Planets (4-8 R_Earth) from P~10 to ~250 days -- Statistics of Kepler Planet Candidates Up to ~0.75 AU
Authors: Subo Dong (IAS), Zhaohuan Zhu (Princeton)

We infer period (P) and size (R_p) distribution of Kepler transiting planet candidates with R_p> 1 Earth radii and P<250 days hosted by solar-type stars. The planet detection efficiency is computed by using measured noise and the observed timespans of the light curves for ~120,000 Kepler target stars. Given issues with the parameters of Kepler host stars and planet candidates (especially the unphysical impact parameter distribution reported for the candidates), we focus on deriving the shape of planet period and radius distribution functions. We find that for orbital period P>10 days, the planet frequency dN_p/d logP for "Neptune-size" planets (R_p = 4-8 Earth radii) increases with period as \propto P^{0.70.1}. In contrast, dN_p/dlogP for Super-Earth-Size (2-4 Earth radii) as well as Earth-size (1-2 Earth radii) planets are consistent with a nearly flat distribution as a function of period (\propto P^{0.110.05}) and \propto P^{-0.100.12}, respectively), and the normalisations are remarkably similar (within a factor of ~ 1.5). The shape of the distribution function is found to be not sensitive to changes in selection criteria of the sample. The implied nearly flat or rising planet frequency at long period appears to be in tension with the sharp decline at ~100 days in planet frequency for low mass planets (planet mass m_p < 30 Earth masses) recently suggested by the HARPS survey.

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Title: Models of Neptune-Mass Exoplanets: Emergent Fluxes and Albedos
Authors: David S. Spiegel (1), Adam Burrows (1), Laurent Ibgui (1), Ivan Hubeny (2), John A. Milsom (3) ((1) Princeton University, (2) Steward Observatory, (3) The University of Arizona)

There are now many known exoplanets with Msin(i) within a factor of two of Neptune's, including the transiting planets GJ436b and HAT-P-11b. Planets in this mass-range are different from their more massive cousins in several ways that are relevant to their radiative properties and thermal structures. By analogy with Neptune and Uranus, they are likely to have metal abundances that are an order of magnitude or more greater than those of larger, more massive planets. This increases their opacity, decreases Rayleigh scattering, and changes their equation of state. Furthermore, their smaller radii mean that fluxes from these planets are roughly an order of magnitude lower than those of otherwise identical gas giant planets. Here, we compute a range of plausible radiative equilibrium models of GJ436b and HAT-P-11b. In addition, we explore the dependence of generic Neptune-mass planets on a range of physical properties, including their distance from their host stars, their metallicity, the spectral type of their stars, the redistribution of heat in their atmospheres, and the possible presence of additional optical opacity in their upper atmospheres.

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