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Register a new userA Population of Very-Hot Super-Earths in Multiple-Planet Systems Should be Uncovered by Kepler
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We simulate a Kepler-like observation of a theoretical exoplanet population and we show that the observed orbital period distribution of the Kepler giant planet candidates is best matched by an average stellar specific dissipation function Q_* in the interval 10^6 ~< Q_* ~< 10^7. In that situation, the few super-Earths that are driven to orbital periods P < 1 day by dynamical interactions in multiple-planet systems will survive tidal disruption for a significant fraction of the main-sequence lifetimes of their stellar hosts. Consequently, though these very-hot super-Earths are not characteristic of the overall super-Earth population, their substantial transit probability implies that they should be significant contributors to the full super-Earth population uncovered by Kepler. As a result, the CoRoT-7 system may be the first representative of a population of very-hot super-Earths that we suggest should be found in multiple-planet systems preferentially orbiting the least-dissipative stellar hosts in the Kepler sample.
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We present a catalog of 11 multi-planet systems from Campaigns 1 and 2 of the K2 mission. We report the sizes and orbits of 26 planets split between seven 2-planet systems and four 3-planet systems. These planets stem from a systematic search of the K2 photometry for all dwarf stars observed by K2 in these fields. We precisely characterized the host stars with adaptive optics imaging and analysis of high-resolution optical spectra from Keck/HIRES and medium-resolution spectra from IRTF/SpeX. We confirm two planet candidates by mass detection and validate the remaining 24 candidates to $>99%$ confidence. Thirteen planets were previously validated or confirmed by other studies and 24 were previously identified as planet candidates. The planets are mostly smaller than Neptune (21/26 planets) as in the Kepler mission and all have short periods ($P < 50$ d) due to the duration of the K2 photometry. The host stars are relatively bright (most have $Kp < 12.5$ mag) and are amenable to follow-up characterization. For K2-38, we measured precise radial velocities using Keck/HIRES and provide initial estimates of the planet masses. K2-38b is a short-period super-Earth with a radius of $1.55 pm 0.16~R_oplus$, a mass of $12.0 pm 2.9~M_oplus$, and a high density consistent with an iron-rich composition. The outer planet K2-38c is a lower density sub-Neptune-size planet with a radius of $2.42 pm 0.29~R_oplus$ and a mass of $9.9 pm 4.6~M_oplus$ that likely has a substantial envelope. This new planet sample demonstrates the capability of K2 to discover numerous planetary systems around bright stars.
Hot super-Earths likely possess minimal atmospheres established through vapor saturation equilibrium with the ground. We solve the hydrodynamics of these tenuous atmospheres at the surface of Corot-7b, Kepler 10b and 55 Cnc-e, including idealized treatments of magnetic drag and ohmic dissipation. We find that atmospheric pressures remain close to their local saturation values in all cases. Despite the emergence of strongly supersonic winds which carry sublimating mass away from the substellar point, the atmospheres do not extend much beyond the day-night terminators. Ground temperatures, which determine the planetary thermal (infrared) signature, are largely unaffected by exchanges with the atmosphere and thus follow the effective irradiation pattern. Atmospheric temperatures, however, which control cloud condensation and thus albedo properties, can deviate substantially from the irradiation pattern. Magnetic drag and ohmic dissipation can also strongly impact the atmospheric behavior, depending on atmospheric composition and the planetary magnetic field strength. We conclude that hot super-Earths could exhibit interesting signatures in reflection (and possibly in emission) which would trace a combination of their ground, atmospheric and magnetic properties.
Simulations predict that hot super-Earth sized exoplanets can have their envelopes stripped by photo-evaporation, which would present itself as a lack of these exoplanets. However, this absence in the exoplanet population has escaped a firm detection. Here we demonstrate, using asteroseismology on a sample of exoplanets and exoplanet candidates observed during the Kepler mission that, while there is an abundance of super-Earth sized exoplanets with low incident fluxes, none are found with high incident fluxes. We do not find any exoplanets with radii between 2.2 and 3.8 Earth radii with incident flux above 650 times the incident flux on Earth. This gap in the population of exoplanets is explained by evaporation of volatile elements and thus supports the predictions. The confirmation of a hot-super-Earth desert caused by evaporation will add an important constraint on simulations of planetary systems, since they must be able to reproduce the dearth of close-in super-Earths.
In this work, we hunt for the best places to find exo-Earths in the currently known exoplanet population. While it is still unclear whether Jupiter had a beneficial or detrimental effect on the creation of the right environment for a habitable Earth to develop, we focus on the 51 multiple planet systems that have at least one Jupiter-like planet and aim to identify which would be good candidates to host an exo-Earth. We conduct a series of numerical simulations to identify dynamically stable regions of the habitable zone of the multiple exoplanet systems capable of hosting an Earth-mass planet. We produce a candidate list of 16 systems that could host such a stable exo-Earth in their habitable zone, and for which the induced radial velocity signal of a hypothetical one, two or four Earth-mass planet on the host star would be detectable with the ESPRESSO spectrograph. We find that whilst the gravitational interactions with the massive planet nearest the habitable zone are critical in determining stability, the secular resonant interactions between multiple planets can also have a dramatic influence on the overall stability of the habitable zone.
The nearest exoplanets to the Sun are our best possibilities for detailed characterization. We report the discovery of a compact multi-planet system of super-Earths orbiting the nearby red dwarf GJ 887, using radial velocity measurements. The planets have orbital periods of 9.3 and 21.8~days. Assuming an Earth-like albedo, the equilibrium temperature of the 21.8 day planet is approx 350 K; which is interior, but close to the inner edge, of the liquid-water habitable zone. We also detect a further unconfirmed signal with a period of 50 days which could correspond to a third super-Earth in a more temperate orbit. GJ 887 is an unusually magnetically quiet red dwarf with a photometric variability below 500 parts-per-million, making its planets amenable to phase-resolved photometric characterization.
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