A remnant planetary core in the hot Neptunian desert

The interiors of giant planets remain poorly understood. Even for the planets in the Solar System, difficulties in observation lead to major uncertainties in the properties of planetary cores. Exoplanets that have undergone rare evolutionary pathways provide a new route to understanding planetary interiors. We present the discovery of TOI-849b, the remnant core of a giant planet, with a radius smaller than Neptune but an anomalously high mass Mp=40.8^{+2.4}_{-2.5} Mearth and density of 5.5 ± 0.8 g cm^{-3}, similar to the Earth. Interior structure models suggest that any gaseous envelope of pure hydrogen and helium consists of no more than 3.9^{+0.8}_{-0.9}% of the total mass of the planet. TOI-849b transits a late G type star (T_{mag}=11.5) with an orbital period of 18.4 hours, leading to an equilibrium temperature of 1800K. The planet's mass is larger than the theoretical threshold mass for runaway gas accretion. As such, the planet could have been a gas giant before undergoing extreme mass loss via thermal self-disruption or giant planet collisions, or it avoided substantial gas accretion, perhaps through gap opening or late formation. Photo-evaporation rates cannot provide the mass loss required to reduce a Jupiter-like gas giant, but can remove a few Earth mass hydrogen and helium envelope on timescales of several Gyr, implying that any remaining atmosphere is likely to be enriched by water or other volatiles from the planetary interior. TOI-849b represents a unique case where material from the primordial core is left over from formation and available to study

An Earth-sized exoplanet with a Mercury-like composition

Accepted preprint in Nature Astronomy. Publisher-edited version available at http://rdcu.be/JRE7 Supplement materials available at https://www.nature.com/articles/s41550-018-0420-5International audienceThe Earth, Venus, Mars, and some extrasolar terrestrial planets have a mass and radius that is consistent with a mass fraction of about 30% metallic core and 70% silicate mantle. At the inner frontier of the solar system, Mercury has a completely different composition, with a mass fraction of about 70% metallic core and 30% silicate mantle. Several formation or evolution scenarios are proposed to explain this metal-rich composition, such as a giant impact, mantle evaporation, or the depletion of silicate at the inner-edge of the proto-planetary disk. These scenarios are still strongly debated. Here we report the discovery of a multiple transiting planetary system (K2-229), in which the inner planet has a radius of 1.165+/-0.066 Rearth and a mass of 2.59+/-0.43 Mearth. This Earth-sized planet thus has a core-mass fraction that is compatible with that of Mercury, while it was expected to be similar to that of the Earth based on host-star chemistry. This larger Mercury analogue either formed with a very peculiar composition or it has evolved since, e.g. by losing part of its mantle. Further characterisation of Mercury-like exoplanets like K2-229 b will help putting the detailed in-situ observations of Mercury (with Messenger and BepiColombo) into the global context of the formation and evolution of solar and extrasolar terrestrial planets

Populations of Extrasolar Giant Planets from Transit and Radial Velocity Surveys

Accepted review for the Handbook of ExoplanetsInternational audienceTransit and radial velocity surveys have deeply explored the population of extrasolar giant planets, with hundreds of objects detected to date. All these detections allow to understand their physical properties and to constrain their formation, migration, and evolution mechanism. In this chapter, the observed properties of these planets are presented along with the various populations identified in the data. The occurrence rates of giant exoplanets, as observed in different stellar environment by various surveys are also reviewed and compared. Finally, the presence and properties of the giant exoplanets are discussed in the regards of the properties of the host star. Over this chapter, the observational constraints are discussed in the context of the dominant planet formation, migration and evolution scenarios