Exoplanets & Exomoons

Kepler's Ultra-Short-Period Planets Formed Water-Poor

By Keith Cowing
October 6, 2016
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Kepler's Ultra-Short-Period Planets Formed Water-Poor

Recent surveys have uncovered an exciting new population of ultra-short-period (USP) planets with orbital periods less than a day. These planets typically have radii <1.5 Earth radii, indicating that they likely have rocky compositions. This stands in contrast to the overall distribution of planets out to ~100 days, which is dominated by low-density sub-Neptunes above 2 Earth radii, which must have gaseous envelopes to explain their size. However, on ultra-short-period orbits, planets are bombarded by intense levels of photo-ionizing radiation and consequently gaseous sub-Neptunes are extremely vulnerable to losing their envelopes to atmospheric photo-evaporation. Using models of planet evolution, I show that the rocky USP planets can easily be produced as the evaporated remnants of sub-Neptunes with H/He envelopes and that we can therefore understand the observed dearth of USP sub-Neptunes as a natural consequence of photo-evaporation. Critically however, planets on USP orbits could often retain their envelopes if they formed with very high-metallicity water dominated envelopes. Such water-rich planets would commonly be >2 Earth radii today, which is inconsistent with the observed evaporation desert, indicating that most USP planets likely formed from water-poor material within the snow-line. Finally, I examine the special case of 55 Cancri e and its possible composition in the light of recent observations, and discuss the prospects for further characterizing this population with future observations.

Born Dry in the Photo-Evaporation Desert: Kepler’s Ultra-Short-Period Planets Formed Water-Poor

Eric D. Lopez
(Submitted on 4 Oct 2016)

Comments: 10 pages, 5 figures, submitted to MNRAS on 9/30/2016
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1610.01170 [astro-ph.EP] (or arXiv:1610.01170v1 [astro-ph.EP] for this version)
Submission history
From: Eric Lopez
[v1] Tue, 4 Oct 2016 20:01:47 GMT (6855kb,D)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻