Exoplanetology: Exoplanets & Exomoons

Carving the Edges of the Rocky Planet Population

By Keith Cowing
Status Report
astro-ph.EP
January 31, 2025
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Carving the Edges of the Rocky Planet Population
A grand summary highlighting all the physical processes that sculpt the small planet population in the radius-period space. Plotted in contour is the Gaussian Kernel density estimation of Kepler candidates from Data Release 25 (Thompson et al. 2018). We do not correct for detection biases as these contours are for illustration purpose. Rocky planets are expected to be found in a region bounded by the evaporation/primordial gap (orange dashed line / light grey region), tides (red dashed line), and magnetic drag (blue dashed line). All the boundaries drawn in this figure assume system age of 5 Gyr, M⋆ = M⊙, and B⋆,i =100 G. Varying these stellar parameters can widen or shrink the allowable region of surviving rocky planets. — astro-ph.EP

Short-period planets provide ideal laboratories for testing star-planet interaction. Planets that are smaller than ∼2R⊕ are considered to be largely rocky either having been stripped of or never having acquired the gaseous envelope.

Zooming in on these short-period rocky planet population, clear edges appear in the mass-period and radius-period space. Over ∼0.2–20 days and 0.09–1.42M⊙, the maximum mass of the rocky planets stay below ∼10M⊕ with a hint of decrease towards ≲1 day, ≳4 day, and ≲0.45M⊙. In radius-period space, there is a relative deficit of ≲2R⊕ planets inside ∼1 day.

We demonstrate how the edges in the mass-period space can be explained by a combination of tidal decay and photoevaporation whereas the rocky planet desert in the radius-period space is a signature of magnetic drag on the planet as it orbits within the stellar magnetic field. Currently observed catastrophically evaporating planets may have started their death spiral from ∼1 day with planets of mass up to ∼0.3M⊕ under the magnetic drag.

More discoveries and characterization of small planets around mid-late M and A stars would be welcome to better constrain the stellar parameters critical in shaping the edges of rocky planet population including their UV radiation history, tidal and magnetic properties.

Eve J. Lee, James E. Owen

Comments: Accepted to ApJL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2501.17241 [astro-ph.EP] (or arXiv:2501.17241v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2501.17241
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Submission history
From: Eve Lee
[v1] Tue, 28 Jan 2025 19:01:27 UTC (427 KB)
https://arxiv.org/abs/2501.17241
Astrobiology,

Keith Cowing

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) 🖖🏻

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