[astro-ph.EP]. Empirical relations and planet formation simulations suggest an upper mass limit for rocky planets of ∼10M⊕, as cores above this will almost certainly undergo runaway gas accretion.
Nevertheless, a small number of planets appear to have densities consistent with rocky Earth-like compositions above this threshold, indicating that unusual formation and evolutionary scenarios are capable of producing extremely massive rocky planets called “Mega-Earths.”
However, all of these planets orbit active stars with logR′HK >-4.75. We find a <1% probability that all literature ultra-massive rocky planets would coincidentally orbit such active stars, and find no astrophysical reason to expect Mega-Earths to preferentially form around active stars.
We therefore conclude that Mega-Earths are artifacts of stellar activity, and we question the reliability of Gaussian Processes to enable mass measurements for rocky planets around active stars.
Planet Mass vs Stellar Activity indicators (here logR′ HK) for 37 planets consistent with rocky planet densities (circles). Planets with a mass below that of the runaway gas accretion threshhold (10 M⊕) are shown in grey while those above this limit are shown in white, delimited by a grey dotted line. Regions of high, very high, and low activity are highlighted (Henry et al. 1996), with the Vaughn-Preston gap delimiting active and inactive stars. Planets consistent with volatile-rich compositions (GJ 523 b, GJ 143 b, TOI-815 c, and HD 18599 b) are shown with lavender squares to distinguish from those in the rocky planet density regime. — [astro-ph.EP]
Casey L. Brinkman, Nicolas Cowan
Comments: Submitted to ApJL 08/3036, resubmitted 10/2026
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2610.12600 [astro-ph.EP](or arXiv:2610.12600v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2610.12600
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Submission history
From: Casey Brinkman
[v1] Thu, 8 Oct 2026 18:22:37 UTC (4,594 KB)
https://arxiv.org/abs/2610.12600
Astrobiology, exoplanet,
