"Mega-Earths" are likely an artifact of stellar activity
Abstract
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.
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BibTeX
@misc{brinkman2026megaearths,
title = {{"Mega-Earths" are likely an artifact of stellar activity}},
author = {Casey L. Brinkman and Nicolas Cowan},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.12600},
eprint = {2610.12600},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.12600},
}
RIS
TY - UNPB
TI - "Mega-Earths" are likely an artifact of stellar activity
AU - Casey L. Brinkman
AU - Nicolas Cowan
PY - 2026
DA - 2026/10/08
PB - arXiv
DO - 10.48550/arxiv.2610.12600
UR - https://arxiv.org/abs/2610.12600
AB - 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.
ER -