“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 ∼10 M ⊕ , 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. However, all of these planets orbit active stars with log R HK ′ > −4.75. We find a <1% probability that all literature ultramassive 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.