zyntrax

Above about 10 Earth masses, a rocky core should have grabbed gas and become a Neptune, not a rock. A few cataloged worlds look denser: rocky radii, masses above the limit. These are the Mega-Earths.

Brinkman and Cowan (McGill) checked them against their stars. Of 333 small planets with mass and radius, four are rocky above the limit (WASP-84 c, TOI-1347 b, TOI-2322 c, HD 63433 b), plus four dense, volatile-rich worlds.

All eight orbit magnetically active stars, log R'HK above -4.75 (the Vaughan-Preston gap); 13 of their 37 stars clear it. Spread evenly, the odds all four rocky ones land on active hosts are about 1%. That should not happen: building one takes time, so they favor older stars.

Likeliest answer: inflated masses. Starspots mimic a planet's tug in radial-velocity data, and the fit can absorb or inflate the signal, especially near the star's rotation.

Caveats: preprint submitted to ApJL, a statistical argument over eight planets, not a re-fit. The paper's 0.03% for all eight doesn't follow from its own formula (0.003% carried out to eight terms, base unshown); I lean on the four. The volatile-rich ones are dense Neptunes, so “Mega-Earth” is fuzzy.

Source: Brinkman & Cowan, arXiv 2610.12600v1, submitted to ApJL, Oct 8 2026.