Astrophysicists Casey Brinkman and Nicholas Cowan have suggested that mass estimates for some unusual exoplanets may have been overestimated due to the magnetic activity of their stars. This calls into question the classification of some of these objects as super-massive rocky «mega-Earths», but does not rule out the existence of the planets themselves, according to Phys.org.
Briefly about the main points
- Scientists have identified eight anomalous objects amongst 333 exoplanets.
- All the selected planets orbit magnetically active stars.
- Four sites were previously considered candidates for rocky mega-earths.
- Star spots can mimic the signal of a planet’s gravitational influence.
- The result is a preprint and requires further observations.
Eight unusual worlds orbiting active stars
The researchers analysed planets for which mass and radius measurements are available and identified eight objects with anomalous characteristics. Four of these — WASP-84 c, TOI-1347 b, TOI-2322 c and HD 63433 b — were previously classified as extremely massive rocky planets.
Four more — GJ 523 b, TOI-815 c, HD 18599 b and GJ 143 b — were characterised by very high density. They may contain significant amounts of water or other heavy volatile substances.
Such rocky environments are difficult to reconcile with existing models of formation planets: According to theoretical calculations, bodies with a mass greater than approximately ten times that of Earth should normally have accumulated a substantial gaseous envelope. All the selected objects turned out to be satellites of magnetically active stars. For the four mega-Earth candidates, the authors estimated the probability of a random coincidence to be less than 1%.
How stellar spots can alter the estimation of a planet’s mass
Masses distant planets are often determined using the radial velocity method. The planet’s gravitational pull causes slight oscillations in the host star, and changes in the spectrum of its light make it possible to estimate the strength of this influence.
However, magnetic activity, sunspots and bright regions on the star’s surface can also produce similar spectral signals. If these are not separated from the planetary signal with sufficient precision, the calculated mass may turn out to be greater than the actual mass.
The authors cited the example of HD 63433 b: one study estimated its mass at approximately 37 Earth masses, whilst other observations placed an upper limit of around 21 Earth masses. According to the researchers, this discrepancy highlights the complexity of working with data from active stars.
The study does not claim that the selected exoplanets do not exist. The doubts relate primarily to the masses assigned to them and the conclusions regarding their entirely rocky composition, which were drawn from previous estimates. The study Brinkman і Forged were published on 8 October 2026 on the arXiv preprint server. Further measurements, taking greater account of stellar activity, are required to verify the findings.
