Astronomers Just Made A Huge Discovery About These Distant 'Mega-Earths'

Two astronomers at McGill University say that a handful of the heaviest rocky planets we know about might not be nearly as heavy as astronomers thought. Casey Brinkman and Nicolas Cowan are the two astronomers behind the research, and they say that the mega-Earths, as they're called, are likely just an "artifact of stellar activity."

Mega-Earths were first discovered more than a decade ago, and are broadly described as planets that "appear to have density consistent with rocky Earth-like compositions," but up to 17 times bigger. One of the original researchers working on the discovery described them as "the Godzilla of Earths!" according to Smithsonian Insider. The new research from McGill suggest they may not be as gigantic as once thought. The team's simulations say a purely rocky planet is unlikely to grow beyond around 10 Earth masses, as a core that big should be pulling a thick envelope of gas from the disk that it formed within.

Brinkman and Cowan say they started with small planets from NASA's Exoplanet Archive and then narrowed that list down to 333 planets, only eight of which were over 10 Earth masses with very little gas around them, and every one of those eight orbits an active star. "Our certainty that these strange objects even exist, however, is only as strong as the quality of the mass and radius measurements," the researchers wrote in their paper, published in October 2026.

Astronomers estimate that the jump from a bare rocky planet to one with a gas envelope usually happens somewhere between 4 and 5 Earth masses. Even one of the most massive super-Earths we've ever discovered, TOI-1075 b, is only about 10 times heavier than Earth.

Four of the eight planets in the study have rocky densities. WASP-84 c, for example, is about 15 Earth masses even though it's less than twice as wide as Earth. The astronomers believe that the other four could hold water or other heavy volatiles but not much hydrogen or helium gas.

What's especially interesting is that the astronomers say that, if these planets are indeed composed the way their research suggests, they would have needed to form in exceptional ways, such as through collisions after the gas disk around their star disappeared or by having once been giant planets that got stripped down to their cores. Now, astronomers have run into some odd planets over the years — like a planet so huge they aren't even sure it's really a planet — but Brinkman and Cowan just don't think these eight are actually mega-Earths.

Astronomers measured these planets with a method known as the radial-velocity method, which tracks how much a planet tugs its star back and forth. "Star spots ... can mimic the signature of an orbiting planet as the star rotates," the researchers explain, and that can push a planet's mass estimate higher than it should be.

The chance that all eight mega-Earths would orbit active stars by coincidence is less than 1%, and the researchers also couldn't find a reason why young, active stars would end up with more of these planets. In fact, Brinkman and Cowan wrote that they "would not expect young stars to preferentially host these worlds."

"The most likely culprit for these larger than expected masses is artificial inflation from stellar activity," the researchers wrote. To fix this, Brinkman and Cowan say that astronomers need to schedule the observations in a way that allows them to catch both the star's activity and the planet's signal. The paper has been submitted to The Astrophysical Journal Letters, but is not officially published yet.