Mysterious ‘Mega-Earth’ Found: A Planet 23.5 Times More Massive Than Earth That Challenges Scientists

Astronomers have discovered an extraordinarily unusual exoplanet, GJ 523b, with a mass about 23.5 times greater than Earth’s — yet, contrary to what planet-formation models predict, it appears to have remained mostly rocky and has not turned into a gas-rich giant, according to the BBC.

The planet is about 2.55 times wider than Earth, and its high density suggests it contains relatively little atmosphere.

It was first identified by NASA’s TESS satellite, which detects exoplanets by recording the tiny changes in stars’ brightness when a planet passes in front of them.

Follow-up measurements were carried out by researchers at the University of Wisconsin–Madison, using the 3.5-meter WIYN telescope at the Kitt Peak National Observatory in Arizona.

Observations showed that GJ 523b has a density of about 126.82 grams per cubic inch and completes an orbit around its star every 17.75 days.

The planetary system is estimated to be around 170 million years old.

A planet that doesn’t follow the expected rules

GJ 523b sits in an unusual zone between rocky planets and gas giants.

In the Solar System, scientists think Jupiter and Saturn began rapidly accumulating large amounts of hydrogen and helium once their cores reached approximately 20 Earth masses.

Yet GJ 523b has passed that threshold without appearing to have developed a thick gaseous envelope.

That makes it a kind of “mega-Earth” — an informal term used for extremely massive rocky planets.

One possibility is that the planet was initially born with a much thicker atmosphere but lost most of it to the young star’s radiation and activity.

Another hypothesis is a giant collision between two planets, which could have created a larger rocky body while expelling most of the gas into space.

However, these are still only hypotheses.

The researchers emphasize that it is difficult to draw general conclusions about planet formation from a single example.

Finding 20–30 similar planets could enable astronomers to understand whether GJ 523b is a rare case or part of a broader class of planets.