Scientists found a ‘Mega-Earth’ 2.5 times the size of Earth with a core 23 times its mass; but it didn’t develop the massive atmosphere expected of a planet this large, and researchers are trying to find out why

Researchers have discovered an unusually dense exoplanet, more than 2.5 times Earth's size, that could challenge existing ideas about how large rocky worlds form and evolve. Named GJ 523b, this ‘Mega-Earth’ exoplanet is nearly 170 million years old and named like this because of its exceptional size and density.

It is the first exoplanet discovered and cataloged by researchers with the Wisconsin Center for Origins Research (WiCOR), a University of Wisconsin–Madison collaboration focused on questions surrounding the origins of life and planetary systems.

The discovery was led by Max Kroft, a graduate student in the lab of Assistant Professor of Astronomy Thomas Beatty. Their paper characterizing the GJ 523b Mega-Earth exoplanet is currently under review and is available through the open-access arXiv archive.


What makes GJ 523b particularly unusual is not simply its size. The planet is more than 2.5 times Earth's diameter but remains extremely dense and is believed to be composed largely of rock, with a massive core. That combination raises a fundamental question for planetary scientists: how did such a large planet avoid developing the enormous hydrogen-rich atmosphere normally associated with planets of this size?

GJ 523b is a Mega-Earth unlike the usual rocky planets

The term “Mega-Earth” has been used by astronomers for more than a decade to describe planets that are substantially larger than Earth yet remain primarily rocky. But according to Beatty, scientists have lacked a planet that could provide a concrete basis for defining the category.

“People have been using the phrase 'Mega-Earth' for more than a decade, but we've never had a planet that let us say concretely what one is,” says Beatty. “GJ 523b finally does. What is also striking is that nailing down the definition of a Mega-Earth isn't something us astronomers can really do by ourselves: We need geologists who understand how iron and rock behave at pressures no laboratory on Earth can reach, and atmospheric scientists who can tell us how much of what we measured is rock at all.”

That makes GJ 523b more than just another addition to the growing catalog of exoplanets. Its unusual combination of size, density and atmospheric properties could help scientists understand where the boundaries between rocky planets and much larger worlds actually lie.

How the GJ 523b Mega-Earth exoplanet was found

The discovery began with NASA’s Transiting Exoplanet Survey Satellite, or TESS, which searches for planets by monitoring stars for periodic changes in brightness. When a planet passes between its star and a telescope, it blocks a small portion of the star’s light. That produces a temporary dip in brightness known as a transit.

“There’s this periodic dipping of the star’s light. We think that’s a planet passing in front of the star and transiting. It’s blocking some of the light from the star, and the star gets dimmer,” says Kroft. TESS has identified more than 8,000 candidate planets, although fewer than one-quarter have been confirmed.

Kroft followed up on the candidate that eventually became GJ 523b using the WIYN telescope in Arizona, equipped with a high-resolution spectrograph.

The initial observations provided clues about the planet’s size and orbit. “We picked out this planet based on what we thought its size and temperature were,” explains Kroft. “A bigger planet makes a bigger dip, so we get an idea of the size, and based on how often that dip happens, we get the distance of its orbit, and we can use that to estimate the temperature of the exoplanet.”

Why GJ 523b is not the Hycean world researchers expected

The discovery was particularly significant for WiCOR because researchers had been searching for Hycean exoplanets, a theorized class of worlds thought to possess large oceans and temperate atmospheres that could potentially support life.

GJ 523b, however, turned out to be something very different. Using the spectrograph along with data collected by the James Webb Space Telescope, the WiCOR team determined the planet’s density and atmospheric composition. The observations indicate that GJ 523b is mostly dense rock with a massive core.

Its core is estimated to have a mass 23 times Earth's, while the planet itself is about 60% the size of Neptune.

“This isn’t what we expected at all,” says Kroft. “Dense planets like this aren’t uncommon, but they’re usually small rocky planets similar to Earth or Mercury. This planet is two and a half times bigger than the Earth.”

That unusual combination is what makes the GJ 523b Mega-Earth exoplanet particularly important. Scientists already know of dense rocky planets, but they are generally much smaller. GJ 523b occupies a much less familiar territory: a planet large enough to raise questions about why it did not evolve into a gas-rich world.

The planet appears to have avoided a massive atmosphere

Planet formation generally begins with a rocky and metallic core. As the core grows, it can accumulate a hydrogen-dominated gaseous atmosphere.

In our own solar system, planets such as Jupiter and Saturn eventually developed enormous atmospheres after reaching roughly 20 times Earth’s size. GJ 523b presents a puzzle because its size would seem to put it in a regime where scientists might expect substantial atmospheric accumulation.

“The question is, why didn’t this planet do that, if it’s 20 times the size of the Earth?” asks Kroft.

The researchers suggest several possible explanations. One possibility is that GJ 523b originally accumulated a larger atmosphere but subsequently lost part of it because the planet orbited too close to its star.

Another possibility is far more violent: the planet could have formed through the collision of two planets, with the enormous impact stripping away much of their atmosphere.

“It kind of blows away,” Kroft explains. “A planet can’t hold on to its atmosphere if it’s really hot, and so you could be left with this big glob of rock made by these two planets with very little atmosphere.”

Why one Mega-Earth is not enough to solve the mystery

Although GJ 523b could provide an important case study, scientists cannot determine the broader rules governing Mega-Earths from a single example.

Kroft hopes that continued searches will identify additional planets with similarly unusual combinations of size and density. Comparing them could eventually reveal patterns in how these worlds form.

“It’s hard to infer things about planet formation in general from a sample size of one,” says Kroft. “We’re not going to get to 10,000 of these over-dense planets, but if we can get to 20 or 30, maybe some trends might pop out, where maybe the heaviest ones have shorter orbital periods, or they tend not to have companion planets.”

The search for more worlds could accelerate as new telescopes come online. NASA is scheduled to launch the Nancy Grace Roman Telescope at the end of August, with the mission expected to identify tens of thousands of potential new planets.



That makes this Mega-Earth exoplanet an unusual laboratory for studying planetary formation and potentially a clue to how some worlds can become enormous without following the evolutionary path of planets like Jupiter and Saturn.