Mega-Earth GJ 523b is 23 times as massive as our planet—but only 2.5 times as wide

It’s nearly 170 million years old. It’s more than 2.5 times the size of Earth. And it’s the first exoplanet discovered and cataloged by researchers with the Wisconsin Center for Origins Research (WiCOR).

Max Kroft, a graduate student in the lab of Assistant Professor of Astronomy Thomas Beatty, is the lead author on a paper that characterizes GJ 523b, a dense exoplanet classified as a “Mega-Earth” because of its massive size. The paper is under review and is available through the preprint server arXiv.

“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 we 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.”

WiCOR’s search for Hycean worlds

WiCOR launched in 2024 as a collaboration of researchers from what are now seven UW–Madison departments (astronomy, biology, chemistry, geoscience, atmospheric and oceanic sciences, physics and bacteriology) pursuing projects related to the origins of life in the universe.

As part of those efforts, WiCOR researchers have been on the lookout for Hycean exoplanets, a theorized type of exoplanet outside our solar system with a large ocean and temperate atmosphere that could potentially support life.

Planetary candidates have been identified by the Transiting Exoplanet Survey Satellite (TESS), a NASA satellite mission launched in 2018. TESS’s main objective is to monitor stars to see if their brightness dips, a possible indication of an exoplanet.

“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, but less than a quarter of them have been confirmed. Kroft used a ground-based telescope in Arizona, WIYN, with a high-resolution spectrograph to follow up on the candidate that eventually became GJ 523b.

“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.”

An unexpectedly dense giant

Using the spectrograph and data collected by the James Webb Space Telescope, Kroft and the rest of the WiCOR team were able to determine information about GJ 523b’s density and atmosphere.

While it’s not a Hycean planet—according to the data, it’s mostly dense rock with a massive core, 23 times Earth’s weight and 60% the size of Neptune—it’s an extremely unusual planet that could shed new light on the ways in which planets form.

“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 the size of Earth.”

Kroft describes the exoplanet’s formation as “a real curveball.” Typically, planet formation includes the formation of a core of rocky material and metals that accretes a gaseous hydrogen-based atmosphere on top. Larger planets within our solar system, like Jupiter and Saturn, developed their massive atmospheres once they reached about 20 times the size of Earth.

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

Kroft and the WiCOR team put forward several possible theories in their paper, including the possibility that the planet had some of its atmosphere ripped off when it orbited too close to its star or that it actually began as two planets that collided and lost atmosphere in the heat of the crash.

“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.”

Looking for more outliers

Kroft is hopeful that as planetary researchers wade through the thousands of potential planets that have already been identified, more dense and oversized planets like GJ 523b will be cataloged, making it more possible to categorize them. At the end of August, NASA is scheduled to launch the Nancy Grace Roman Telescope, the next flagship telescope expected to find tens of thousands of potential new planets.

“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 overdense 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.”

Susanna Widicus Weaver, the Vozza Professor of Chemistry and Astronomy and director of WiCOR, expects the exoplanet to be just the first of many key discoveries.

“We are thrilled to find WiCOR’s first exoplanet!” she says. “The UW co-authors on this manuscript span four different departments in L&S. That kind of collaborative effort is exactly what WiCOR is designed to support and promote. Without WiCOR facilitating the collaboration, we would not have been able to fully interpret these results at the level of detail that we have.”

Who’s behind this story?

Sadie Harley

BSc Life Sciences & Ecology. Microbiology lab background with pharmaceutical news experience in oil, gas, and renewable industries.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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