Discovery of rare "Mega-Earth" raises questions about existing planetary theories

American researchers have identified an unusual planet outside our solar system that they describe as a “Mega-Earth”, a world whose size and composition challenge traditional ideas about how planets form.

The planet, known as GJ 523b, is more than 2.5 times the size of Earth and is believed to be around 170 million years old, according the scientist’s press release through their institution.

Researchers from the Wisconsin Center for Origins Research (WiCOR) discovered and catalogued the planet, which could provide new insights into the formation and evolution of large rocky worlds.

According to Max Kroft, a graduate student in the laboratory of University of Wisconsin–Madison Assistant Professor of Astronomy Thomas Beatty, who is the lead author of a paper on the planet, astronomers have used the term “Mega-Earth” for decades without having a planet that could definitively establish what the category actually means.

“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,” says Beatty.

The WiCOR project was launched in 2024 as a collaboration between researchers from seven UW–Madison departments: astronomy, biology, chemistry, geoscience, atmospheric and oceanic sciences, physics and bacteriology.

The centre is particularly interested in finding Hycean exoplanets — a theorized class of worlds outside our solar system thought to have large oceans and temperate atmospheres that could potentially support life.

Many of the planetary candidates studied by researchers have been identified by NASA’s Transiting Exoplanet Survey Satellite (TESS), which was launched in 2018.

TESS monitors stars for periodic changes in their brightness. A temporary decrease in brightness can indicate that a planet is passing in front of its host star.

“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 a quarter have been confirmed.

Kroft used the ground-based WIYN telescope in Arizona, equipped with a high-resolution spectrograph, to conduct follow-up observations of 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.”

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

The planet is not a Hycean world. Instead, the data suggest it is primarily composed of dense rock and has a massive core with a mass around 23 times that of Earth. Its diameter is approximately 60% that of Neptune.

Despite not fitting the original search criteria, GJ 523b is an exceptionally unusual planet that could help scientists better understand how large 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 bigger than the Earth.”

Challenge to planet formation theories

Kroft described the planet’s formation as “a real curveball”. Under conventional models, planets begin with a rocky and metallic core that gradually accumulates a gaseous atmosphere, primarily composed of hydrogen.

In our own solar system, larger planets such as Jupiter and Saturn developed their enormous atmospheres after reaching roughly 20 times Earth’s mass.

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

Researchers have proposed several possible explanations. One possibility is that GJ 523b once had a substantial atmosphere that was stripped away after the planet moved too close to its host star. Another theory is that the planet formed from the collision of two smaller planets, with the enormous impact generating enough heat and energy to strip away much of their atmospheres.

By Nazrin Sadigova