A huge, unusually dense planet more than twice the size of Earth is puzzling scientists because it has very little atmosphere, despite being massive enough that researchers would expect it to have accumulated a thick layer of gas.
The planet, named GJ 523b, is around 170 million years old and more than 2.5 times the size of Earth. It has approximately 23 times Earth’s weight and is about 60 percent the size of Neptune, according to researchers at the University of Wisconsin-Madison. A release from the university described it as mostly dense rock surrounding a massive core.
"This isn’t what we expected at all," lead author Max Kroft, a graduate student in the laboratory of UW-Madison Astronomy professor Thomas Beatty, said in the release. "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."
"Our planet formation models are very good at telling you what usually happens: a planet that grows this big should pull in gas and end up looking like Neptune in our own solar system," Beatty told Newsweek.
"GJ 523b didn't, and now we get to figure out why. Maybe it formed after that gas was already gone, or maybe a giant collision blew its atmosphere off. Now that we know this planet is there, we can observe it in more detail with telescopes like JWST to try and discover the answer.
"But, however this planet formed, it's a great example of how we often end up learning more from the one planet that breaks the rules than from a hundred that follow them."
What Is a Mega-Earth?
A Mega-Earth is an exceptionally large and dense terrestrial planet outside our solar system. It is more massive than a typical "super-Earth" and is thought to consist largely of solid material such as rock and metal. Beatty said GJ 523b could help scientists define the category more clearly.
"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," Beatty said. "GJ 523b finally does."
Understanding the planet will require expertise beyond astronomy, he added. Researchers need geologists who can understand how iron and rock behave under pressures that cannot be reproduced in laboratories on Earth, as well as atmospheric scientists who can help determine how much of the planet is actually rock, Beatty said.
GJ 523b was first identified as a candidate by NASA’s Transiting Exoplanet Survey Satellite, or TESS. The satellite searches for planets by monitoring stars for regular dips in brightness, which may happen when an orbiting planet crosses in front of its star.
"There’s this periodic dipping of the star’s light," Kroft said. "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."
Kroft followed up on the candidate using the WIYN telescope in Arizona. The team also used data collected by the James Webb Space Telescope to investigate the planet’s density and atmosphere.
Why Is Its Atmosphere So Small?
The release noted that scientists generally expect a growing core of rock and metal to pull in a hydrogen-rich atmosphere. In our solar system, planets such as Jupiter and Saturn developed massive atmospheres after reaching around 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?" Kroft said.
The researchers have proposed several possible explanations, the university said. GJ 523b may once have orbited so close to its star that extreme heat stripped away much of its atmosphere. Alternatively, it could have formed when two planets collided, with the impact heat stripping away the atmosphere.
"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," Kroft said.
Update, 8/26/26, 11:43 a.m. ET: this article was updated with comment from Thomas Beatty.
Contact Newsweek editors on this story: Matthew Robinson and Sam Wilson.