Astronomers have discovered an unusually massive, dense exoplanet that challenges conventional ideas about how rocky planets form.
Called GJ 523b, the exoplanet is about 2.5 times wider than Earth but packs roughly 23 times our planet’s mass into that relatively compact size. Worlds this large and dense are sometimes referred to as “mega-Earths,” an informal term for unusually massive, predominantly rocky planets. GJ 523b’s high density suggests it contains relatively little atmosphere despite being large enough that astronomers would normally expect it to have accumulated a substantial gaseous envelope, according to a statement from the University of Wisconsin–Madison.
“This isn’t what we expected at all,” Max Kroft, lead author of the study, said in the statement. “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.”
GJ 523b was initially identified as a candidate by NASA’s Transiting Exoplanet Survey Satellite (TESS), which searches for periodic dips in starlight caused when planets cross, or transit, their host stars. Researchers followed up with observations from the ground-based WIYN 3.5-meter Telescope at Kitt Peak National Observatory in Arizona, using a spectrograph to measure the planet’s gravitational tug on its star.
Combining those observations, the team calculated that GJ 523b has a mass about 23.5 times that of Earth, a radius 2.55 times larger and a density of about 126.82 grams per cubic inch. It circles its star every 17.75 days. The planetary system is also relatively young, at an estimated nearly 170 million years old, according to the study.
That combination of size, mass and youth poses a puzzle for planet formation models. Planets begin by building cores of rock and metal, which can then pull in hydrogen and helium from the disk of gas and dust surrounding a young star. In our own solar system, giant planets such as Jupiter and Saturn are thought to have begun rapidly accumulating their massive gaseous envelopes once their growing cores reached roughly 20 times Earth’s mass.
GJ 523b is already about 23 times Earth’s mass, putting it beyond that threshold. Yet instead of developing into a gas-rich world, its unusually high density suggests it has relatively little gas and remains predominantly rocky — raising the question of why it followed such a different path.
“The question is, why didn’t this planet do that, if it’s 20 times the size of Earth?” Kroft said in the statement.
One possibility is that GJ 523b initially formed with a thick atmosphere that was later stripped away. Another is that it formed through a collision between two planets, creating a larger rocky world while blasting much of their gaseous envelopes into space, according to the statement.
Astronomers have used the term “mega-Earth” for more than a decade to describe exceptionally massive rocky worlds, though it has never represented a formally established class of exoplanets. Finding more examples like GJ 523b could help reveal whether these unusual worlds are rare exceptions or part of a broader population.
“It’s hard to infer things about planet formation in general from a sample size of one,” Kroft said in the statement. “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.”
The findings have been submitted to The Astronomical Journal and are currently available on the preprint server arXiv and have not yet been peer reviewed.
