A Surprising Discovery
Astronomers have discovered an unusually massive and dense exoplanet, named GJ 523b, that challenges conventional ideas about how rocky planets form. The planet is about 2.5 times wider than Earth and packs roughly 23 times our planet's mass, according to reports from NewsBytes and Space.
Worlds this large and dense are sometimes referred to as "mega-Earths," an informal term for unusually massive, predominantly rocky planets. The term has been used for more than a decade but is not a formal class of exoplanets. 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.
"This isn't what we expected at all," Max Kroft, lead author of the study, said in a statement from the University of Wisconsin–Madison. "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."
The Planet's Profile
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.
A Puzzle for Formation Theory
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 started rapidly accumulating their massive gaseous envelopes once their growing cores reached about 20 times Earth's mass.
However, GJ 523b already exceeds this threshold but has not evolved into a gas-rich world. This discrepancy raises questions about how such a planet could form and remain predominantly rocky.
Possible Explanations
The unusual high density of GJ 523b raises questions about its evolution. 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.
Kroft acknowledged the limitations of drawing broad conclusions from a single finding. "It's hard to infer things about planet formation in general from a sample size of one," he said, expressing hope that future discoveries of similar planets will provide more context.
The findings have been submitted to The Astronomical Journal and are available on the preprint server arXiv, but have not yet been peer reviewed.