Massive Rocky 'Mega-Earth' Found 23 Times Heavier Than Our Planet
Exoplanet GJ 523b has a density that baffles astronomers because it hasn't accumulated a gaseous envelope despite crossing the critical mass threshold.
Exoplanet GJ 523b has a density that baffles astronomers because it hasn't accumulated a gaseous envelope despite crossing the critical mass threshold.
Astronomers have stumbled upon an exceptionally massive and dense exoplanet that challenges established theories about how rocky worlds form. The planet, named GJ 523b, is about 2.5 times wider than Earth but packs roughly 23 times its mass into that relatively compact body, according to a statement from the University of Wisconsin-Madison.
Worlds of such dimensions and density are sometimes informally dubbed "mega-Earths," referring to extremely massive, predominantly rocky planets. The high density of GJ 523b indicates very little atmosphere, even though the planet is large enough that one would expect it to have accumulated a substantial gaseous layer.
"This is not at all what we expected. Dense planets like this are not uncommon, but they are usually small rocky planets similar to Earth or Mercury. This planet is two and a half times larger than Earth," said Max Kroft, the lead author of the study, in the statement.
According to the study, planets begin forming by building cores of rock and metal, which can subsequently attract hydrogen and helium from the disk of gas and dust surrounding a young star. In our solar system, gas giants like Jupiter and Saturn are thought to have started rapidly accumulating their massive gaseous envelopes only once their cores reached roughly 20 times Earth's mass.
GJ 523b has already surpassed that threshold with a mass of about 23 Earth masses. Yet instead of evolving into a gas-rich world, its extremely high density suggests it has little gas and remains predominantly rocky.
"The question is why this planet didn't do the same, if it has a mass 20 times that of Earth?" Kroft said.
GJ 523b was first recorded as a candidate by NASA's Transiting Exoplanet Survey Satellite (TESS), which detects periodic dimming of stars caused by planets passing in front of them. The researchers then conducted additional observations with the 3.5-meter WIYN telescope at Kitt Peak National Observatory in Arizona, using a spectrograph to measure the planet's gravitational influence on its host star.
Combining these data, 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 approximately 126.82 grams per cubic inch, or 7.74 grams per cubic centimeter. It completes an orbit around its star every 17.75 days. The system is also relatively young, with an estimated age of nearly 170 million years.
One hypothesis is that GJ 523b originally formed with a dense atmosphere that was later stripped away. Another is that it formed from a collision of two planets, creating a larger rocky world while much of their gaseous envelopes were ejected into space, the statement notes.
The term "mega-Earth" has been used by astronomers for over a decade to describe extremely massive rocky worlds, although it has never been established as a formal class of exoplanet. Discovering additional examples like GJ 523b could show whether these unusual worlds are rare exceptions or part of a broader population.
"It's hard to draw conclusions about planet formation in general based on a sample of just one object. We won't get 10,000 of these over-dense planets, but if we get 20 or 30, maybe some patterns will emerge," Kroft concluded.
The findings have been submitted to The Astronomical Journal and are currently available on the preprint server arXiv. The study has not yet undergone peer review.
