Photo: NASA/JPL-Caltech/R. Hurt (SSC)
Astronomers have discovered an exceptionally massive and dense planet that could challenge existing theories about how rocky worlds form, Space.com reports.
The exoplanet, GJ 523b, is about 2.5 times larger than Earth but has roughly 23.5 times its mass. Objects with such extreme properties are sometimes informally called “mega-Earths” — unusually massive planets believed to be predominantly rocky.
Researchers at the University of Wisconsin–Madison said GJ 523b’s density suggests it has retained only a relatively thin atmosphere, even though a planet of its mass would normally be expected to accumulate a substantial envelope of hydrogen and helium.
“This is not at all what we expected,” said lead study author Max Croft. He noted that extremely dense planets are not unusual, but they are typically much smaller, such as Earth or Mercury.
GJ 523b was initially identified as an exoplanet candidate by NASA’s Transiting Exoplanet Survey Satellite (TESS). Researchers then conducted follow-up 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 star.
Combining the observations, the team determined that GJ 523b has a radius about 2.55 times that of Earth and orbits its star every 17.75 days. The system is relatively young, with an estimated age of around 170 million years.
The combination of the planet’s size, mass and age presents a challenge for current planet-formation models.
Planets are thought to begin by forming rocky and metallic cores that can then accumulate hydrogen and helium from the disk of gas and dust surrounding a young star. In our Solar System, Jupiter and Saturn are believed to have rapidly accumulated their massive gas envelopes once their growing cores reached roughly 20 times Earth’s mass.
GJ 523b is already more than 23 times Earth’s mass, meaning it has surpassed that threshold. Yet its high density indicates that it contains relatively little gas and remains predominantly rocky.
Scientists are considering several possible explanations. The planet may have originally formed with a thick atmosphere that was later stripped away. Another possibility is that it formed through a massive collision between two planets, producing a larger rocky world while ejecting much of their gaseous envelopes into space.
The discovery could provide new insight into the boundary between rocky planets and gas-rich worlds and force scientists to reconsider how massive planetary cores evolve.
