Astronomers Find Super-Earth Around One of Milky Way's Oldest Stars

The exoplanet is about 50% larger than Earth, with three times its mass.
Fabienne Lang
Artist's rendition of TOI-561bAdam Makarenko/W.M. Keck Observatory

A hot and rocky "super-Earth" has been discovered orbiting one of the oldest stars in our Milky Way galaxy, taking scientists by surprise. 

In a new study led by scientists at the University of California, Riverside (UC Riverside), who worked alongside Keck Observatory astronomers in Hawai'i, shed light on the exoplanet that's about 50% bigger than Earth and has three times its mass. 

Their findings were published in The Astronomical Journal on Monday.


Known as TOI-561b, the newly-discovered planet takes under 12 hours to orbit its star. "For every day you’re on Earth, this planet orbits its star twice," said UC Riverside planetary astrophysicist and team member Stephen Kane.

Being so close to its star means TOI-561b is toasty hot with an average temperature over 2,000 degrees Kelvin — meaning 3,138 degrees Fahrenheit (1,726 degrees Celcius). And even though the team measured the planet's rough mass to be around three times more than Earth, it's density is about the same as our home planet. 

"This is surprising because you’d expect the density to be higher," Kane explained. "This is consistent with the notion that the planet is extremely old."

TOI-561b potentially formed over 10 billion years ago, the scientists suggest, because the older a planet is, the less dense it usually is as not as many heavy elements were available when it was formed. 

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"The rocky planet orbiting TOI-561 is one of the oldest rocky planets yet discovered. Its existence shows that the universe has been forming rocky planets almost since its inception 14 billion years ago," said Dr. Lauren Weiss, Beatrice Watson Parrent Postdoctoral Fellow at the University of Hawai'i and leader of the team that discovered the TOI-561 planetary system.  

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The old star around which the exoplanet orbits, TOI-561, is part of a rare group of stars called the galactic thick disk, as the study noted. These stars have a chemical distinction, with fewer heavy elements such as magnesium and iron that are typically associated with planet building. 

"Information about a planet’s interior gives us a sense of whether the surface of the planet is habitable by life as we know it," Kane said. "Though this particular planet is unlikely to be inhabited today, it may be a harbinger of a many rocky worlds yet to be discovered around our galaxy’s oldest stars."

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