Ancient Rocky Planet Orbits Its Star Twice a Day

Artistic rendering of TOI-561, one of the oldest and most metal-poor planetary systems discovered to date in the Milky Way. WM Keck Observatory / Adam Makarenko

Researchers have identified one of the oldest planetary systems ever discovered, and it’s a strange one. A rocky planet called TOI-561b orbits a 10 billion year old star, more than twice the age of our sun, showing that planets have formed since the beginning of the universe.

The planet, which is about 1.5 times the size of Earth and therefore known as super-earth, orbits its star more than twice in a day on Earth. It’s lashing around its star so quickly because it’s very close to it, meaning the planet has a seriously high surface temperature of over 1,700 degrees Celsius.

The super-fast year and high temperatures aren’t the only notable features of this extraterrestrial curiosity, however. The planet also has an unusually low density for its size, because although it is roughly three times the mass of the earth, it is the same density as our planet. That suggests that it is very old, according to the study’s authors.

Older planets are less dense because they have less heavy elements such as metals. These heavy elements are formed in stars as they age and eventually explode in a supernova. They distribute the elements in the space around them from which planets are formed. Earlier in the universe there were fewer star explosions, and so planets with less heavy elements formed.

“TOI-561b is one of the oldest rocky planets discovered to date,” said Lauren Weiss, postdoctoral researcher and team leader at the University of Hawaii, in a statement. “Its existence shows that the universe has formed rocky planets almost since its inception 14 billion years ago.”

The team discovered the planet using NASA’s Transiting Exoplanet Survey Satellite (TESS) and confirmed its presence using the WM Keck Observatory in Hawaii.

The study’s lead author, Stephen Lane of the University of California, Riverside, said this finding could just be the beginning of many more rocky planets around older stars to be discovered in upcoming missions.

“While this particular planet is unlikely to be inhabited today, it may be a harbinger of many rocky worlds yet to be discovered around the oldest stars in our galaxy,” said Kane.

The research was presented at the American Astronomical Society meeting in 2021 and will be published in the Astronomical Journal.

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