Rocky Planet Discovered Orbiting Ancient Star

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Artist’s rendition of TOI-561, one of the oldest, most metal-poor planetary systems discovered yet in the Milky Way galaxy. This 10 billion-year-old system, has a hot, rocky exoplanet (center) that’s one and a half times the size of earth as well as two gas planets (to the left of the rocky planet) that are about twice as large as Earth.
Credit: W. M. Keck Observatory/Adam Makarenko

University of Hawai‘i astronomers using the WM Keck Observatory on Maunakea have discovered an ancient rocky planet in Earth’s home galaxy.

It’s one of the first confirmed rocky exoplanets found in the Milky Way (most planets that are known are in the galactic thin disk). This 10 billion-year-old planet is also one of the oldest rocky planets yet discovered.

The planet orbits the star TESS Object of Interest (TOI) 561, named for the ongoing NASA TESS planet hunting mission, according to a WM Keck press release. TOI-561 belongs to a rare population of stars called the galactic thick disk. Thick disk stars are chemically distinct, with fewer trace heavy elements (and especially less iron) than typical stars of the Milky Way, suggesting they formed early, approximately 10 billion years ago. They also have wandering motions that can lift them out of the galactic plane, providing an epic view of our own spiral galaxy.


“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 UH IfA and leader of the team that discovered the TOI-561 planetary system.

News of this discovery was just announced at a press conference Monday at the January 2021 virtual meeting of the American Astronomical Society.

The rocky planet orbiting TOI-561 transits its star, meaning that the planet passes in front of its star as seen from Earth, blocking a fraction of the starlight. The planet is small, with a radius only one and a half times that of Earth. As a result, the reduction of light it causes is minuscule, just 0.025% of the star’s brightness.


TOI-561 has at least two other planets transiting the star, both of which have about twice Earth’s radius and are too large and low-mass to be rocky.

Although the past 10 billion years of the planet’s history are murky, it likely does not host life now, officials say.

“The planet orbits its star twice every Earth day, so close to its host star that the estimated average surface temperature is over 3,100 degrees Fahrenheit (or about 1,726 Celsius) — far too toasty for life as we know it,” the release states. “However, this rocky, magma planet is perhaps a harbinger of a population of rocky worlds yet to be discovered around our galaxy’s oldest stars.”

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