

The exoplanet, called that because it's located outside our solar system, is about 50% larger than Earth and has three times the mass -- which makes it a super-Earth by astronomy standards.


"For every day you're on Earth, this planet orbits its star twice," said Stephen Kane, study coauthor and astrophysicist at the University of California, Riverside, in a statement.


This super-Earth's proximity to its host star creates an average surface temperature on the planet exceeding 2,000 Kelvin, or 3,140 degrees Fahrenheit.

The planet was discovered using NASA's planet-hunting TESS mission, which launched in 2018. The name "TESS" is short for Transiting Exoplanet Survey Satellite. The mission regularly surveys parts of the sky and observes nearby stars to determine if there are planets orbiting them.

The astronomers found TOI-561b -- "TOI" is short for TESS Object of Interest -- in the thick galactic disk of the Milky Way. This rare population of stars is known for having less heavy elements, like iron or magnesium, than are often associated with planet creation.

To confirm their finding, the researchers used the W.M. Keck Observatory in Hawaii and were able to determine the planet's mass, radius and density.

Astronomers, however, were surprised to discover that despite the planet's mass, its density is about the same as that of Earth.

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

Older planets are actually less dense because there weren't as many heavy elements present in the universe when these planets formed. Heavy elements are created as stars age and explode -- which seeds the creation of new stars and planets across the universe.

"TOI-561b is one of the oldest rocky planets yet discovered," said lead study author Lauren Weiss, a University of Hawai'i postdoctoral fellow, in a statement. "Its existence shows that the universe has been forming rocky planets almost since its inception 14 billion years ago."


The Milky Way galaxy was likely formed 12 billion years ago. This star and its planets are 10 billion years old -- while our sun is only 4.5 billion years old.

"This planet formed at a time when the majority of stars in our galaxy were first beginning to shine," Weiss said at the conference on Monday.

Understanding the planet's mass and radius allow astronomers to learn more about its internal structure.

"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 many rocky worlds yet to be discovered around our galaxy's oldest stars."

Even though TOI-561b is a rocky, or terrestrial planet, like Earth, it's far too hot to support life. But given that this rocky planet is so old, and the first confirmed rocky planet around such an old star, astronomers wonder if the planet was habitable at some point in the past.


Stars in the thick galactic disk "have motions that take them high above and far below the plane of the galaxy, providing them with what might be stunning views, if anyone were there to see them," Weiss said.