In 6 billion years, the Sun will expand to become a red giant. That process should consume Mercury and perhaps Venus. We have long thought that it could also incinerate the Earth.
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But perhaps all is not lost for planet Earth (although it may have long since become uninhabitable).
Scientists have discovered a rocky world orbiting another star that has already passed through its red giant phase. This planet now orbits a white dwarf, the smallest stellar body left after a star goes out. Significantly, The planet appears to have orbited the star in the same position that the Earth currently travels around our Sun, and did so until it was pushed into a more distant orbit, twice the Earth-Sun distance, before the dying giant could devour him. This makes it the first potential rocky world observed orbiting a white dwarf.
“We don’t know if the Earth can survive,” said Keming Zhang, an astrophysicist at the University of California, San Diego, who led the study published in the journal Nature Astronomy. “If so, it will end up somewhere like this system.”
The planet is about 4 thousand light years from us. It was discovered in 2020 with a network of Korean telescopes through a process called microlensing. The Korean team had observed how the planet’s star passed in front of another star, which from the background increased the amount of light that was directed to the telescope a thousand times.
This event was a one-time event, limiting the possibility of detailed follow-up observations until new powerful telescopes can better observe the planet’s star in the future. But Zhang’s team worked at the Keck Observatory in Hawaii last year and was able to identify it as a white dwarf. The data that the researchers managed to collect allowed them to calculate that there were at least two objects orbiting the white dwarf.
One was a suspected brown dwarf, a failed star that was never ignited by nuclear fusion, located at a great distance from the star. But the other object was a planet about 1.9 times Earth’s mass that orbited much closer to the star, suggesting it was a rocky planet.
By modeling the evolution of the star system, the team calculated that the planet could once have had the same habitable orbit as Earth. The star was probably also similar in size to ours.
But when the star ran out of fuel, it lost mass, causing the rocky planet’s orbit to lengthen. This allowed it to escape the star’s expanding red giant phase and survive into the white dwarf phase.
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