In six billion years, the Sun will become a red giant. That process would consume Mercury, and perhaps Venus. We have long thought it could incinerate Earth as well.
But perhaps not all is doomed for planet Earth (although it may be an uninhabitable world for a long time).
Scientists have discovered a rocky world orbiting another star that has already passed its red giant phase. This planet now orbits a white dwarf, the smallest stellar body left after a star burns up. The planet appears to have orbited the star in the same position as Earth orbits our Sun, until it was pushed into a farther orbit, twice the distance between Earth and the Sun, before the dying giant could devour it. This makes it the first potentially rocky world observed orbiting a white dwarf.
“We don’t know if Earth can survive,” said Keming Zhang, an astrophysicist at the University of California, San Diego, who led the work published Thursday in the journal Nature Astronomy . “If it does, it will end up somewhere like this system.”
The planet is located about 4,000 light-years away from us. It was discovered in 2020 with a network of Korean telescopes using a process called microlensing. The Korean team had observed how the planet’s star passed in front of another star, which from the background magnified 1,000 times the amount of light that was directed towards the telescope.
This specific occurrence was an isolated event, limiting the ability to make detailed follow-up observations until new, powerful telescopes can better observe the planet’s star in the future. However, Zhang and his team were able to conduct additional work at the Keck Observatory in Hawaii last year and identify the star as a white dwarf.
The data that researchers were able to gather 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 never ignited with nuclear fusion, located at a very large distance from the star. But the other object was a planet about 1.9 times the mass of Earth that orbited much closer to the star, suggesting it was a possible rocky planet.
By modeling the evolution of the star system, the team calculated that the planet could have once had the same habitable orbit as Earth. The star was also likely to have been similar in size to our own. “We think it had a mass similar to that of the Sun,” Zhang said.
However, when the star ran out of fuel, it lost some of its 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.
A handful of gas planets have been found orbiting white dwarfs, but they either had more distant orbits or had migrated inward after the red giant phase. But if Zhang’s detection is correct, this would be the first known rocky planet orbiting such a star, said Susan Mullally, an astronomer at the Space Telescope Science Institute in Maryland. “This is certainly the smallest, rockiest thing we’ve ever found around a white dwarf,” Mullally said.
Stephen Kane, an astronomer at the University of California, Riverside, said he was “really excited” when he saw the paper. But Kane, who has previously investigated whether planets can survive a star’s red giant phase, said the presence of the brown dwarf posed complications. “If the brown dwarf was closer and then moved away, that changes the whole dynamical environment of the system,” he said. “Maybe there were other planets that were ejected, and what we see is what survived.”
NASA plans to launch the Nancy Grace Roman Space Telescope no earlier than 2027, and it is expected to find many more planets through microlensing, including some around white dwarfs. “Some of them might be close enough to investigate further,” Zhang said.
For now, Zhang’s system remains a possible crystal ball into our future that we manage to glimpse for a fleeting moment.
