A huge planet about the size of Jupiter has been spotted orbiting a tiny white dwarf star 80 light-years away. The discovery by an international team of astronomers is puzzling because the planet should have been swallowed-up long ago, when the star expanded to become a red giant before contracting to a white dwarf.
This is the first planet known to orbit a white dwarf and its existence suggests that at least some of the Sun’s planets could survive when our star becomes a red giant in five billion years.
The white dwarf (called WD 1856+534) and the giant planet (WD 1856b) were studied using NASA’s Transiting Exoplanet Survey Satellite – which looks for fluctuations in starlight that occur when a planet passes in front of its star. TESS spotted the planet whizzing around the star once every 34 h in an extremely tight orbit that is about twenty times closer to its star than Mercury is to the Sun. Another extraordinary thing about this system is that the white dwarf is about the size of Earth, so the planet is much larger in size than the star it orbits.
“We were using the TESS satellite to search for transiting debris around white dwarfs, and to try to understand how the process of planetary destruction happens,” says Andrew Vanderburg at the University of Wisconsin-Madison, who is part of the team that made the discovery. “We were not necessarily expecting to find a planet that appeared to be intact.”
The team then studied the system in more detail using the Gemini Near-Infrared Spectrograph (GNIRS) on the Gemini North telescope in Hawaii. While the star could be seen by GNIRS, no light from an orbiting debris field, nor from the giant planet was detected.
Siyi Xu of the Gemini Observatory explains, “Because no debris from the planet was detected floating on the star’s surface or surrounding it in a disk we could infer that the planet is intact”. She adds, “because we didn’t detect any light from the planet itself, even in the infrared, it tells us that the planet is extremely cool, among the coolest we’ve ever found”. Indeed, NASA’s Spitzer Space Telescope was used to put an upper limit of 17 °C on the temperature of WD 1856b – which is similar to the average temperature of Earth. As a result, it is possible that life exists on the planet.
“I think the most exciting part of this work is what it means for both habitability in general – can there be hospitable regions in these dead solar systems – and also our ability to find evidence of that habitability,” says Vanderburg.
WD 1856+534 was once a star like the Sun before it ballooned out to become a red giant – which would have consumed WD 1856b had it been in its current orbit. Instead, astronomers believe that the planet was in a much larger orbit when the star was in its red-giant phase, so instead of being consumed, it was knocked into an eccentric orbit.
Eventually, the red giant burnt out, leaving the cool white dwarf behind. At that point, the planet could have wandered into its current tight orbit – possibly though gravitational interactions with other surviving planets. This journey would have taken billions of years, but astronomers believe that WD 1856+534 is almost 6 billion years old. Because the system is relatively close to Earth, its possible that astronomers could spot the other planets in the future.