One day, in about five billion years, the Sun will run out of fuel, swell up 100 times bigger than it is now into a red giant star, and swallow up Mercury, Venus and maybe Earth. Whether the more distant planets, especially gas giants Saturn and Jupiter, will survive is unknown.
New research by an international team of astronomers, including Cornell’s Victoria Boehm, used the James Webb Space Telescope to observe a Jupiter-sized exoplanet that did survive its star’s death, offering the first window into the future of planets like Jupiter after the death of the Sun.
The study, which published July 1 in Nature ("Aerosols and Hydrocarbons in the Atmosphere of a White Dwarf Planet,") used Webb to watch the exoplanet WD 1856 b pass in front of its star in a so-called grazing transit where the very top of the planet partly overlapped the star. The transit yielded unique information about the planet’s mass and temperature, enabling the scientists to estimate the planet at between four and eleven times Jupiter’s mass. Light from the star passing through the planet’s atmosphere picked up information about the atmosphere’s chemical composition, as well.
"We’re used to looking back in time when we use telescopes, but this is the first time we have been able to look forward to what might happen to the outer planets around the remnant of a Sun-like star. It's like using a time machine to peer into the distant future of our Solar System,” said lead author Ryan MacDonald, at the University of St. Andrews in the United Kingdom and a former postdoctoral researcher in the laboratory of Nikole Lewis, associate professor of astronomy in the College of Arts and Sciences.
The observations were only possible because of Webb’s extraordinary capabilities, said Boehm, a doctoral candidate in astronomy and member of the Carl Sagan Institute (A&S). She analysed the data from Webb to extract the planet’s spectrum. "White dwarfs like WD 1856 are exceptionally dim compared to the planet-hosting stars we normally observe with Webb. To make things even harder, the planet's transit only lasts eight minutes, so it's very much if you blink you miss it! Capturing enough light to see WD 1856’s spectrum, while also doing so quickly enough to not miss the transit, is something only Webb can do," she said.
WD 1856 b was discovered in 2020 by scientists using the Transiting Exoplanet Survey Satellite (TESS), the Spitzer Space Telescope and several large ground-based telescopes. It orbits the white dwarf WD 1856+534 about 80 light-years from Earth. This was the first such discovery of an intact planet closely orbiting a white dwarf.
“The planet is quite the oddball,” said MacDonald. “It’s about the size of Jupiter, but the white dwarf it orbits is the size of Earth, so the planet is seven times larger than its star."
WD 1856 b is also unusual for its extremely close orbit around its host star, a distance 50 times closer than Earth orbits the Sun. If WD 1856 b had originally been orbiting at that distance, it would have been obliterated while the star was a red giant.
The scientists examined two theories for how the planet ended up so close to its star: the planet may have been engulfed by the host star as it was dying but somehow managed to survive; or that it migrated inward to the white dwarf star later from its original position.
The Webb data they gathered indicated that the planet has a temperature of about 400 Kelvins, or 126°C — about 240 degrees hotter than it would be if its only source of heat was the light from the white dwarf. This puzzling discovery turned out to be the key fact which indicated how the planet must have reached its current orbit.
The researchers realised that there was no source of energy present to generate that heat today, so it must be residual energy from an earlier time when the planet was heated, either from being engulfed by the red giant or during an inward migration. Using models of how sub-stellar objects like WD 1856 b cool down over time, coupled with the new data from Webb about the planet’s mass and its current temperature, the team was able to project its temperature back in time and deduce how long ago the heating must have happened.
They concluded that the heating most likely happened between 3 and 5.5 billion years after the star became a white dwarf. In this scenario, the planet was on a wide orbit that kept it safe from the star during its destructive red giant phase and only migrated to its present location later on.
“The white dwarf is part of a triple star system, and the outer companion stars could have influenced WD 1856 b’s orbit,” said co-author Christopher O’Connor of Northwestern University in Illinois. “As the planet moved inwards, its interactions with the strong gravity of the white dwarf will have caused it to warm up considerably, and it has been cooling ever since.”
The researchers also found signs of molecules present in the planet’s atmosphere. “Our Webb observations saw the telltale signatures of small cloud particles and hydrocarbons, most likely methane, which is the first time we have seen an atmosphere on a planet transiting a dead star,” said Boehm. “We recently observed four more transits of WD 1856 b with Webb to take a deeper look into its atmospheric chemistry and can’t wait to see the results.”
