New research from the University of St Andrews has given a new window into what happens to planets after the death of their star, giving a foresight into the future of planets like Jupiter after the death of the Sun, billions of years into the future.
A team of international astronomers used the NASA/ESA/CSA James Webb Space Telescope to watch the Jupiter-sized exoplanet WD 1856 b transit its 'dead' host star, a white dwarf, measuring the planet's mass and temperature and even detecting its atmosphere. The researchers found that the planet is significantly warmer than expected and determined how it most likely reached its very tight orbit around the white dwarf.
In approximately five billion years, the Sun will run out of hydrogen fuel in its core and swell up more than 100 times larger than it is now into a red giant star. It will then shed its outer layers and end its life as a white dwarf star. Mercury, Venus, and possibly the Earth will be destroyed by the red giant. However, the fate of the more distant planets, particularly the gas giants, is unclear. Finding and studying planets in orbit around the remnants of Sun-like stars after their death is a means of learning what might happen in our own Solar System in the far future.
WD 1856 b was discovered in 2020 by scientists using the Transiting Exoplanet Survey Satellite (TESS) and the Spitzer Space Telescope, orbiting the white dwarf WD 1856+534 about 80 light-years from Earth.
Lead Author, Dr Ryan MacDonald from the University of St Andrews, said: "The planet is quite the oddball. 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."
What is so unusual about WD 1856 b is its extremely close orbit around its host star, a distance 50 times closer than Earth orbits the Sun. This was the first such discovery of an intact planet closely orbiting a white dwarf. If WD 1856 b had originally been orbiting at that distance, it would have been obliterated while the star was a red giant. How did it survive the death of its host star and end up in its current position?
The new study, published today (1 July) in Nature, used Webb to watch the planet passing in front of its star in a so-called grazing transit where the very top of the planet partly overlapped the star [1]. The transit yielded unique information about the planet's mass and temperature, estimating the planet at between four and eleven times as massive as Jupiter. Light from the star passing through the planet's atmosphere picked up information about the atmosphere's chemical composition.
Dr MacDonald added: "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."
During the transit, light from the star was partly blocked, but infrared light was blocked less than other wavelengths. The difference was infrared light emitted by the planet from its own heat. The data 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.
Co-author on the paper, Dr Christopher O'Connor of Northwestern University was responsible for tracing the temperature of the planet back in time. Dr O'Connor said: "The big question is how WD 1856 b ended up where it is today, and there are two theories. One is that the planet was swallowed by the host star as it was dying, and managed to survive on the inside. The other is that the migration took place due to the gravitational effect of other objects in the system. The white dwarf is part of a triple star system, and the outer companion stars could have influenced WD 1856 b's 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 where 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.
Dr O'Connor added: "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."
These observations were only possible because of Webb's extraordinary capabilities.
Co Author Dr Victoria Boehm of Cornell University, who analysed the data from Webb to extract the planet's spectrum, said: "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 8 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."
Aside from deducing how the planet most likely survived, the transmission spectrum [2] also revealed signs of the molecules present in the planet's atmosphere.
Dr Boehm added: "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, 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."
University of St Andrews' Dr Ryan MacDonald, added: "This is just the beginning of our exploration of planets orbiting dead stars with Webb, and the search for further planets orbiting white dwarfs is ongoing. Our results show that stellar death is not the end - some planets experience a vibrant and lively future after the death of their star."
