Artists impression of exoplanet WD 1856 b transiting its dead host star. (European Space Agency via SWNS)
By Stephen Beech
Earth will be destroyed when our sun dies - around 5 billion years from now, suggests new research.
The study by astronomers from the University of St Andrews in Scotland gives fresh insight into what happens to planets after the death of their star.
They say it was like using a "time machine" to discover the future of the solar system.
The research team used the NASA/ESA/CSA James Webb Space Telescope to watch a Jupiter-sized exoplanet, called WD 1856 b, transit its "dead" host star.
They were able to measure the planet's mass and temperature and even detect 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.
The team explained that 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 around 5 billion years from now.
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, according to the study published in the journal Nature.
But the research team say the fate of the more distant planets, particularly the gas giants, is unclear.
By using spectroscopy, the light that passed through the planet's atmosphere during the transit can be split up into its constituent wavelengths. (ESA / Ryan MacDonald via SWNS)
They explained that finding and studying planets in orbit around the remnants of sun-like stars after their death is a way 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 around 80 light-years from Earth.
Study lead author 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."
He says that 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.
It was the first such discovery of an intact planet closely orbiting a white dwarf.
(Photo by Amy Law via Pexels)
But MacDonald said if WD 1856 b had originally been orbiting at that distance, it would've been obliterated while the star was a red giant.
The researchers wanted to know how it survived the death of its host star and end up in its current position.
The new study 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.
The transit yielded unique information about the planet's mass and temperature, estimating the planet at between 4 and 11 times as massive as Jupiter.
Light from the star passing through the planet's atmosphere picked up information about the atmosphere's chemical composition.
MacDonald said: "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."
(Photo by Sanjeev Khatarkar via Pexels)
During the transit, he said 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 around 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.
That puzzling discovery turned out to be the key fact which indicated how the planet must have reached its current orbit.
Study co-author Christopher O'Connor, of Northwestern University, was responsible for tracing the temperature of the planet back in time.
He 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 research team realized 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 Webb data 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 billion and 5.5 billion years after the star became a white dwarf.
In that 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.
O'Connor said: "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."
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."