Earth will be destroyed when our Sun dies, around five billion years from now, suggests new research from astronomers at the University of St Andrews in Scotland. The study gives fresh insight into what happens to planets after the death of their star, using observations from the James Webb Space Telescope.
Time Machine to the Future
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 planet is significantly warmer than expected, and the team determined how it most likely reached its very tight orbit around the white dwarf.
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."
Fate of the Solar System
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 five billion years from now. It will then shed its outer layers and end its life as a white dwarf star. Mercury, Venus, and possibly Earth will be destroyed by the red giant, according to the study published in the journal Nature. The fate of more distant planets, particularly gas giants, remains unclear.
WD 1856 b was discovered in 2020 by scientists using the Transiting Exoplanet Survey Satellite (TESS) and the Spitzer Space Telescope. It orbits the white dwarf WD 1856+534 around 80 light-years from Earth.
Surviving the Death of 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. If it had originally been orbiting at that distance, it would have been obliterated while the star was a red giant. The researchers wanted to know how it survived the death of its host star and ended 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 four and 11 times as massive as Jupiter. Light from the star passing through the planet's atmosphere picked up information about its chemical composition. By tracking how much extra infrared heat the planet gives off during its transit, scientists found it is around 126°C, letting them rewind its temperature history to work out how it ended up so close to its white dwarf star.
Two Theories for the Planet's Migration
Dr MacDonald explained: "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 three and 5.5 billion years after the star became a white dwarf.
Researchers think the planet survived its star's red-giant phase on a distant orbit before later spiraling inward, heating up in the white dwarf's gravity and slowly cooling — evidence some worlds can endure long after a star dies.
