An international team of astronomers has used the NASA/ESA/CSA James Webb Space Telescope to watch the Jupiter-sized exoplanet WD 1856 b transit its host star, measuring the planet’s mass and temperature and even detecting its atmosphere. The results offer our first window into the future of planets like Jupiter after the death of the Sun, billions of years into the future.
Billions of years ago, a Sun-like star nearing the end of its life swelled into a red giant before ejecting its outer layers, leaving a hot, remnant core known as a white dwarf. As a red giant, the star should have engulfed and destroyed any nearby planets. Yet, astronomers have found WD 1856 b orbiting the white dwarf every 34 hours at a separation of less than 3 million kilometres. The planet was discovered in 2020 using NASA’s TESS and Spitzer Space Telescope, orbiting a white dwarf about 80 light-years from Earth.
Ryan MacDonald of the University of St Andrews, lead author, stated:
“The planet is 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 orbits 50 times closer than Earth orbits the Sun. If it had originally been at that distance, it would have been obliterated while the star was a red giant. The new study used Webb to watch the planet transiting its star, yielding information about its mass — between four and eleven times the mass of Jupiter.
The team also determined the planet’s temperature. The data indicated that the planet has a temperature of approximately 126 degrees Celsius — significantly hotter than expected. This discovery proved key to understanding how the planet reached its current orbit.
Christopher O’Connor of Northwestern University, co-author, added:
“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 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.”
Using cooling models and Webb data, the team determined 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 remained safe on a wide orbit during the red giant phase and only migrated to its present location later.
Victoria Boehm of Cornell University, co-author, stated:
“We 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.”
In approximately five billion years, the Sun will become a red giant, destroying Mercury, Venus, and possibly Earth. The fate of the gas giants remains unclear. Discovering and studying planets around white dwarfs provides a glimpse into our Solar System’s distant future.
Ryan MacDonald concluded:
“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.”
