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The Sun still has a long life ahead of it, say about five billion years. But our star is still on borrowed time. When its inevitable end comes, the sun’s core will exhaust the last remnants of hydrogen fuel and cause the first stellar death pangs. At that point, our yellow sun will begin to swell into a red giant star about 100 times its current size. Over the next billion to two billion years, its outer layers will begin to peel away as it passes into its final phase as a white dwarf. The closest neighboring planets (Mercury, Venus and possibly even Earth) will be destroyed by the resulting collateral damage.
But what about the fate of the other five known planets orbiting the solar system? The answer is much darker, especially for gas giants. To learn more about the distant future potential of our cosmic neighborhood, astronomers conducted a case study on a recently discovered gas giant located about 80 light years from Earth. Their findings, published today in the journal Naturereveal surprising insights into what the future of life in the solar system will look like. That is, if there is any form of life left.
“Our results show that stellar death is not the end: some planets experience a vibrant, life-filled future after the death of their star,” Ryan MacDonald, an astronomer at the University of St. Andrews and co-author of the study, said in a statement.
Cosmic life after death
The cosmic subjects in question are the white dwarf WD 1856+534 and its orbiting gas giant, WD 1856 b, which MacDonald describes as “pretty weird.”
“It 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,” he explained.
Astronomers first detected WD 1856 b using the Spitzer Space Telescope and NASA’s Transiting Exoplanet Survey Satellite (TESS) in 2020. The gas giant orbits its star at an unexpectedly close distance, about 50 times closer than Earth’s orbit around the sun. It’s such a strange feature that it actually marks the first known example of an intact planet surviving so close to a white dwarf. But if WD 1856 b had always orbited so close to its star, the gas giant would undoubtedly have been destroyed during the red giant era.
So this raises an important question about how the WD 1856 b got there. According to Northwestern University astronomer Christopher O’Connor, there are currently two main theories.
“One is that the planet was swallowed by the host star while it was dying, and managed to survive inside,” he said. “The other is that the migration occurred due to the gravitational effect of other objects in the system.”
Those “other objects” are the other two stars in the WD 1856+534 triple star system. Its stellar companions may also have influenced the orbit of WD 1856 b at some point in its history.
Then there’s the question of the planet’s temperature, which was assessed with the help of the ever-busy James Webb Space Telescope (JWST). Using JWST, researchers observed that WD 1856 b partially overlapped its star during what is known as a grazing transit. This allowed them to analyze crucial information not only about the gas giant’s temperature, but also its mass and atmospheric chemical composition.
Heated attraction
The results suggest that the planet exhibits a temperature of 400 Kelvin, or about 260 degrees Fahrenheit. That is a lot hotter than it should be if its only heat source is a white dwarf. Since there are no other heat generators nearby, that energy must be a residual effect from a previous era. The planet warmed up while engulfed during the red giant phase, or began to warm up as gravity pulled it closer to the resulting white dwarf.
Although the extra heat was puzzling, it also turned out to be a critical detail in determining how WD 1856 b got so close to its star. By combining the planet’s mass data with its current temperature, astronomers calculated that things started to warm up between 3 and 5.5 billion years after its star became a white dwarf. This means that the exoplanet originally resided safely outside the diameter of the red dwarf.
“As the planet moved inward, its interactions with the white dwarf’s strong gravity caused it to heat up considerably, and it has been cooling ever since,” O’Connor said.
Beyond its travel history, the team also detected signs of molecules still within its atmosphere. The JWST readings revealed clear signatures of small cloud particles and hydrocarbons such as methane, another first for an exoplanet orbiting a dead star.
A time machine telescope
WD 1856 b still has a lot to tell astronomers and they are ready to continue digging. Even more atmospheric data from four additional recent JWST transits will soon be available. The entire project highlights the mind-blowing beauty and potential history lessons of peering into the depths of the cosmos.
“It’s like using a time machine to look into the distant future of our solar system,” MacDonald said.
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