NASA’s James Webb Space Telescope gives us new insights into the distant future of solar systems like ours, as the agency continues to reveal the secrets of the universe and our place in it. Billions of years ago, a Sun-like star nearing the end of its life grew enormously in size to become a red giant before expelling its outer layers, leaving a hot remnant core known as a white dwarf. As a red giant, the star should have engulfed and destroyed all nearby planets. However, astronomers have found a Jupiter-sized exoplanet orbiting the white dwarf every 34 hours at a separation of less than 3 million kilometers (2 million miles).

To solve the mystery of how this exoplanet survived, an international team of astronomers used NASA’s James Webb Space Telescope to watch the Jupiter-sized exoplanet WD 1856 b transit its host star, measuring the planet’s temperature and detecting molecules in its atmosphere. They found that the planet is significantly warmer than expected and determined how it probably reached its very tight orbit around the white dwarf star. The results are a window into the future of planets like Jupiter after the death of the Sun, billions of years in the future.

The results were published Wednesday in the journal Nature.

WD 1856 b was discovered in 2020 by scientists using NASA’s TESS (Transiting Exoplanet Survey Satellite) and the retired Spitzer Space Telescope. It orbits the white dwarf WD 1856+534, which is about 80 light years from Earth. “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,” said lead author Ryan MacDonald of the University of St. Andrews in the United Kingdom.

WD 1856 b orbits extremely close to its host star, a distance 50 times closer than the Earth orbits the Sun. 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 you survive the death of your host star and end up in your current position?

The new study used Webb to observe the planet passing in front of its star. This transit yielded unique information about the mass of the planet, which is between four and eleven times the mass of Jupiter.

The team was also able to determine the temperature of the planet. During the transit, light from the star was partially blocked, but infrared light was reduced less than other wavelengths. The difference was the infrared light emitted by the planet from its own heat. The data indicated that the planet has a temperature of about 260 degrees Fahrenheit (126 degrees Celsius), significantly hotter than it would be if its only heat source were the white dwarf’s light. This puzzling discovery turned out to be the key fact that demonstrated how the planet must have reached its current orbit.

Christopher O’Connor of Northwestern University in Illinois, a co-author of the paper, was responsible for tracking the planet’s temperature over time. 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 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 companion stars could have influenced the orbit of WD 1856 b.”

The researchers realized that there was no energy source present to generate that heat today, so it must be residual energy from an earlier time when the planet was warming. Using models of how substellar objects like WD 1856 b cool over time, along with Webb’s new data, the team was able to project their temperature over time and deduce how long ago the warming must have occurred. The timing is key to determining whether the warming was due to the red giant engulfing it or occurred during an inward migration.

They concluded that the warming probably occurred between 3 and 5.5 billion years after the star became a white dwarf. In this scenario, the planet was in a wide orbit that kept it safe from the star during its destructive red giant phase, and only later migrated to its current location. “As the planet moved inward, its interactions with the white dwarf’s strong gravity would have caused it to heat up considerably, and it has been cooling ever since,” O’Connor said.

Light from the star passing through the planet’s atmosphere also collected information about its chemical composition. “We saw the telltale signatures of small cloud particles and hydrocarbons, most likely methane, and it’s the first time we’ve seen an atmosphere on a planet transiting a dead star,” said co-author Victoria Boehm of Cornell University. “We recently observed four more transits of WD 1856 b with Webb to take a deeper look at its atmospheric chemistry and we are eager to see the results.”

In about five billion years, the Sun will run out of hydrogen fuel in its core and swell more than 100 times larger than it is now to become a red giant star. 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. However, the fate of the more distant planets, particularly the gas giants, is unclear. Finding and studying planets orbiting the remains of Sun-like stars after they die is one way to learn what could happen to our own solar system in the distant future.

“We’re used to looking back in time when we use telescopes, but this is the first time we’ve been able to anticipate what might happen to the outer planets around the remnant of a Sun-like star,” MacDonald said. “It’s like using a time machine to look into the distant future of our solar system.”

The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond it to distant worlds around other stars, and exploring the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and CSA (Canadian Space Agency).

For more information about Webb, visit:

https://science.nasa.gov/webb

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