Webb studies how a planet survived death of its star

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An international team of astronomers have 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. They found that the planet is significantly warmer than expected and determined how it most likely reached its very tight orbit around the star, a white dwarf. The results are our first window into the future of planets like Jupiter after the death of the Sun, billions of years into the future.

An orange gas giant planet at left, taking up about one-third of the frame, facing a star, which appears at top right as a far smaller bright dot. The planet has subtle orange cloud bands. The star illuminates the right side of the planet like the crescent of a waxing moon. Both are on the black background of space. The words “artist’s concept” are in the bottom right corner. Image credit: NASA, ESA, CSA, R. Crawford (STScI)

Webb is giving us new insight into the far-future of Solar Systems like our own. Billions of years ago, a Sun-like star nearing the end of its life swelled tremendously in size to become 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 a Jupiter-sized exoplanet orbiting the white dwarf every 34 hours at a separation of less than 3 million kilometers.

The results were published today in the journal Nature.

WD 1856 b was discovered in 2020 by scientists using NASA’s Transiting Exoplanet Survey Satellite (TESS) and the Spitzer Space Telescope, orbiting the white dwarf WD 1856+534 about 80 light-years from Earth. “The planet 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, at a distance 50 times closer than 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 it survive the death of its host star and end up in its current position?

This image shows a graph of the amount of light blocked by percent on the y-axis and wavelength of light in microns on the x-axis. The y-axis ranges from 55.2% to 56.5% with tick marks every 0.1% and labels at 55.5 and 56.0. The x-axis ranges from 0.5 to 4.0 microns with tick marks every 0.5 microns. A thick purple line outlined with two semi-translucent bands has an inner line that’s darker and an outer line that’s lighter. The purple line is wavy and runs higher, in the top third, until about 3.5 microns, where it drops to 55.2 on the y-axis and 4.0 on the x-axis. Five humps are highlighted by vertical red bars, indicating the presence of methane. White circles representing data points are scattered above and below the purple line. A key shows that the purple line is the best fit model, red highlights methane, and white circles represent data. Image credit: NASA, ESA, CSA, J. Olmsted (STScI)

How big, how hot
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The new study used Webb to watch the planet passing in front of its star. This transit[^] yielded unique information about the planet’s mass, which is between four and eleven times the mass of Jupiter.

The team also was able to determine the planet’s temperature. During the transit, light from the star was partly blocked, but infrared light was reduced 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 about 126 degrees Celsius — significantly hotter than it would be if its only source of heat was the light from the white dwarf. This puzzling discovery turned out to be the key fact that proved how the planet must have reached its current orbit.

How?
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Christopher O’Connor of Northwestern University in Illinois in the United States, a co-author on the paper, was responsible for tracing the temperature of the planet back in 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 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 researchers realised that there was no source of energy present to generate that heat today, so it must be residual energy from an earlier time when the planet was heated. Using models of how sub-stellar objects like WD 1856 b cool down over time, coupled with the new data from Webb 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. The timing is key to determining whether the heating was from being engulfed by the red giant or during an inward migration.

They concluded 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 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. “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.” said O’Connor.

The light
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Light from the star passing through the planet’s atmosphere also picked up information about its chemical composition. “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,” said co-author Victoria Boehm of Cornell University in the United States. “We recently observed four more transits of WD 1856 b with Webb to take a deeper look into its atmospheric chemistry and can’t wait to see the results.”

Solar System’s possible future
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In approximately five billion years, 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. 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. However, the fate of the more distant planets, particularly the gas giants, is unclear. Finding and studying planets in orbit around the remnants of Sun-like stars after their death is a means of learning what might happen in our own Solar System in the far future.

“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,” said MacDonald. “It’s like using a time machine to peer into the distant future of our Solar System.”

Citation
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The study Aerosols and hydrocarbons in the atmosphere of a white dwarf planet was published today in Nature. Authors: Ryan J. MacDonald, Christopher E. O’Connor, Victoria A. Boehm, E. M. May, David K. Sing, Elijah Mullens, L. C. Mayorga, Trevor O. Foote, Simon Blouin, Logan A. Pearce, Nikole K. Lewis, Jeff Valenti, Natasha E. Batalha, Maura Lally, Joshua D. Lothringer, Mark S. Marley, Ishan Mishra & Susan E. Mullally

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[^]: A transit occurs when a planet passes in front of the star it is orbiting from our point of view, blocking some of the light from the star. Many exoplanets have been detected by looking for the small decrease in brightness of a star caused by a transiting planet. Comparing the light of the star to the light that passes through the transiting planet’s atmosphere also offers information about the atmospheric composition.