A Jupiter-size planet that escaped its star's death - Ars Technica

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WD 1856 b is the only confirmed case of a planet that survived the death of a Sun-like star. It’s a Jupiter-size world orbiting a white dwarf—the burned-out remnant of a Sun-like star. Now, a team of astronomers has used the James Webb Space Telescope to take a closer look at this planet for the first time, and what they found makes an already strange system even stranger.

A feeding frenzy

WD 1856 b was an accidental discovery. Astronomers pointed the TESS observatory at a sample of roughly 2,000 white dwarfs in 2020. These stars are the remains of a Sun-like star that have already gone through a red-giant phase, leaving behind an Earth-size body that’s primarily composed of elements like carbon and oxygen. The TESS team was searching for small objects like comets or asteroids that might transit across the face of these dead stars.

What they found in the WD 1856 system was a gas giant. “As soon as they looked at it, they said, okay, that’s weird,” said Christopher O’Connor, a theoretical astrophysicist at Cornell University and co-author of the recent Nature study on WD 1856 b.

The white dwarf is about seven times smaller than the gas giant circling around it. Its brightness should be dropping to nearly nothing each time the planet crosses in front of it, but instead it’s dipping by about half. O’Connor thinks the reason is a grazing transit, where only the edge of the planetary disk clips the face of the star. “That’s a very unlikely viewing angle,” he said, “but it’s the only way to explain what we actually see.”

What’s more, the planet orbits at about 0.02 AU from the white dwarf, which goes against our ideas of how the death of a star should reshape its system. “When the star expands to become a red giant, it consumes the inner planets,” O’Connor explains. Then, in the process of shrinking down to a white dwarf, it loses about half of its original mass, which means its gravitational pull becomes weaker. “The outer planets, like gas giants, should migrate outward by about a factor of two,” O’Connor said.

WD 1856 b, though, apparently did not migrate outward. It got closer.

The discovery immediately has the science community buzzing. “It sent theoretical astrophysicists into a feeding frenzy,” O’Connor said. “When you find something that’s totally bizarre, totally in the wrong place, totally unexpected from any previous way of thinking about things—that’s the Universe inviting us to get creative.” First, though, scientists needed more data to get creative with, so O’Connor’s team booked time on the James Webb Space Telescope to take a closer look at what was going on in the WD 1856 system.

Eight minutes of light

The JWST observations were done on April 27, 2023, and captured a single transit that lasted just eight minutes. The viewing angle and the unusual size mismatch between the star and its planet posed an immediate technical problem. Standard exoplanet transmission spectroscopy assumes a smaller planet is entirely silhouetted against the face of a much larger star, which was not the case here.

To get around it, the team developed new equations to express the transmission spectrum as the time-varying area of the planet overlapping the star’s disk. Then, they modified POSEIDON, software for reconstructing exoplanets’ atmospheres based on JWST data to account for the grazing transit geometry (the software had been developed by Ryan MacDonald, the lead author of the study). When the scientists were done crunching numbers, WD 1856 b’s atmosphere proved somewhat surprising.

It turned out the planet is shrouded in aerosol hazes, and its atmosphere contains methane. It is also far hotter than the team expected. WD 1856 b apparently emits roughly 25 times more energy into space than it receives from its cooling host star. Even though its star, according to O’Connor, has been dead for about 6 billion years, the planet is glowing.

This extraordinary temperature, O’Connor argues, tells us a lot about WD 1856 b’s history.

Running hot

“We expected this planet to be roughly as hot as Jupiter, but it wasn’t,” O’Connor said. At about 0.02 AU from a white dwarf that has been cooling for 6 billion years, WD 1856 b should be somewhere between 150 and 200 Kelvin, close to the temperature of Jupiter’s cloud tops. Instead, it is around 400 Kelvin. “Whatever is causing this planet to glow, it must be an internally derived heat rather than just re-radiating energy from the star,” O’Connor said.

The...