Astronomers discover how giant planet survived its star’s death

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Astronomers discover how giant planet survived its star's death

Planet offers an unprecedented glimpse into our solar system's future

EMBARGOED UNTIL 11 A.M. EDT (U.S) ON WEDNESDAY, JULY 1, 2026

• Gas giant orbits its dead star so closely that it takes just 1.4 days to complete a full revolution
• At this proximity, planet shouldn't have survived its star's red giant phase
• Using the James Webb Space Telescope, astrophysicists measured the planet's atmosphere, mass and temperature
• They found the planet orbited at a safe distance when its star died and migrated closer billions of years after its death

EVANSTON, Ill. - When astronomers discovered a giant planet orbiting a dead star in 2020, they wondered how it survived its star's violent demise. Now, observations from NASA's James Webb Space Telescope (JWST) may finally explain the planet's unlikely escape from destruction.

In a new study, an international team of scientists - including a Northwestern University astrophysicist - analyzed the planet's atmosphere for the first time. Using measurements of the planet's atmosphere, mass and temperature, the researchers reconstructed the planet's journey. They found the planet (called WD1856b) originally orbited its star from a safe distance. But, billions of years after the star died, the planet migrated toward its dead companion.

The findings give an unprecedented glimpse into the distant future of planetary systems - including our own.

The study will be published on Wednesday (July 1) in the journal Nature.

"Our findings have bearing on the long-term fate of our solar system," said study co-author Christopher O'Connor of Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics. "In roughly five billion years, our sun will die, and we don't know precisely what will happen to the planets at that time. The fact that planets can survive into that final stage of the stellar life cycle really widens the range of possibilities for where and when habitable planets might exist in the universe."

O'Connor is a CIERA Postdoctoral Fellow at Northwestern, where he studies stellar and planetary astrophysics and dynamics. The study was led by Ryan J. MacDonald, a lecturer in extrasolar planets at the University of St. Andrews in Scotland.

An impossible orbit

When astronomers first discovered WD1856b, they were instantly puzzled. Located just 80 light-years from Earth, the gas giant is between four and 11 times more massive than Jupiter. While most stars are vastly larger than their planets, WD1856b orbits a relatively diminutive Earth-sized white dwarf. White dwarfs are dense stellar remnants left behind after a sun-like star exhausts its fuel and dies.

"This is one of the most bizarre planetary systems we know of," O'Connor said. "The planet's radius is about eight times larger than the white dwarf, and it orbits at an extremely close distance - completing a full revolution every 1.4 days."

The close proximity is especially baffling because WD1856b shouldn't have survived its star's red giant phase. When sun-like stars run out of fuel, they balloon to more than 100 times their original size. Red giants often engulf nearby planets before collapsing into white dwarfs. When our sun dies, for example, it will swallow Mercury, Venus and possibly Earth.

"The big question is how WD1856b ended up where it is today, and there are two theories," O'Connor said. "One is that the planet was swallowed by its host star as it was dying and managed to survive on the other side. 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 WD1856b's orbit."

A lingering fever

To investigate those possibilities, the research team used JWST to measure the planet's atmosphere, temperature and mass. Among their findings, they discovered the planet is significantly hotter than expected - about 400 Kelvin (127 degrees Celsius or 260 degrees Fahrenheit), roughly 240 degrees hotter than can be explained by the white dwarf's light alone.

By combining the JWST measurements with models of how giant planets cool over time, the team reconstructed the planet's history. Because giant planets cool at predictable rates, O'Connor could effectively trace the planet's temperature back in time. The team found the planet likely heated up as it moved toward its host - 3 to 5.5 billion years after the star became a white dwarf. In this scenario, the planet remained at a safe distance during its star's destructive red giant phase and migrated to its present location much later.

"As the planet moves inward, its interactions with the strong gravity of the white dwarf caused it to warm up considerably," O'Connor said. "It has been cooling ever since."

'Stellar death is not the end'

This planetary system could offer a preview of our own solar system's distant fate. Rather than ending when the sun dies, our system may continue evolving for billions of years afterward.

"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," MacDonald said. "It's like using a time machine to peer into the distant future of our solar system."

The observations also revealed methane and clouds in the planet's atmosphere. The study marks the first time scientists have characterized the atmosphere of a planet orbiting a dead star.

"This is just the beginning of our exploration of planets orbiting dead stars with JWST, and the search for further planets orbiting white dwarfs is ongoing," MacDonald said. "Our results show that stellar death is not the end - some planets experience a vibrant and lively future after the death of their star."

The study, "Aerosols and hydrocarbons in the atmosphere of a white dwarf planet," was supported by NASA and the National Science Foundation.

Study images

Christopher O'Connor

Coauthor

CIERA postdoctoral fellow

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