How a Jupiter-sized exoplanet escapes its star's 'kiss of death'

In the evolution of the vast universe, when a sun-like star burns out its fuel and comes to an end, it typically expands into a red giant, swallowing everything in its orbit. However, a planet called WD 1856 b breaks this cosmic norm. As the only confirmed planet to survive the death of a sun-like star, it is orbiting a white dwarf star. Recently, astronomers used the James Webb Space Probe (JWST) to conduct the first in-depth observation of this strange celestial system, and the results shocked scientists.

The discovery of WD 1856 b was an accident. In 2020, the research team used the Transiting Exoplanet Survey Satellite (TESS) to observe about 2,000 white dwarfs, aiming to find small objects that pass by the surfaces of these dead stars. To the surprise of astronomers, they discovered a gas giant planet in the WD 1856 system. The white dwarf is only one-seventh the size of the planet orbiting it, but its brightness halves during the planet's transit, suggesting that the planet is undergoing an extremely rare "predatory" transit, in which only the edge of the planet's disk shears across the star's surface.

According to conventional theory, when a star evolves into a red giant, the inner planets will be engulfed. Since the star will lose about half of its mass in the process of evolving into a white dwarf, its gravity will weaken, which should cause the outer gas giant planets to migrate outward. But what’s confusing is that instead of moving away, WD 1856 b is orbiting closely to the white dwarf at a distance of only about 0.02 astronomical units.

To unravel the mystery, Christopher O’Connor, a theoretical astrophysicist at Cornell University, and his team applied for time with the Webb Telescope to conduct observations. Because of the system's special transit geometry, the researchers developed entirely new algorithms to process the data. The analysis found that WD 1856 b was shrouded in a layer of aerosol clouds, the atmosphere contained about 7% methane, and the temperature was abnormally high. The data shows that the planet emits about 25 times more heat than it receives from its star. Although its host white dwarf star has cooled for about 6 billion years, the planet itself continues to shine.

The research team believes that this excessive temperature is not residual heat from the formation, but evidence that the planet was heated during its migration. By working backwards from the planetary cooling model, the researchers ruled out the "common envelope model" of planets surviving in the star's red giant phase in favor of the "high eccentricity migration model." This means that WD 1856 b was originally located in a farther orbit, and was later affected by the gravitational disturbance of the other two distant companion stars in the WD 1856 system. After many violent passes over billions of years, it finally fell into its current orbit.

Currently, WD 1856 b's atmospheric composition ratios differ from conventional models, which may have contributed to deviations in cooling model predictions. O'Connor pointed out that more accurate evolutionary models of the planet's atmospheric properties will be needed in the future. With the system only 75 light-years away from Earth, researchers hope to see it as a breakthrough and search for more similar "survivor" planets to reshape mankind's understanding of the later years of stars and the evolution of their planetary systems. At present, the team has obtained more follow-up observation data, and this exploration of the survival and destruction of the universe is still continuing.