WD 1856 b: The Jupiter-size Planet That Survived Its Star's Death

WD 1856 b has become the only confirmed case of a planet that survived the death of a Sun-like star, after astronomers utilized the James Webb Space Telescope for analysis on April 27, 2023. This Jupiter-sized planet orbits a white dwarf, the remnant of a Sun-like star, revealing unexpected characteristics that challenge existing theories of planetary evolution.

Discovery of WD 1856 b

Initially discovered by the TESS observatory in 2020, astronomers were examining around 2,000 white dwarfs for small celestial objects. Instead of finding comets or asteroids, they stumbled upon a gas giant. The unusual observation prompted Christopher O’Connor, a theoretical astrophysicist at Cornell University, to remark, “As soon as they looked at it, they said, okay, that’s weird.”

The brightness of the white dwarf, which is about seven times smaller than the gas giant, should diminish significantly during the planet's transit. However, the brightness dips by about half, suggesting a grazing transit where the planetary disk barely clips the star's face.

Unusual Orbital Characteristics

WD 1856 b orbits at approximately 0.02 AU from the white dwarf, a finding that contradicts prior expectations of planetary migration following a star's death. O’Connor explains that typically, a star expanding into a red giant phase would consume its inner planets and cause outer planets to migrate outward due to reduced gravitational pull. Yet, WD 1856 b defies this logic by moving closer instead.

This unexpected positioning has sparked significant interest in the scientific community. O’Connor noted, “It sent theoretical astrophysicists into a feeding frenzy.” The peculiar nature of this system is inviting new, creative theories about planetary formation and evolution.

Insights from the James Webb Space Telescope

The JWST observations presented several challenges due to the odd size discrepancy between the star and its planet. The team developed new equations to analyze the transmission spectrum, accounting for the grazing transit geometry. Their findings revealed that WD 1856 b is enveloped in aerosol hazes and contains methane, with an astonishing temperature of around 400 Kelvin.

The planet emits approximately 25 times more energy into space than it receives from its cooling host star, which has been dormant for nearly 6 billion years. This extraordinary heat indicates an internal source of energy rather than merely re-radiating warmth from the star.

• WD 1856 b’s orbit: 0.02 AU from the white dwarf
• Temperature: Around 400 Kelvin
• Energy emission: 25 times more than received
• White dwarf age: Approximately 6 billion years

O’Connor and his team theorized two possible scenarios for the planet's current orbit. The first is a common-envelope model, where the planet survived engulfment during the star's red giant phase. The second is a high-eccentricity migration model, which suggests the planet spiraled inward over billions of years due to gravitational interactions.

Ultimately, the research on WD 1856 b opens new avenues for understanding not only this unique system but also the broader implications for planetary systems around dying stars.