Discovery of Nearest Candidate Exoplanet Orbiting a White Dwarf

What will happen to Earth and the other planets in our solar system when the Sun dies? Finding exoplanets around white dwarfs can help answer this pressing question, and researchers have discovered a possible giant exoplanet around a nearby white dwarf.

At the End of the Solar System

When the Sun exhausts its supply of core hydrogen, its interior will undergo a series of transitions that transform our home star into a cool and puffy red giant. Eventually, the Sun will shed its outer layers and leave behind a scorching, crystallized, Earth-sized remnant called a white dwarf. How the planets in our solar system will weather these changes is an open question (though experts are in agreement that Mercury and Venus will be engulfed by the expanding star, sadly).

One way to probe the answer to this question is to study planetary systems orbiting white dwarfs. This may reveal the distances at which planets find safe harbor from the red giant, as well as whether any planets change or undergo migration as the result of their host star’s evolution. Historically, detecting planets around white dwarfs has been challenging — but luckily, JWST makes this task much easier.

Discovering Exoplanets with MEOW

The MIRI Exoplanets Orbiting White dwarfs (MEOW) survey is one of several surveys using JWST’s sensitive infrared instruments to search for planets around white dwarfs. In a recently published article, a team led by Mary Anne Limbach (University of Michigan) reported early results from the MEOW survey, focusing on a candidate planet around the nearby white dwarf WD 0310–688.

The survey is designed to uncover white-dwarf exoplanets through either direct imaging or detection of infrared excess: an unexpectedly large flux at infrared wavelengths that indicates the presence of an object cooler than the white dwarf, like a planet. Limbach and coauthors detect an infrared excess around WD 0310–688 that is most easily explained by a cold (248K) planet companion with a mass of about 3 Jupiter masses. Curiously, the observations place this potential planet between 0.1 and 2 au from the white dwarf — even though planets interior to 2 au are thought to be destroyed when their stars balloon into red giants. This may suggest that the planet migrated to its current location after its host star’s red giant phase.

Candidate Considerations

Researchers have discovered a handful of exoplanets around white dwarfs already — what makes this discovery special? WD 0310–688 is only 34 light-years away, making it the closest white dwarf with a planet candidate, and no planet has ever been discovered 0.1–2 au from a white dwarf. Additionally, this marks the first planet around any type of star to be discovered with the infrared-excess method.

However, the authors cautioned that a planet isn’t the only possibility for the observed infrared excess; a small, cold disk of debris could also be responsible. If the object is a disk, it would be one of the coldest disks ever found around a white dwarf, making this possibility intriguing in its own right.

Follow-up spectroscopy is needed to discern between the giant planet and cold disk hypotheses. Spectral features commonly found in the atmospheres of exoplanets would support the giant planet hypothesis, while a silicate feature would point toward the debris disk. Future work should illuminate the nature of this planet candidate as well as bring us new results from the MEOW survey!

Citation

“The MIRI Exoplanets Orbiting White dwarfs (MEOW) Survey: Mid-infrared Excess Reveals a Giant Planet Candidate around a Nearby White Dwarf,” Mary Anne Limbach et al 2024 ApJL 973 L11. doi:10.3847/2041-8213/ad74ed