WD 0806−661 B

WD 0806−661, or Maru, is a white dwarf star of the spectral type DQ. The metal-poor composition of its planetary-mass companion could explain its spectral type, as it is theorized that hydrogen-deficient stars of the asymptotic giant branch could evolve into white dwarfs of spectral type DB and then DQ as they cool down.^[6] WD 0806-661 is estimated to be 1.5−2.7 billion years old,^[7] and likely used to be an A-type main sequence star of 2.1 ± 0.3 solar masses before reaching the end of its life and becoming a white dwarf.^[8] WD 0806-661 B may have formed closer to the star, but migrated further away as it reached the end of its life.

Because it orbits very far away from its star, WD 0806−661 B is likely very dark, receiving almost no light from its star. However, due to the object's high mass, internal heat keeps the temperature hotter than that of Earth.^[4] The object's radius is estimated to be 12% larger than that of Jupiter,^[9] and is likely the same age as the star. Despite having temperatures comparable to that of Earth, WD 0806−661 B is a poor candidate for extraterrestrial life due to high surface gravity and lack of starlight. Because of its large mass and distance from its star, WD 0806−661 B could host many large exomoons.

WD 0806−661 B was observed by the Mid-Infrared Instrument (MIRI) aboard the James Webb Space Telescope, which probed its atmosphere with medium-resolution data. The results were published in 2025. The MIRI spectrum is dominated by absorption of water vapor, ammonia and methane. Carbon monoxide and carbon dioxide were not detected, and only upper limits of their abundance can be determined. The atmosphere of WD 0806−661 B is mostly consistent with theoretical models, but some results are at odds with the model predictions, such as the non-detection of water clouds and the mixing ratio of ammonia. The retrieved mass is smaller than expected, possibly hinting at a younger age or an incorrect retrieved mass.^[3] Additional data was taken with the NIRCam and NIRSpec instruments of JWST, and published in 2026. The study found that vertical mixing decreased with higher altitude. This is based on the measured abundance of carbon monoxide and ammonia, which are present at 47 bars and carbon dioxide, which is present at lower pressures of 22 bars (or higher altitudes). The study also measured the abundance of water, hydrogen sulfide and methane. Based on the retrieval, the researchers found an elevated carbon/oxygen ratio, sub-solar metallicity and nearly solar carbon/sulfur ratio in the atmosphere.^[2]

• WD 0806−661
• COCONUTS-2b − Another planetary-mass companion with a large separation from its star
• List of Y-dwarfs