| Caption: | JWST image of WD 0806−661 B and its white dwarf star |
| Discoverer: | Luhman et al. |
| Mean Radius: | 1.12 |
| WD 0806-661 b / Ahra | |
| Discovered: | 2011 |
| Discovery Method: | Direct imaging |
| Alt Names: | Ahra, GJ 3483 B |
| Semimajor: | 2,500 AU |
| Star: | WD 0806−661 |
| Mass: | 7-9 |
| Surface Grav: | 16.17 g (Estimate) |
| Single Temperature: | 325-350 K |
WD 0806-661 B, formally named Ahra, is a planetary-mass companion of the white dwarf star WD 0806−661, or Maru.
This object was discovered in 2011 by the Spitzer Space Telescope. It has a mass of between 7-9 MJ, putting it as a gas giant planet. At the time of its discovery, WD 0806-661 b was the coldest brown dwarf ever discovered, with a temperature of 325-350 Kelvin (52-77 °C or 125-170 °F) and also had the largest separation from its star at about 2,500 AU at the time of its discovery. The photometric colors of the object suggest it is metal-poor.[1]
As of 2021, WD 0806-661 b is a potential target for study from the James Webb Space Telescope.[2]
See main article: WD 0806−661. 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.[3] WD 0806-661 is estimated to be 1.5-2.7 billion years old,[4] 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.[5] 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.[6] The object's radius is estimated to be 12% larger than that of Jupiter,[7] 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.
The WD 0806−661 system is planned to be studied by the James Webb Space Telescope, which will probe the atmosphere of the object, as well as search for other planetary-mass bodies in the system.
There is no consensus as to whether WD 0806-661 b should be considered an exoplanet or a sub-brown dwarf. Based on its large distance from the white dwarf, this object likely formed like a star rather than in a protoplanetary disk, and it is generally described as a brown dwarf in the scientific literature. However, the IAU considers objects below the limiting mass for deuterium fusion that orbit stars (or stellar remnants), with M2/M1 < 1/25 to be planets, no matter how they formed. Additionally, WD 0806-661 b has been named Ahra through the IAU's NameExoWorlds exoplanet naming campaign, and is included in databases such as the NASA Exoplanet Archive.