WD 0806−661 B

WD 0806-661 B, also 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 M J, putting it in between the sizes of a gas giant planet and a brown dwarf. 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.

As of 2021, WD 0806-661 b is a potential target for study from the James Webb Space Telescope.

Host star
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. WD 0806-661 is estimated to be 1.5-2.7 billion years old, 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. WD 0806-661 B may have formed closer to the star, but migrated further away as it reached the end of its life.

Characteristics
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. The object's radius is estimated to be 12% larger than that of Jupiter, 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.

Type of object
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 $~13 Jupiter mass$ limiting mass for deuterium fusion that orbit stars (or stellar remnants) 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.