astronomy

WD 0806−661 B

WD 0806−661 B is a astronomy topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand WD 0806−661 B rather than just read about it. In short: WD 0806−661 B or 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.

Key takeaways

WD 0806−661 B belongs to astronomy; place it in that map before memorising details.
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Reference excerpt

WD 0806−661 B or 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 in the range 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.

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. 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. 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.

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 MJ 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.

See also WD 0806−661 COCONUTS-2b − Another planetary-mass companion with a large separation from its star List of Y-dwarfs 2MASS J2126−8140

Notes

References

Illustrations

Worked examples

Example 1 — a first encounter with WD 0806−661 B

Start with the simplest possible case. Write down what WD 0806−661 B claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to WD 0806−661 B before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about WD 0806−661 B ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of WD 0806−661 B

In research: WD 0806−661 B appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses WD 0806−661 B in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: WD 0806−661 B is common in secondary-school and first-year university syllabi. It links to neighbouring topics Exoplanets detected by direct imaging, Exoplanets discovered in 2011, Exoplanets with proper names, so understanding it makes those chapters shorter.
In everyday life: Look for WD 0806−661 B outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study WD 0806−661 B in 20 minutes

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Frequently asked questions

What is WD 0806−661 B in simple terms?

WD 0806−661 B or 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.

Why does WD 0806−661 B matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study WD 0806−661 B?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on WD 0806−661 B.