Object 44: WD 0806-661B
Podcast release date: 05 April 2021
Right ascension: 08:07:14.7
Corresponding Earth location: An area of ocean just offshore from the Blair Glacier in Wilkes Land, Antarctica
So, you've probably never heard of the constellation Volans, and neither have I. This is a constellation located very far south, so it's really difficult to see unless you are in the southern hemisphere. This was one of a dozen constellations made up in the late sixteenth century when Europeans first started mapping the southernmost constellations , and it is supposed to depict a flying fish. The outline actually does vaguely look like a flying fish, although all of the stars are so faint that this constellation would seem line a relatively indistinct area of the night sky.
In any case, the object for this episode is WD 0806-661B. This is an object that was identified as a brown dwarf when it was discovered in orbit around a white dwarf. The white dwarf is named WD 0806-661; the brown dwarf was given the same name but with a B added to the end to indicate that it was the second brightest object in the star system.
Brown dwarfs are basically objects made of gas that are larger than planets but smaller than stars and not massive enough to trigger the fusion of hydrogen into helium in their cores. They can be thought of as failed stars in a way. Consequently, they generally look cooler than normal stars. They don't radiate much light in the visible part of the electromagnetic spectrum, but they do radiate infrared light, so astronomers search for brown dwarfs using infrared telescopes.
WD 0806-661B was discovered in 2011 by Kevin Luhman, Adam Burgasser, and John Bochanski in an analysis of archived infrared images from the Spitzer Space Telescope (or, in other words, the leftovers from other people's research) . The object is separated from the white dwarf by a distance of about 2500 astronomical units (AU) , where 1 AU is equivalent to distance from the Earth to the Sun and 30 AU is equivalent to the distance from the Sun to Neptune. This means that WD 0806-661B is really far away from the star that it's orbiting. Both objects are located 62.7 light years (19.2 pc) away from Earth [3,4].
Finding a brown dwarf by itself is usually rather impressive. Theories from the 1960s had proposed that brown dwarfs exist , but they were not found until the 1990s , and they're still rather challenging to find. However, WD 0806-661B is not an ordinary brown dwarf, which is rather strange to say given how unusual brown dwarfs are. First of all, this object is really, really cool (in terms of temperature and not just in terms of popularity). A lot of brown dwarfs still radiate heat that was trapped inside them when they initially formed out of interstellar gas clouds, so they may still have surface temperatures of hundreds of degrees Celsius up to even one or two thousand degrees Celsius. WD 0806-661B has a surface temperature of about 20 to 30 degrees Celsius, which is about room temperature . If it wasn't a giant ball of hydrogen gas, it might be a nice place to visit.
So, the people who discovered this object were really excited to find such a cool brown dwarf, and this was what they highlighted in their first science paper about WD 0806-661B. It is potentially the coolest brown dwarf ever found. However, this actually skips over one other interesting aspect of this object, and that is its mass. It's only seven times more massive than Jupiter . This is so low that it may be more appropriate to actually think of WD 0806-661B as an exoplanet.
One definition that a lot of people like to use to separate planets from brown dwarfs is whether the objects are large enough to trigger the fusion of deuterium atoms with normal hydrogen atoms into helium atoms in their cores . This is a little different from the standard fusion processed within normal stars, which involves combining individual hydrogen atoms that contain no neutrons. Deuterium atoms are hydrogen atoms that have single neutrons and which are much rarer than standard hydrogen atoms, so the fusion process doesn't generate that much energy in brown dwarfs. This deuterium fusion process can only take place in objects that are about 13 times the mass of Jupiter or larger, although the exact numbers depend on the specific situation . WD 0806-661B is below this threshold, which means that it may not qualify as a brown dwarf.
Because it's smaller than a brown dwarf and because it orbits a star, WD 0806-661B should probably be considered an exoplanet instad. Moreover, this is an exoplanet that was directly imaged with a telescope, which is incredibly difficult to do. Most exoplanets are found by looking at how stars jiggle around slightly as planets orbit these stars and exert gravitational forces on them or how stars drop in brightness slightly when planets pass in front of them. Directly imaging an exoplanet is exceptionally difficult and exceptionally rare; one recent paper described how less than 50 exoplanets have been found this way . However, the people who found WD 0806-661B decided to call it a very cool, very low mass brown dwarf instead, which, while very interesting, still isn't quite as exciting as finding an exoplanet. I'm not sure why these people were so reluctant to call it an exoplanet. Maybe they just wanted to be a little conservative about their discovery in case they miscalculated something, or maybe they weren't certain whether this was small enough to call it a planet, or maybe the excitement would have been too much for them. I definitely think that this discovery was really undersold.
 Luhman, K. L. et al., Discovery of a Candidate for the Coolest Known Brown Dwarf, 2011, Astrophysical Journal Letters, 730, L9
 Gaia Collaboration et al., The Gaia mission, 2016, Astronomy & Astrophysics, 595, A1
 Gaia Collaboration et al., Gaia Early Data Release 3: Summary of the contents and survey properties, 2020, arXiv e-prints, arXiv:2012.01533
 Kumar, Shiv S., Study of Degeneracy in Very Light Stars., 1962, Astronomical Journal, 67, 579
 Nakajima, T. et al., Discovery of a cool brown dwarf, 1995, Nature, 378, 463
 Spiegel, David S. et al., The Deuterium-burning Mass Limit for Brown Dwarfs and Giant Planets, 2011, Astrophysical Journal, 727, 57
 Swastik, C. et al., Host Star Metallicity of Directly Imaged Wide-orbit Planets: Implications for Planet Formation, 2021, Astronomical Journal, 161, 114