Brown dwarfs form like stars, collapsing directly out of a gas cloud like a star in a top-down process. Clearly they are intended to become stars, but something happens along the way that causes them to become runts of the stellar litter, smaller and cooler than even the chilled-out red dwarf stars. Actually, the universe seems to favour smaller objects. The so-called mass function describes the distribution of masses of the objects that are created in a star-forming nebula. A handful will be massive stars that will one day die as supernovae. More will be Sun-like stars. Even more will be red dwarfs, smaller and cooler than our Sun. And the most common type of object that will form in a nebula will be brown dwarfs. This is backed up by observations with the Hubble Space Telescope of the Orion Nebula, which discovered 50 brown dwarfs amid the newborn stars of the Trapezium Cluster. There will undoubtedly be more brown dwarfs in the Orion Nebula, but they are difficult to spot because they are so cool and dim. Hubble used its near-infrared camera to find the brown dwarfs because at their low temperature, brown dwarfs give out most of their light in thermal infrared.
In fact, so difficult are they to spot, the first brown dwarf was not discovered until the late-Eighties, when astronomers Ben Zuckerman and Eric Becklin of the University of California, Los Angeles, found a suspected brown dwarf called GD165B, although there remains some lingering doubt that it could just be a very low-mass star.
Astronomers then had to wait another ten years before finding more brown dwarfs, with Teide 1 in the Pleiades star cluster being discovered in 1995. Of course, brown dwarfs had been theorised to exist long before the Eighties, and it was actually Jill Tarter of SETI fame who came up with the name 'brown dwarf in 1975. Previously they had been known as black dwarfs, but this caused confusion with the other black dwarfs, which are what white dwarfs will eventually become when they cool down over trillions of years. Besides that, in actual fact brown dwarfs are not black, or even really brown - they are more of a magenta shade.
Huge advances in our understanding of brown dwarfs have been made in recent years, mainly thanks to WISE, which is NASA's Wide-field Infrared Survey Explorer satellite. WISE spent a whole year scanning the sky in mid-infrared light, wavelengths in which cool brown dwarfs should just pop into view. "The brown dwarfs jump out at you like big, fat, green emeralds," says WISE deputy project scientist. Amy Mainzer. They appear green in WISE's images because their temperatures are coded to false colours. WISE has proven itself as a prolific brown dwarf discoverer, and it has ended up finding the coolest brown dwarfs discovered so far. They are so cool, in fact, that astronomers have had to come up with a whole new classification for them.
Stars are grouped into types dependent on their temperature and luminosity. The hottest, most luminous stars are termed O-types. Next are B-types, then A-types, then F-types, G-types (like the Sun), K-types and M-types, the latter of which are red dwarfs. But brown dwarfs are even cooler than red dwarfs, so new types were needed to describe them, namely L and T-types. In 2014 though, astronomers using WISE found a brown dwarf called W0855-0714, which was so cold that it had a temperature of between -48 to -13 degrees Celsius (-54.4 to 8.6 degrees Fahrenheit). This brown dwarf was described as Y-type, and a dozen or so more have subsequently been identified. Scientists have not even been able to rule out the possibility of one or more brown dwarfs lying closer to the Sun than the current nearest star, Proxima Centauri, which is 4.2 light years away.
Traditionally, brown dwarfs are described as being between 13 and 80 times the mass of Jupiter, but W0855-0714 comes in below that - weighing only as much as between three to ten Jupiters - showing how difficult it is to define what a brown dwarf is. at least based on its mass. It has been speculated that it could be an escaped planet but astronomers suspect it is more likely to be a brown dwarf, simply because there should be so many brown dwarfs that the odds are against it being a rogue planet.
Brown dwarfs may form like stars, but they look more like planets, to the extent that they even have weather and clouds. For example, one brown dwarf, called ULAS J222711. appeared redder than other normal brown dwarfs. Under further inspection, astronomers from the University of Hertfordshire found that it was clouds scattering sunlight that were giving ULAS J222711 its red hue, but these were certainly not clouds like the fluffy water-vapour versions we have in Earth's sky.
"The thick clouds on this particular brown dwarf are mostly made of mineral dust, like enstatite and corundum," says the University of Hertfordshire's Federico Marocco. "Not only have we been able to infer their presence, but we have also estimated the size of the dust grains in the clouds." These dust grains were calculated to be about 0.5 micrometres (0.5 millionths of a metre or 20 millionths of an inch) across. On another brown dwarf that we have already mentioned. W0855, there is also evidence for frozen clouds of sulphides and water-ice, while gases such as methane, hydrogen sulphide and ammonia are taken as a given. "If you could bottle up a gallon of a brown dwarf's atmosphere and bring it back to Earth, smelling it wouldn't kill you but it would stink pretty bad, like rotten eggs with a hint of ammonia," says Amy Mainzer.
Most brown dwarfs could also be stormy, further cementing their similarities with Jupiter-like worlds. WISE's infrared predecessor, the still-operational Spitzer Space Telescope, has found signs of patchiness in the cloud cover of brown dwarfs, which could equate to roiling storm regions that sport terribly strong winds, enormous lightning strikes and rainfalls not of water, but of molten sand and iron. "What we see here is evidence for massive, organised cloud systems, perhaps akin to giant versions of the Great Red Spot on Jupiter; says Professor Adam Showman of the University of Arizona.
By teaming up Spitzer with Hubble, astronomers are able to look at brown dwarfs in different wavelengths of infrared light, which are able to peer down into different layers of a brown dwarf's atmosphere. As the brown dwarf rotates, variations in the amount of cloud cover and the size of the storms affects the brightness that Hubble and Spitzer space telescopes see. "These out-of-sync light variations provide a fingerprint of how a brown dwarf's weather systems stack up vertically. The data suggests regions on a brown dwarf where the weather is cloudy and rich in silicate vapour deep in the atmosphere coincide with balmier, drier conditions at higher altitudes - and vice versa." says Showman.
But where do brown dwarfs get the energy from to drive weather and planet-sized storms? On Earth, the energy for our weather systems comes from heat emitted by the Sun. Certainly, some brown dwarfs are found orbiting stars, but that does not explain where brown dwarfs without stellar companions get their heat from. A world like Jupiter, which is far from the Sun, still retains some residual heat within its core from the days when it was formed, and some of the heat of brown dwarfs will come from the same source. However, brown dwarfs have an advantage over worlds like Jupiter. Although they are lacking too much mass to ever have the required pressures and temperatures in their cores to instigate nuclear fusion of hydrogen into helium, they can for a short while ignite nuclear fusion reactions of deuterium. The most massive brown dwarfs are also able to fuse lithium - lithium does not exist in any significant quantities in normal stars, so a search for lithium is a good test of whether an object is a brown dwarf or not. The smallest brown dwarfs that are less than 13 times the mass of Jupiter are not hot enough in their cores for any fusion reactions. Nevertheless, those that are able to start the reactions can, for a short while, produce heat and energy this way, which resides in the star for billions of years after the fusion reactions have actually run themselves out.
Inside a star like the Sun. there are two zones. The innermost is the radiative layer around the nuclear core, where energy produced by fusion reactions is transported through radiation. It is this energy that holds the Sun up against the pull of its gravity. Above the radiative layer is the convective layer, where convection currents transport the energy the rest of the way to the Sun's surface. Brown dwarfs, however, are suspected to only have convective layers. This leads to their interiors being 'springy', so they can become more compressed with greater mass. This results in brown dwarfs that, astonishingly, are not much larger than the diameter of Jupiter, despite some having dozens of times more mass.
This could result in the surprising scenario where a planet orbiting a brown dwarf is actually bigger than the brown dwarf! Given that brown dwarfs are not proper stars, it had been uncertain as to whether planets could form around them. However, the Atacama Large Millimeter/submillimeter Array (ALMA), in the Chilean desert, has discovered a disc of dust and rubble around a brown dwarf, just like the planet-forming dust discs that astronomers find around young stars. The disc around the brown dwarf, which is known as ISO-Oph 102 and which has 60 times the mass of Jupiter, contains millimetre-sized dust grains. In the planet-forming discs around young stars, these grains gradually begin to stick together, growing larger and larger until they build up into rocky planets.
"We were completely surprised to find millimetre-sized grains in this thin little disc," says Luca Ricci, from the California Institute of Technology in Pasadena, who headed the team of astronomers who used ALMA to find this disc. "Solid grains of that size shouldn't be able to form in the cold outer regions of a disc around a brown dwarf, but it appears that they do. We can't be sure if a whole rocky planet could develop there, or already has, but we're seeing the first steps."
This leaves brown dwarfs facing something of an identity crisis. They form in the same way as stars but are not stars, unable to fuse hydrogen into helium. They look like planets with weather systems but are more massive and don't form like planets, yet they may be able to form proper planets orbiting around them. They are also likely the most common type of object in the universe - some scientists even suspect that there could be enough unknown brown dwarfs to account for some of the missing mass that has been attributed to dark matter. Yet despite all of this they will always be seen as failures, objects that couldn't make the grade to become stars, when really we should see them as super-planets that take on some star-like qualities. The brown dwarf is truly a unique breed of object, capable of taking on the role of both planet and star, while possibly revealing more about our hidden universe.