AS IF space wasn’t lonely enough, pity the brown dwarf. Compared with their stellar siblings, these astronomical objects are something of a failure. And while they have much in common with planets, they don’t seem to fit in there either.
This awkward status as cosmic in-betweener means brown dwarfs are often overshadowed by their flashier counterparts, such as alien worlds or fiery supernovae. Yet not fitting in is precisely what makes brown dwarfs far more interesting and useful than we once thought.
As new evidence of these celestial outcasts emerges, they are challenging our ideas about the differences between planets and stars. Some have weather unlike anything seen before, from molten iron falling as rain to silicate snow. And the traits they share with exoplanets means that we can learn things that telescopes pointed at alien worlds cannot reveal. The most unloved destinations in space are fast becoming the new cosmic “it” objects.
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The existence of brown dwarfs was first suggested in 1962 by Shiv Kumar at the NASA Goddard Institute for Space Studies in New York City, who had wondered how small a star could be. Below a certain size, Kumar calculated, you would end up with degenerate objects with too little mass to sustain hydrogen fusion, and they would fizzle out.
Kumar called these hypothetical objects “black dwarfs”, but the name proved problematic. In the 1970s, astronomer Jill Tarter pointed out that the term also referred to a dark, cooling star near the end of its life. Various other names had been proposed, such as “planetar”, “still-born star” or “substar”, but Tarter argued for “brown dwarf”. She knew they couldn’t actually be brown (see “True colours“), but since their actual colour was going to be difficult to observe due to their feeble radiation.
Tarter was half right: their feeble radiation meant none was spotted for another 20 years. In this period, brown dwarf research stalled. Then, in 1995, Gliese 229b popped into view. About 19 light years away, this brown dwarf was found thanks to advances in infrared telescopes and by switching the search to younger star systems, within which any objects are brighter and easier to spot.
The dwarf had a mass between 20 and 50 times that of Jupiter, and a relatively cool surface temperature of 680 x°C. Did this thrust brown dwarfs into vogue at last? Not quite. After three decades of speculation, , but it was announced at the same conference as the discovery of the first exoplanet, so was overshadowed.
“After years of speculation, the first brown dwarf might have enjoyed more fanfare, but it was overshadowed”
Still, for a small group of dedicated researchers, it offered an intriguing puzzle: Gliese 229b was a star, but it had a planet’s atmosphere. “It was clearly very different… its luminosity, its spectral analysis, the fact that it had methane in it,” says Ben Oppenheimer, assistant curator at the American Museum of Natural History, who was part of the team who found it. The only thing they had to compare it with was Jupiter, but it wasn’t a gas giant.
As more brown dwarfs emerged with similarly puzzling traits – – it sparked debate about how to classify them. Since humans first looked to the heavens, there has always been a separation between stars and planets. Brown dwarfs challenge these ideas. “It depends on what question you ask, as to whether they are a planet or a star. They blur the distinction between the two,” says of the University of Hertfordshire, UK.
Brown dwarfs are born from the collapse of a gas cloud, just like stars, so share some features with their stellar relations. They have magnetic spots like stars, and some even emit radio emissions like pulsars. They are also dense enough to fuse a finite tank of deuterium at the start of their lives. This produces faint infrared radiation, as does the conversion of gravitational energy into heat. Gradually, though, they cool through their lifetimes, says Burningham, “like an ember plucked from a fire”. This makes them much colder – some as cool as 27 °C.
So should we think of them more like planets? Brown dwarfs are significantly more massive than most planets – between 13 and 75 times the mass of Jupiter. Only 3 to 4 per cent of are so hefty. But according to of Stony Brook University in New York state, “we truly don’t know how low in mass the lowest-mass star-like object can be and how massive a planet can be.” And in terms of actual size, most are not far off the diameter of Jupiter.
Cosmic relations
Brown dwarfs share many traits with gas giant planets, too, with a boiling atmosphere made of a toxic brew of carbon monoxide, hydrogen sulphide and water, or methane and ammonia. “The more data we collect, the more obvious the connection between brown dwarfs and planets,” says Oppenheimer. Brown dwarfs, then, represent a bridge between stellar and planetary science.
Recently, we have encountered an intriguing new twist: they have weather. This realisation has sparked a shift in focus among brown dwarf aficionados, from simply searching the sky for more of them to characterising known objects in detail.
It was always suspected that brown dwarfs had clouds, because their internal heat would prompt gases to rise and then condense, as happens in the atmospheres of planets in the far reaches of our solar system. But recently we have been able to watch this weather change over time. In the past few years, astronomers realised that variations in the levels of infrared light emitted by brown dwarfs pointed to shifting patterns within the atmosphere. By training their telescopes on a target for months at a time, they deduced that certain changes in infrared emission were caused by huge storms.
If these brown dwarfs had a . We know from studying the chemical composition of stars that the atmospheres of hotter brown dwarfs contain gaseous iron and silicate, which would eventually condense as it rises and cools. Imagine rain drops of molten iron, with swirling clouds made of hot grains of sand that gradually fall as silicate snow. “We think it would be very similar to the water cycle,” says Mark Marley at NASA’s Ames Research Center in Mountain View, California.
Arguably, brown dwarfs have provided the first detailed insights into weather beyond our solar system. Weather on exoplanets, by contrast, is harder to see. “Exoplanets are too faint and we can’t always get the spectra, because we need a huge telescope and a way to remove the starlight,” says of the University of Leicester, UK. What we are learning from brown dwarfs could inform our knowledge of exoplanet climates, and help hone the techniques required to probe them (see diagram). “Brown dwarfs are an excellent proxy for extrasolar giant planets,” says Metchev.
For example, the fact that when a brown dwarf’s atmosphere cools its cloud systems suddenly transform in character – often almost dissipating – suggests that we will see similar patterns on gassy exoplanets. “They’ve taught us that there is great diversity among substellar objects, and we should certainly expect even more as we study extrasolar planets,” says Marley.
The coolest brown dwarfs have weather patterns we associate with our own planet – a few may even have clouds made of water vapour. “They are fully comparable in temperature to terrestrial zone planets in the solar system,” says Metchev.
Other brown dwarfs promise to provide clues about the weather experienced by planets heated on only one side. Casewell, for example, studies systems in which a brown dwarf is paired with a white dwarf, locked so that only one side of the brown dwarf is bathed in radiation. “We’re working on a lot of the atmospheric effects,” she says. “Does only one hemisphere get hot? If not, are there large winds transferring heat from the hot side to the cold?” Understanding how such supersonic winds would affect weather patterns could help us work out the potential habitability of rocky exoplanets.
Perhaps the most tantalising revelation is that brown dwarfs can be accompanied by planets. Indeed, last month a team of astronomers claimed to have found . The planet is roughly twice the mass of Jupiter.
“It’s quite possible that planets may form around brown dwarfs by standard planet-formation mechanisms,” says of Imperial College London. While last month’s discovery is a gas giant, many of the future planets we find are likely to be small and rocky, says Mohanty, since young brown dwarfs have less material surrounding them than more massive stars. Life, then, could exist on a world orbiting a brown dwarf.
Given that brown dwarfs have now finally come of age, perhaps we should stop trying to pigeonhole them as planets or stars. It’s time to put them in a class of their own.
Additional reporting by Richard Fisher
True colours
What colour is a brown dwarf? Well, not really brown.
Brown dwarfs received their drab name to differentiate them from other celestial objects: observed with optical telescopes, blue stars tend to be hot, red stars cooler. Brown was chosen as it is a mongrel shade, which some felt appropriate given that the colours of brown dwarfs were expected to be tricky to pin down.
When astronomers showcase images of brown dwarfs they use representative colours. Most brown dwarfs are observed using infrared telescopes, with various filters to record data at specific wavelengths. To produce a representative colour in a red-green-blue palette, astronomers assign the shortest wavelength filter they use to blue and the longest to red, and then stack them together. This commonly creates a magenta shade, although occasionally you get wilder colours, like green.
So what would a brown dwarf look like to the naked eye? Zoom past in a spaceship and you may well fail to see it because it would produce so little visible light. Peer closer, though, and you might see a faint glow in regions where it is still hot enough to produce light – but it might be more of a very dark orange.
This article appeared in print under the headline “Fallen star? Lonely planet? Best of all worlds”
