YET another planetary system in the making has been discovered 鈥� but this one is a bit different from the many other extrasolar systems discovered so far. Instead of surrounding a sun-like star, it is centred on a brown dwarf that is itself barely bigger than a giant planet. And despite the host star lacking the nuclear reactions that make our sun shine, its disc could one day spawn habitable Earth-sized planets.
The discovery calls for a rethink of how many different kinds of planetary systems there might be. 鈥淲e just think of planets forming around stars about the mass of our sun,鈥� says Kevin Luhman of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. 鈥淏ut they could form in more exotic situations around very small brown dwarfs. There might be little mini solar systems out there.鈥�
Brown dwarfs are failed stars with masses of between 15 and 70 times that of Jupiter. They are thought to form like ordinary stars, from a collapsing cloud of gas and dust. But unlike ordinary stars, they don鈥檛 generate high enough temperatures to trigger the sustained fusion of hydrogen nuclei that is the source of the tremendous amounts of energy released by larger stars.
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Earlier studies have shown that several medium-sized brown dwarfs are surrounded by protoplanetary discs, but astronomers using ground-based telescopes find it hard to study the discs because they are fainter than the infrared radiation coming from the brown dwarf itself.
NASA鈥檚 Spitzer Space Telescope, launched in August 2003, gets round this problem. Spitzer is far more sensitive than ground-based telescopes to longer-wavelength infrared light, and at these wavelengths the cool disc around a brown dwarf looks brighter than the star itself. Luhman and his team found that an exposure of just 20 seconds was enough to reveal signs of a disc around a brown dwarf. He reported the discovery this week at a conference on planet formation and detection in Aspen, Colorado.
The brown dwarf itself was identified five years ago, and lies about 500 light years away in a star-forming region called Chamaeleon I. Weighing in at 15 times the mass of Jupiter, it is scarcely heavier than some of the larger extrasolar planets that have been discovered.
鈥淏rown dwarfs take a long time to cool, so a planet could remain habitable long enough for life to evolve鈥�
Luhman鈥檚 observations suggest that the inner rim of the protoplanetary disc lies about 700,000 kilometres from the dwarf鈥檚 centre, or about three times the star鈥檚 radius. Discs around brown dwarfs typically have about one-tenth of the mass of the star itself, so in this case the disc probably contains about two Jupiter masses of planet-building material. 鈥淚鈥檇 speculate that it could build a Saturn, or maybe a few smaller Earth-sized planets,鈥� Luhman says.
It is conceivable that such planets, if they form, could be habitable. The surface temperature of the mini brown dwarf is 2300 kelvin, so a planet 1.5 to 7 million kilometres away could host liquid water. Planets very close to a brown dwarf would be scorched at first, but would become hospitable as the star cooled. Brown dwarfs take a long time to cool, so the planet could remain habitable long enough for life to evolve.
Luhman hopes to find out whether even smaller objects 鈥� maybe just five times as heavy as Jupiter 鈥� can reign at the centre of planetary systems. 鈥淚t鈥檚 still an open question as to how small you can go, but hopefully we will be able to soon answer that.鈥� His stunning 20-second snapshot suggests longer exposures will allow the Spitzer scope to uncover many smaller, fainter examples.
鈥淪pitzer really is a killer instrument for detecting this kind of object,鈥� agrees Andrew Collier Cameron, who studies extrasolar planets at the University of St Andrews in the UK. Brown dwarfs could be a fertile hunting ground for planets, and it might be possible to see these planets directly. Extrasolar planets orbiting ordinary bright stars are usually hard to see because of the stellar glare.
