PLANETS either form very rarely, or the prevailing model of how they form is wrong, say researchers who have been studying the dusty discs from which they are born.
Their results exacerbate a longstanding quandary. The theory is that small particles in the discs of dust and gas surrounding a star gradually collect together due to gravity. But this process would take 10 to 20 million years to make gas giants such as Jupiter and Saturn, and even longer, perhaps 100 million years, to make rocky planets such as Earth. Yet such discs of debris haven’t been seen around stars that are more than about 5 million years old. So how do planets manage to form at all?
The latest results, from one of the most detailed surveys of protoplanetary discs, have made things even worse. Elizabeth Lada, Richard Elston and eight other astronomers from the University of Florida used their infrared survey of dust clouds to find seven stars with discs that are edge-on, revealed in silhouette, doubling the number of discs known. By monitoring infrared radiation from the stars that has been reflected off their discs, researchers should eventually be able to glean details about the mass, size, temperature and accretion rate of the discs.
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In its observations so far, the team has seen two distinctive infrared signatures from the stars – one wavelength from a thin, inner disc, and one from a more massive outer disc. Only the outer disc contained enough mass to form a planet. And while the inner discs were still around for stars up to 6 million years old, the larger rings didn’t even make it to 3 million years – nowhere near enough time for a planet to form by ordinary accretion.
If these results are representative, planets must be forming some other way. There is another theory for how they could form, based on instabilities in the disc itself, which could potentially create a planet within a few million years. But many researchers are sceptical of this dramatic scenario. Lada and Elston now hope to study the edge-on discs they have identified in greater detail and at other wavelengths, to try to prove or disprove the theory.
Meanwhile, David Weintraub of Vanderbilt University in Nashville, Tennessee, has another way to solve the problem of short-lived discs. He reports evidence that the discs might last much longer than we thought, but simply become invisible to our telescopes. He points out that infrared surveys only pick up the tiniest particles. So as matter forms bigger clumps, the disc becomes more difficult to detect. Weintraub and his colleagues looked for hydrogen instead, thought to be the last thing left behind during the accretion process.
Sure enough, they found hydrogen emissions from a 10-million-year-old star where no disc had previously been visible. Maybe the discs really are long-lived after all, says Weintraub.
