THE birth of a planet has been caught on camera. The infant planet, circling a young star the size of our Sun, hasn鈥檛 settled into a steady orbit yet and astronomers think they can see it migrating inwards towards its parent star.
Before a planet can form, dust and rocks surrounding a star start to collect in a flat orbiting disc. Astronomers believe planets appear when gravitational attraction starts to make parts of the disc clump together and move in towards the star. But no one knows exactly how this happens or how long it takes.
Now observations of a star called KH15D are shedding light on the problem. The star was first noticed in 1997 and is 2400 light years away in the constellation of Monoceros, near the famous Cone nebula 鈥� the dark notch in the bottom left of the picture.
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Blobs of orbiting material pass between us and the star every 48 days, blocking all but 4 per cent of its light. Graduate student Catrina Hamilton from Wesleyan University in Connecticut organised continuous monitoring of the star from September 2001 to April 2002. During that time, the length of the eclipse increased from 16 days to 18 days. 鈥淭he eclipse is widening as we鈥檝e been watching,鈥� says Hamilton. That means the orbiting material is probably moving in closer to the star rapidly enough that we can see it.
The star takes 2.5 days to dim and brighten. That suggests a wave crest of material is passing in front of the star, rather than a sharp edge, says Hamilton鈥檚 supervisor William Herbst, who announced the results last week at a conference on extrasolar planets in Washington DC. The star also brightens slightly during the middle of the eclipse, suggesting the blobs are being shaped by a massive central object, around which the dust is sparse.
Alternate eclipses looked similar, so Herbst and Hamilton believe there are actually two clumps of material, shepherded by a single object that鈥檚 circling the star in a 97-day orbit. The object鈥檚 gravity tugs at the disc, forming ridges that stick out far enough from the plane of the disc to block our line of sight to the star as they pass by (see Graphic).
The big uncertainty is over the object鈥檚 mass. So far the researchers can only say that it must be less than 130 times the mass of Jupiter. That means it could be a very small star in the making. But it is likely to be much smaller, and if it鈥檚 less than 10 times the mass of Jupiter it must be a planet. 鈥淭hat would be very exciting,鈥� says astronomer Alan Boss of the Carnegie Institution of Washington.
That鈥檚 because until now, theorists have had no hard data on which to base their theories of how planets develop. 鈥淭his object might turn out to be something of a Rosetta Stone for deciphering the nature of planet formation,鈥� says Boss. Astronomers believe that giant planets form far out from their star, then migrate inwards. But the object forming around KH15D is even closer in than Mercury is to our Sun. 鈥淚f it鈥檚 a planet, it probably formed further out, and it may still be in the process of migrating,鈥� Boss told 快猫短视频. Timing its orbit over the next few years should help pin down the small changes caused by migration, he says, giving scientists their first clue to how fast this actually happens.
But the biggest surprise is perhaps that the object has had time to form at all, as the star is only 3 million years old. Theorists had thought that it would take much longer than this for dust to coalesce into a planet, so now they are going to have to find ways to explain how this could happen so quickly.