The discovery, a first for astronomers, has been hailed by scientists who believe it will shed crucial light on the frequency of solar systems throughout the galaxy.
Researchers have recently pinpointed scores of planets in orbit round stars, raising hopes of finding life elsewhere in the universe. But the exact mechanism of the formation of planets has remained a mystery.
However, observations of star KH15D by a group of students using a simple university telescope have raised hopes of a breakthrough. 'This is going to add a whole new dimension to astronomy,' said Nasa physicist Steven Maran.
The astronomers, from Wesleyan University, Connecticut, spotted KH15D - a young star the size of our sun, sited in the constellation Monocero - six years ago when a routine survey showed it was behaving oddly. For 32 days it shone brightly. Then it dimmed dramatically for the next 16.
'Basically, the star is winking at us,' said project leader Professor William Herbst. 'Something is passing between it and Earth. However, it cannot be another star or a large planet. That would produce much briefer eclipses. These last for more than two weeks.'
Herbst concluded that a collection of smaller objects - dust grains, rocks and asteroids, strung out in a great clumpy arc - must be orbiting KH15D. As these swaths are exactly what astronomers would expect to find in a system giving birth to planets, a major programme of observations of KH15D was launched. According to theory, a star starts to spin as it is born, causing surrounding clouds of dust and grain to form a disc, just as a flat pizza is created when a chef whirls a piece of dough. The grains in the disc bash into each other, accrete into bigger and bigger lumps - and eventually become asteroids and finally planets. The trouble is astronomers have never observed this process. Hence their excitement about KH15D and its swaths of dust.
Only lengthy observations would provide the information astronomers needed - which ruled out the use of large expensive telescopes that only allocate brief periods of observing to astronomers. So the Wesleyan team ran programmes, using the campus's modest 24-inch telescope, for graduate and undergraduate students who maintained bouts of uninterrupted star-gazing. In addition, astronomers in Uzbekistan, Ger many, Israel and Spain joined in to keep a watch on KH15D.
The results, revealed at a conference in Washington a few days ago, produced several surprises. First, the eclipse was found to have lengthened from 16 to 18 days, indicating that the orbiting dust clouds were moving closer to the KH15D. Second, in the middle of each eclipse brief interludes of blue light emerged from KH15D which astronomers believe come from a large, single object, most probably a planet, that seems to have formed inside the star's dust cloud. 'We can't tell exactly what size this object is yet, but we should be able to in a year or so,' said Herbst. 'However, something very unusual is happening around KH15D.'
This excitement is shared by other astronomers. 'Theory predicts that, as a planet forms, it will be followed around in its parent disk by waves of denser material, and that seems to be what we are seeing here,' said Edinburgh astronomer Dr Jane Greaves, of the Royal Astronomical Society. 'Planet formation is a short-lived process, tiny compared to the lifetime of the star. We are only seeing this happen round KH15D because we are looking at exactly the right moment. That is why it is so exciting.'
The star is only three million years old, compared to our own sun, which is billions of years old. For a planet to have formed so quickly is surprising. However, the surface of this other world will not provide a home for life-forms for a long time. Earth was battered for hundreds of millions of years by orbiting debris left over from its creation, making life untenable. And even after Earth cooled down, and primitive life had appeared, it still took another three billion years to evolve into intelligent beings.