Planet's impact forms dust disc

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Planet's impact forms dust disc

BBC News
July 20, 2005

An unusually thick ring of dust around another star could hold clues about planet formation, say astronomers.


The warm dust around the star is
thought to be from recent collisions

The dustiest disc ever seen around a nearby star is probably the result of a collision between two small planets less than 1,000 years ago, they say.

Writing in the journal Nature, the researchers believe the collision may have been similar to the impact on the primitive Earth that formed our Moon.

The star has been studied by the giant Keck I and Gemini North telescopes.

Pulverised planets

"We were lucky. This set of observations is like finding the proverbial needle in the haystack," said Gemini Observatory astronomer Inseok Song.

The Sun-like star, designated BD +20 307, is slightly more massive than our Sun and lies in the constellation Aries about 300 light-years away.

The large dust disc that surrounds the star has been known since astronomers detected an excess of infrared radiation with the Infrared Astronomical Satellite in 1983.

The disc or warm dust around the star is thought to be from recent collisions of rocky bodies at distances from the star comparable to that of the Earth from the Sun.

"The amount of warm dust near BD+20 307 is so unprecedented I wouldn't be surprised if it was the result of a massive collision between planet-size objects, for example; a collision like the one which many scientists believe formed Earth's moon," said Benjamin Zuckerman, professor of physics and astronomy at the University of California at Los Angeles (UCLA), US.

"Whatever massive collision occurred, it managed to totally pulverise a lot of rock," said team member Alycia Weinberger of the Carnegie Institute of Washington, US.

More rocky planets

Because the star is estimated to be about 300 million years old, any large planets that might orbit BD +20 307 must have already formed.

However, the orbits of rocky remnants left over from the planetary formation process might be influenced by the planets in the system, as Jupiter did in our early Solar System.

Given the properties of this dust, the team estimates that the collisions could not have occurred more than about 1,000 years ago.

A longer history would give the fine dust (about the size of cigarette smoke particles) enough time to be dragged into the central star.

"What is so amazing is that the amount of dust around this star is approximately one million times greater than the dust around the Sun," said UCLA team member Eric Becklin.


Interplanetary dust reflects sunlight
- the so-called Zodiacal Light

The observations support the idea that comparable collisions of rocky bodies occurred early in our Solar System's formation about 4.5 billion years ago. They could also lead to more discoveries of this sort which would indicate that the rocky planets and moons of our inner Solar System are not as rare as some astronomers believe.


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