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The Sun Steals Comets from Other Stars

By David Reneke

Dave Reneke brings news from the space and astronomy communities around the world.

The next time you see a comet blazing across the night sky, consider this: it’s a stolen pleasure. If astronomers are reading things right, our Sun has snatched comets from neighbouring stars’ backyards.

This kind of thievery accounts for most of the comets in the Oort Cloud at the edge of our solar system. “The Sun was born within a huge community of other stars that formed in the same gas cloud,” says Hal Levison of the Southwest Research Institute.

According to the “thief” model, comets accompanied the nearest star when the birth cluster blew apart. The Sun made off with quite a treasure – the Oort Cloud was swarming with comets from all over the “neighbourhood”.

The Oort Cloud is an immense cloud of comets orbiting the Sun far beyond Pluto. For astronomers it explains the origin of comets sometimes seen falling into the inner solar system. Although no confirmed direct observations of the Oort Cloud have been made, most astronomers believe that it is the source of all long-period and Halley-type comets. To be fair, our Sun came by these comets honestly.

Comets in the Oort Cloud are typically about 3 km across, and they’re so far away that estimating their numbers is no easy task. Levison and his team think that there should be something like 400 billion comets there.

“These comets are in very odd orbits that take them far from our Sun into remote regions of space,” Levinson said. “They couldn’t have been born in orbit around the Sun. They had to have formed close to other stars and then been hijacked here.”

This means that comets can tell us not only about the early history of the Sun, but also about the history of other stars.

Big Game Planet Hunter
The premier observatory of the 21st century, the James Webb Space Telescope, will hopefully launch in 2014 in search of the first stars and galaxies ever formed in the universe. But there’s a growing awareness among astronomers that the world’s largest infrared telescope is going to be a canny hunter of planets circling faraway stars – if NASA can survive the cost overruns and technical hitches that have plagued its development.

Astronomers are finding planets at a prodigious rate, with close to 500 being bagged so far. The Holy Grail of extrasolar astronomy is to find another planet that resembles our own – a kind of twin Earth.

The Webb telescope will be able to carry out precise follow-up studies of newly found planets that are hiding in the glare of their own suns. It’s like trying to spot a firefly in front of a searchlight from a great distance, but there are ways to do it.

One way is called the transit method, which studies the light from a star when a planet passes in front of it. Doppler measurements help us to determine the planet’s mass and radius, and then astronomers can start to think about the planet’s composition.

We can also use spectroscopy during the transit by measuring the spectrum of the starlight before the transit and then again when the starlight is filtered through the planet’s atmosphere. This way we can learn a little about the planet’s make-up, temperature and other vital signs.

Can Webb find signs of life on a planet like Earth? The answer is probably not. A true Earth twin would be too small to emit enough infrared light for analysis.

Still, whenever scientists make statements like that, someone proves them wrong. It’s hard to say exactly what wonders Webb’s hunt will turn up.

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at www.davidreneke.com