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RE: Exosolar comets
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Space missions looking for other Earths could be fooled into seeing them where none exist.
At first glance, such planets would be indistinguishable from comets orbiting around the same star, even though comets are minuscule compared to planets.
Comets are dirty snowballs of rock and ice anywhere from 1 to 20 kilometres across. Most astronomers had assumed their small size would make it impossible to see them around other stars.
Not so, says Mike Jura of the University of California at Los Angeles.



As a comet nears the sun, its dust-laden tail can stretch 50 million kilometres, reflecting sunlight and appearing as a ghostly fan-shaped glow in the night sky. Jura reasoned that although the immense distance from Earth would mean the tail of an extra-solar comet would not appear as a streak, it could still be seen from Earth as a point of light.
His calculations show that a large comet like Hale-Bopp, which was seen from Earth in 1997, would appear twice as bright as an Earth-sized world orbiting the same star.

"There is nothing contentious about the maths of this, but the result is surprising" - Mike Jura.
And since no one knows whether our solar system's comets are typical, it's possible extra-solar comets could appear brighter still. The dust trailing in our comets is larger-grained than interstellar dust.
If a comet around an alien star had dust grains of the same size as those of interstellar space, the tail would scatter more light and could appear a hundred times brighter than an Earth-sized planet, making it look like a giant planet
Malcolm Fridlund, the study scientist on the European Space Agency's proposed planet-finding mission Darwin, agrees that Jura's work raises concerns.
"This is highly interesting. We will fold these ideas into our analysis of how we plan to confirm our planet detections"

He says that analysing the reflected light from potential planets at different wavelengths could settle the matter, because a planetary atmosphere absorbs certain wavelengths that comets do not.
But he agrees that longer observation will be needed to identify planets without an atmosphere.

"We plan to look at each system three times during Darwin's five-year mission. In that time, comets will change brightness and move on highly elliptical orbits, but planets will not. So I don't think there will be any confusion"
Hale-Bopp-sized comets cross our solar system once a century, on average. Darwin should therefore only expect to see one or two comets as it trawls through a few hundred stars, unless it chances upon a star caught in a comet shower.

"It's possible we might just pick up a star suffering one of those" - Mike Jura.


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If comets exist around other stars, astronomers should be able to see them directly, says Michael Jura, astronomer at the University of California in Los Angeles (UCLA). All they need to do is use the same observatories that promise to reveal Earth-size extrasolar planets.
Astronomers have discovered more than 100 planets around other stars, but all these worlds are either giant planets orbiting Sun-like stars or small planets orbiting a pulsar. The ultimate goal of planet hunters is the discovery of an Earth-size planet circling a Sun-like star, because such a world might harbour life. To find an analogue to Earth, astronomers have proposed building space-based observatories like the Terrestrial Planet Finder and Darwin.

These observatories should see more than just Earth's twin.
"Any system which will be able to obtain direct images of an analogue to the Earth also will be able to detect bright comets."

Comets are much smaller than Earth, but as they approach the Sun, its radiation heats their surfaces and causes them to sprout two types of tails: dust and gas. A comet's dust tail scatters much sunlight. In fact, a bright comet observed from another planetary system would be at least as bright as Earth.
For example, Comet Hale-Bopp, a great comet that passed Earth in 1997, actually outshone Earth. Any extraterrestrial astronomer able to see Earth also should have seen the comet.

On the other hand, Comet West, a great comet that passed Earth in 1976, might have escaped detection. Although Comet West also was bright, it passed so close to the Sun that seeing it would have been hard for an extraterrestrial observer.

The best stars for extrasolar comet hunters to examine are those somewhat brighter than the Sun. The more luminous the star, the more starlight both a planet and a comet reflect. However, the comet gets an additional boost because as more radiation hits it, more of its ice vaporizes and more dust launches into space.

A comet like Hale-Bopp would fail to outshine an Earth analogue around a star that had less than half the Sun's luminosity. This luminosity corresponds to spectral type G8, the same spectral type as the nearby star Tau (τ) Ceti.

Only about 1 percent of the time would a comet as large as Hale-Bopp be visible around the Sun. Thus, if other stars have comet clouds like the Sun's, astronomers would need to observe about a hundred stars to see one extrasolar comet.

However, no one yet knows what percentage of stars have comets like the Sun's. Some stars may have no comets. By discovering extrasolar comets astronomers can estimate just how common comets are.


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