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. 鈥淭here is nothing contentious about the maths of this, but the result is surprising,鈥� Jura says.
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And since no one knows whether our solar system鈥檚 comets are typical, it鈥檚 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鈥檚 proposed planet-finding mission Darwin, agrees that Jura鈥檚 work raises concerns. 鈥淭his is highly interesting,鈥� he says. 鈥淲e will fold these ideas into our analysis of how we plan to confirm our planet detections.鈥�
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. 鈥淲e plan to look at each system three times during Darwin鈥檚 five-year mission,鈥� he says. 鈥淚n that time, comets will change brightness and move on highly elliptical orbits, but planets will not. So I don鈥檛 think there will be any confusion.鈥�
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. 鈥淚t鈥檚 possible we might just pick up a star suffering one of those,鈥� says Jura.
