Sunday, February 6, 2011

Earth-like Exoplanets Are Hard to Detect

Recent discoveries of exoplanets, or extrasolar planets, suggest that they are common in the universe. For these planets to have life—at least life as we know it—they need to be quite similar to Earth. Planets larger than Earth often have enough gravity to retain thick and crushing atmospheres, while planets much smaller than Earth would let atmospheres escape into space. Assuming a planet had the proper size, it could probably only harbor life if it was in its star's habitable zone—the band of space around a star where temperatures would be suitable for liquid water.

Artist conception of exoplanet Kepler-10b
Artist's rendering of the exoplanet
Thus the search for exoplanets is focused on Earth-like planets within their star's habitable zone. The methods of detecting exoplanets, however, are biased in favor of large planets that orbit close to their star; these kinds of planets are often called "hot Jupiters" because of their size and scorching proximity to their suns.

Hot Jupiters, which are not likely to have life, are commonly discovered because it is much easier to detect an exoplanet indirectly—by its effect on its star—than through a direct observation. In one method, astronomers look for a wiggle in a star's location that could only be caused by the gravitational influence of planets. Contrary to common vernacular, planets don't orbit stars. Instead, both planet and star orbit their collective center of mass. Since stars are very massive, they move only slightly compared to their planets. Astronomers can pick up on this movement and infer the presence of planets—but only if the planets are large and close enough to significantly influence their star. Smaller and more distant planets are less likely to be detected.

Another indirect method of detection is called the transit method. It watches for planets that transit, or pass in front of, their parent stars. Astronomers can't see the planets in detail, but they measure the reduction in brightness of the star being transited. Larger reductions mean larger planets. The transit method is quite capable of finding Earth-sized planets, but it still is better at finding those that orbit close to their suns. The problem is that it requires a solar system's orbital plane—the plane in which planets orbit—to be near perfectly aligned with Earth's line of sight. Only in this condition would a planet transit its star from our point of view. The further out a planet orbits, the more perfect this alignment must be. Since all these detection methods favor finding hot Jupiters, Earth-like exoplanets in the habitable zone should be even more common than our discoveries suggest.

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