These tugs cause the star to surge toward Earth and then away, creating periodic variations that we can detect by analyzing the spectrum of light from the star. The first method, known as the wobble method, looks for changes in a star's relative velocity caused by the gravitational tug of a nearby planet. Luckily, astronomers have other means at their disposal, and they all call for sophisticated telescopes armed with photometers (devices that measure light), spectrographs and infrared cameras. Our Earth-based telescopes can't resolve a faraway planet as a dot separate from its host star. Most are simply too small and too far away to be observed directly. One of the great problems in the search for exoplanets is detecting the darn things. This change must be periodic if it is caused by a planet. Transits by terrestrial planets produce a small change in a star's brightness of about 1/10,000 (100 parts per million, ppm), lasting for 2 to 16 hours.
When a planet crosses in front of its star as viewed by an observer, the event is called a transit. Now the search is on to find another planet in the Goldilocks zone of another solar system.
In between, the conditions are just right so that liquid water remains on the planet's surface without freezing or evaporating out into space. Earth, of course, fills that bill, while Venus roasts in a runaway greenhouse effect and Mars exists as a frozen, arid world. Also called the habitable zone or life zone, the Goldilocks region is an area of space in which a planet is just the right distance from its home star so that its surface is neither too hot nor too cold. Terrestrial planets are also more likely to lie in the Goldilocks zone. Terrestrial planets tend to stick close to their host stars, which means they have smaller orbits and thus much shorter years. Astronomers refer to these pipsqueaks as terrestrial planets because they possess heavy-metal cores surrounded by a rocky mantle.
Smaller planets, including Earth and super-Earth lookalikes, are much more likely to become incubators of life. Gas giants, with their stormy, multicolored atmospheres, may offer spectacular sights, but they'll never make good digs. In fact, behemoths like Jupiter and Saturn are known as gas giants because they're nothing more than giant balls of hydrogen, helium and other gases with little or no solid surface.
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