NASA's Kepler spacecraft has detected its first known exoplanet orbiting in its star's habitable zone. Kepler-22b orbits its star in the narrow region where liquid water is potentially possible on its surface.
Exoplanets are planets orbiting distant stars, far from our own solar system. Since the discovery of the first exoplanet 1992, over 700 more confirmed exoplanets have been found, with new discoveries coming in continuously. Given the methods used for finding them, most exoplanets to date have been exotic when compared to the Earth. Most known exoplanets are massive, and lie in extreme orbits either very close to or very far from their stars.
This diagram compares our own solar system to Kepler-22, a star system containing the first "habitable zone" planet discovered by NASA's Kepler mission. The habitable zone is the sweet spot around a star where temperatures are right for water to exist in its liquid form. Liquid water is essential for life on Earth. (NASA)
Now, NASA"s space telescope Kepler has found a planet which, like Earth, lies in its star's habitable zone. Orbiting the star Kepler-22, the planet (named Kepler-22b) lies in what some astronomers call the "Goldilocks region" which is generally neither too hot nor too cold for liquid water. Because many biologists believe liquid water is essential for the development of the kind of chemistry that can support life, scientists expect that extrasolar life-bearing, Earth-like planets -- if any exist at all -- are likely to be found in this habitable zone.
The Kepler spacecraft finds planets by means of the effect they have on their star's light. As a planet passes in front of its star, it blocks out some of the starlight as seen from Earth. If these drops in light are caused by an orbiting planet, the drops in starlight will occur at regular intervals. Because a planet's speed around its star is determined by the distance at which it orbits, by measuring the period of a planet's orbit astronomers can calculate its distance from its star. Kepler-22b orbits once every 290 days, fairly close to the Earth's own 365-day orbit around the Sun. This places Kepler 22-b within its star's habitable zone. By contrast, Mercury orbits the Sun in only 88 days, while Neptune takes just under 165 years to make a trip around the Sun. In addition, by timing the drops in light, astronomers can estimate the planet's size. Kepler 22b seems to be about 2.4 times the size of Earth, making it the smallest known planet to orbit in its star's habitable zone.
Just because it orbits in its star's habitable zone does not mean that Kepler-22b is necessarily anything like Earth. In the first place, Kepler's methods do not allow astronomers to find information such as a planet's total mass or composition, so it remains to be seen how Kepler-22b compares in character to Earth. Even though it is similar in size and orbit to Earth, many other factors will affect its ability to hold liquid water. Within our own solar system, the planets Venus and Mars both lie within the habitable zone, but neither has liquid water on its surface. Venus bears a thick toxic atmosphere that creates a burning hot surface, hot enough to melt even metals. Mars, to the contrary, is so cold that any water on its surface is immediately frozen into ice. Despite its tantalizing similarities to our own watery planet, astronomers still need more information in order to make any guesses about whether Kepler-22b does in fact hold any liquid water on its surface.