NASA found 715 new planets using data collected from the now-crippled Kepler space observatory. This bonanza of new discoveries, in 305 star systems, doubles the number of confirmed planets found outside our solar system. This discovery could quadruple the number of recognized Earth-sized planets.

The $600 million dollar mission was launched in 2009, in order to detect planets orbiting around 150,000 stars. The observatory looked for tiny dips in the brightness of stars, made as planets sass between their stellar companion and the Earth. The patch of sky Kepler examined can be seen from Earth, between the constellations Lyra and Cygnus.

In May 2013, the Kepler spacecraft stopped working, as its fine-pointing system went out of commission. Using a new technique, astronomers may be able to find hundreds of new planets from data collected by the malfunctioning telescope.

Until recently, planetary candidates discovered by Kepler had to be confirmed using second method. This was due to some multi-star systems acting like planets. But, researchers found a way to eliminate cases where a stellar orbit would have been unstable. Those identified had to be planets, doubling the number of known planets in one day.

"The fact that you can't have multiple star systems that look like planetary systems is the basis of verification by multiplicity," Jack Lissauer, of the Ames Research Center, said in a statement.

Each of the new discoveries were made in stellar families containing more than one planet. Each of the newly-found worlds is smaller than Neptune.

Four of the planets recently discovered are classified as super-Earths, each two-and-a-half times the size of Earth. They lie within the habitable zone around their stars, where water is likely to exist in a liquid state. They have been named Kepler 174d, 296f, 298d and 309c.

Before the discovery of exosolar planets, many astronomers believed other solar systems would be much like our own. The smallest members planetary family - Mercury, Venus, Earth and Mars - are closest to the Sun, While gas giants Jupiter, Saturn, Uranus and Neptune are found further away. Observations from Kepler and other observatories revealed planetary families around other stars are usually far different from our own solar system.

Planets orbiting distant stars are difficult to see directly, as the image is washed out by the light from its companion star. A new generation of telescopes being developed will be able to occult, or block out, light from the star.

The new data suggests as many as one in five stars may be accompanied by one or more worlds the size of the Earth. This could suggest billions of possible habitable worlds in the Milky Way.

Details of the study will be published in the Astrophysical Journal.

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