Data from the Kepler Observatory was used to discover another exoplanet, this one with the longest year yet found - 704 days. Kepler 421b has an orbit just slightly shorter than Mars, which revolves around the sun once every 780 days.

Planets around other stars are almost impossible to directly image. Their presence is usually determined from the effects they have on their local stars. Astronomers on Earth measure the slight gravitational push and pull between a planet and star to find new worlds. Other researchers find alien worlds when they pass between a star and our home world, temporarily blocking some of the light. Each of these methods is easier to detect from planets closer to their suns than those farther out.

The Kepler Spacecraft was launched on 9 March 2009, on a mission to discover worlds around alien stars. So far, astronomers have found roughly 1,800 new and suspected worlds outside the solar system. Due to the nature of the detection methods, these stars are nearly all much closer to their own stars than Kepler 421b.

"Finding Kepler-421b was a stroke of luck. The farther a planet is from its star, the less likely it is to transit the star from Earth's point of view. It has to line up just right," David Kipping from the Harvard-Smithsonian Center for Astrophysics, and lead author of the study, said.

Kepler 421 is an orange star, cooler than our own sun. The newly-discovered planet is about the size of Uranus, and orbits approximately 110 miles from its local star. These distances create frigid conditions - infrared light from Kepler 421b show a temperature of just - 135 degrees Fahrenheit.

Kepler observed a single patch of sky for a period of four years, recording the dimming and brightening of stars, as planets orbited. Despite this ultra-long exposure, the extreme length of the year meant Kepler 421b only passed between its star and our own twice.

Many astronomers believe that gas giants, like Jupiter and Saturn, form farther out from stars than rocky planets, triggered by the formation of ice crystals. This line between inner and outer worlds is called the "snow line."

The discovery of vast numbers of alien solar systems has revealed families of planets where gas giants orbit close to their suns. In fact, these are the easiest planets to find, as they have the greatest effect on the home stars. This could suggest that gas giants form outside the snow line, then spiral toward their suns over time. Oddly, Kepler 421b appears to violate that rule, likely forming in its present position.

An article announcing the discovery of Kepler 421b was accepted for publication in an upcoming edition of the Astrophysical Journal.

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