Astronomers have discovered the smallest planet beyond the solar system: a mini-Mercury that is hellishly hot, probably rocky and lacking atmosphere and water.

The planet, known as Kepler-37b, is one of a trio of planets orbiting a yellow star similar to the sun that is located about 210 light-years away in the constellation Lyra.

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The planet circles its parent star every 13 days. Mercury’s orbit, by comparison, is 88 days. Sibling planets Kepler-37c, which is slightly smaller than Venus, and Kepler-37d, about twice the size of Earth, have orbital periods of 21 and 40 days respectively. The whole system would fit within the orbit of Mercury.

"We're really finding there is a great diversity in planetary systems," Thomas Barclay, with NASA's Ames Research Center in Mountain View, Calif., told Discovery News.

"When we first found exoplanets, they were all much larger than anything we have in the inner solar system. We didn't know of anything that was smaller, so we didn't know about the architecture of other star systems at the low range. This is the first time we’ve been able to probe the smallest range, smaller than anything we have in our solar system,” Barclay said.

Kepler-37 is a sun-like star, though slightly smaller and cooler than our sun, a G-type star. Its innermost planet, Kepler-37b, which is about the size of Earth's moon, is estimated to have a surface temperature of about 800 degrees Fahrenheit.

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“This particular one is nowhere near habitable. It’s 10 times closer to its star than we are,” astronomer Eric Ford, with the University of Florida in Gainesville, told Discovery News.

The planets were found with NASA’s Kepler space telescope, launched in 2009 to look for Earth-sized planets positioned in so-called habitable zones where liquid water, believed to be necessary for life, can exist on their surfaces.

The telescope monitors about 150,000 sun-like stars for minute changes in the amount of light coming from the stars. The observations are then analyzed to see if the dips are caused by orbiting planets passing in front of the stars, relative to the telescope’s point of view.

The discovery lends weight to the belief that planet occurrence increases exponentially with decreasing planet size, Ford noted in a paper published in this week’s Nature.