The little rock planet is the size of our moon and every 13 days it celebrates New Year's as it completes an orbit of a star slightly smaller than our sun. Scientists say it's too hostile to support life (in terms of angry hotness, it's a lot like our mercury), though most likely the planet is just shy and would not be unfriendly once you got to know it. Any water on its surface would disappear very quickly, due to its close proximity to its sun. The planet's name is Kepler-37b, but you can call it Keppy.
NASA scientists were very excited to discover the little rock planet, not because they'd never seen one before, but because it's smaller than any planet they've ever discovered, either inside our solar system or out. They didn't even realize they could find such small planets so far away from Earth until they found this one; normally their telescopes only pick up big Jupiter-sized monsters.
Wee planets like the little rock planet obscure such a small amount of light from their stars when they pass in front of them, that most of the time, the resulting dimming could be mistaken for natural variability in the star's burning. The little rock planet dims its star's light by 0.002% every time it passes in front of it. It is the doll clothes of planets.
The little rock planet was discovered as a result of NASA's Kepler Mission, which explores other solar systems to locate planets comparable to Earth (Earth-sized or smaller) in the "habitable zone" of their suns, where liquid water (and possibly life) could exist.
The scientists say they can report with "a confidence of 99.95 percent" that the little rock planet is actually a planet and not just a mistake they made.