Kepler-37b is the smallest planet ever discovered outside of our solar system.
Astronomers made the discovery using the Kepler telescope that NASA launched in 2009 to search for earth-like planets. Since then, 861 planets have been spotted and only recently have scientists been able to detect planets that are similar in size to Earth or smaller.
Kepler-37b is slightly larger than our moon and about one-third the size of Earth.
University of California, Berkeley astronomer Geoff Marcy, one of the founding fathers of the planet-hunting field, said the discovery gives us an idea of how jam-packed the universe is. Marcy likened it to jelly beans in a jar.
Scientists estimate the surface temperature of the new planet to be a sizzling 700 degrees Fahrenheit - too hot to support life. It also lacks an atmosphere and water on its rocky surface.
Discoverer Thomas Barclay of the NASA Ames Research Center in Northern California was so excited when he spied the moon-sized planet that for days, he said he recited the "Star Wars" movie line: "That's no moon." It took more than a year and an international team to confirm that it was a valid planet.
The discovery is detailed in Thursday's issue of the journal Nature.
Scientists are looking for an Earth-size planet that's in the so-called Goldilocks zone, which is the sweet spot that's not too hot and not too cold where water, which is essential for life, could exist on the surface.
The Associated Press contributed to this report.