A hot, rocky planet discovered using the Kepler space telescope is the smallest ever found outside our solar system.

The new planet, called Kepler-37b, has a radius less than a third the size of Earth’s, making it roughly the size of the moon, a paper published online in the journal Nature reported Wednesday.

"The thing that really I find astounding about this is we’ve managed to find a planet that is smaller than any that we know of in our own inner solar system,” Thomas Barclay, a researcher at NASA-Ames Research Center, who led the study, said in an interview with CBC News.

Barclay noted that many of the first planets found outside our solar system were larger than the planets found in our own solar system, showing that stellar systems could be quite different from our own.

"Now we know that things are not only larger than what we have in our own solar system, but also smaller,” he said.

Kepler-37b is “significantly smaller” than Mercury, which has officially been the smallest planet in our solar system since Pluto was demoted to a “dwarf planet” by the International Astronomical Union in 2006.

Kepler-37b, shown in this artist's conception, orbits its host star every 13 days and likely has a surface temperature of in excess of 400 C. (NASA/Ames/JPL-Caltech )

Kepler-37b is the inner-most of three planets detected orbiting a star called Kepler-37, located about 210 light years away from Earth in the constellation Lyra.

The star is sun-like, but cooler and a little bit smaller than the sun, according to the paper.

Kepler-37b is thought to be rocky, with no atmosphere, like Mercury. Because it is very close to its star, its surface temperature is estimated to be a scorching 400 C. The little planet is also a speedy traveller, completing its journey around its star once every 13 days.

The two other planets in the system are Kepler-37c, which has a radius about 70 per cent the size of Earth’s, and Kepler-37d, which has a radius about double that of the Earth’s.

The Kepler space telescope, launched in 2009, is pointed at stars in the constellations Cygnus and Lyra located between a few hundred and a few thousand light years away, within our own Milky Way galaxy.

The telescope detects planets by measuring the brightness of stars over time and detecting dips in the brightness caused by planets passing in front of the star during the course of their orbits. The smaller the planet relative to the size of the star, the smaller the change in brightness and the more difficult the planet is to detect — even though smaller planets are expected to be more common than larger planets.

Lucky strike

Barclay said it was a bit of luck that allowed his research team to detect Kepler-37b from an extremely tiny signal. The star is relatively bright and doesn't have a lot of activity on its surface such as star spots and flares that could cause other variations in brightness. Such phenomena would make it more difficult to detect the dimming caused by planets.

"For most stars we are not so lucky," Barclay added.

While it's hard to extrapolate how common planets this size are from a single sample, he said, "the fact that we found one around one of the very few stars that we could find one is suggestive that small planets may be very common.”

In order to confirm that Kepler-37b is actually a planet and not a “false positive” caused by some other phenomena, such as planets passing in front of other stars in the background, the researchers took extra high-resolution images and measurements using ground-based telescopes.

They also used a computer to simulate possible scenarios that could generate a signal that looked like a planet, allowing them to rule out almost all those possibilities.

"The result was a 99.95 per cent confidence that Kepler-37b is a bona fide planet," Barclay said.

As of Wednesday, the Kepler mission had confirmed the detection of 105 planets.