NASA’s Kepler space telescope has detected two extrasolar planets that are not much larger than Earth, and possibly smaller, making them the smallest exoplanets found to date.

The two planets orbit the same star, which astronomers have named Kepler-20. Located about 945 light-years away in the constellation Lyra, Kepler-20 is approximately 90 percent the size of our sun.

The two newly discovered exoplanets — designated Kepler-20 e and Kepler-20 f — have radii of 1.03 and 0.87 times the Earth and zip around their parent star in just 6.1 and 19.6 days, respectively. The previous record holder for smallest exoplanet had a radius 1.42 times the Earth.

The findings represents a milestone for astronomers.

“What we’re announcing today really demonstrates that the Kepler mission has the sensitivity to find Earth-sized planets,” said astronomer David Charbonneau from Harvard University, who is a member of the Kepler team, during a NASA press briefing on Dec. 20.

Models suggest that the planets are rocky, and mainly composed of iron and silicon. Both are too close to their parent star, and therefore too hot, to contain life. But if the more distant exoplanet, Kepler-20 f, had liquid water on its surface when it formed, the researchers said it might have held on to that water for billion of years, opening intriguing possibilities to extraterrestrial life’s formation.

The star, Kepler-20, is also known to host three other planets, with radii between two to three times that of Earth, making them more like Neptune than our home planet. All the exoplanets orbit closer to their parent star than Mercury does to our sun. Taken together, the Kepler-20 system looks like our own solar system done in miniatures.

But unlike our own solar system, where rocky planets like Earth and ice giants like Neptune are segregated into separate regions, the Kepler-20 system has these two types of planets freely mixing.

“It’s the first time we see something like this,” said Charbonneau, who added that explaining why the two planet types occur next to one another will be a major challenge to astronomers.

Earlier this month, the Kepler team announced the discovery of planet in the habitable zone, though that exoplanet was too large to be considered truly Earth-like.

Image: NASA/Ames/JPL-Caltech

Video: NASA