Kepler finds three smallest exoplanets so far

Artist's conception of the KOI-961 system. Credit: NASA/JPL-Caltech

The Kepler space telescope has been finding exoplanets by the thousands, and as the mission progresses, it has been able to detect smaller and smaller planets over time. Indeed, it was just recently announced, among other significant discoveries, that the smallest exoplanets have (again) been found.

These three planets all orbit one star, KOI-961, a red dwarf star only about one-sixth the diametre of our Sun. They all orbit very close to the star, taking less than two days to complete an orbit, and are only about 0.78, 0.73 and 0.57 times Earth’s radius. The smallest is actually close to the size of Mars!

According to Doug Hudgins, Kepler program scientist at NASA Headquarters in Washington, “Astronomers are just beginning to confirm thousands of planet candidates uncovered by Kepler so far. Finding one as small as Mars is amazing, and hints that there may be a bounty of rocky planets all around us.”

Size comparison of smallest known exoplanets to Earth and Mars. Credit: NASA/JPL-Caltech
Scale comparison of KOI-961 planets to Jupiter and its four largest moons. Credit: NASA/JPL-Caltech

The discovery was made by a team led by astronomers from the California Institute of Technology in Pasadena.

The easiest exoplanets to detect are larger gas giants, like Jupiter or Saturn, and planets with very tight orbits around their stars (and smaller, dimmer stars). As Kepler accumulates more data, it is able to detect both smaller planets like Earth or even smaller as in this case, and planets that orbit farther out from their stars.

“This is the tiniest solar system found so far,” said John Johnson, the principal investigator of the research from NASA’s Exoplanet Science Institute at the California Institute of Technology in Pasadena. “It’s actually more similar to Jupiter and its moons in scale than any other planetary system. The discovery is further proof of the diversity of planetary systems in our galaxy.”

Interestingly, the new planets were found by comparing KOI-961 to another very similar star, Barnard’s Star. This aided in determining the planets’ sizes.

Red dwarfs are the most common type of star in the galaxy, so discoveries such as this help to reinforce earlier conclusions that smaller rocky planets are common. If this one red dwarf star has at least three planets, than how many other red dwarfs do also?

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