The tiny worlds are clustered around a red M-dwarf star called KOI-961 that is itself among the more diminutive stellar objects in the universe. The star is just a sixth as wide as our sun, or about 70 percent bigger than Jupiter.
"It's almost like you took your shrink gun and set it to seven and zapped the planetary system—the whole thing shrunk," said study co-author John Johnson, a researcher with NASA's Exoplanet Science Institute at the California Institute of Technology.
All three of the KOI-961 planets are thought to be rocky, like Earth and Mars. But they circle fairly close to their star, taking roughly two days to complete their orbits.
That means, even though the host star is dimmer than our sun, the planets are far too hot for liquid water—and thus life as we know it—to exist on their surfaces.
"The surface temperatures of these planets range from 720 Kelvin to 450 Kelvin [836°F to 350°F] ... so the coolest one is almost a factor of two too hot" to be habitable, Johnson said.
Still, the discovery hints that many more small, rocky planets exist across our Milky Way galaxy.
"It's like if you see one cockroach, you've seen ten—they're hiding," Johnson said.
"If you see one transiting planet, then you see ten transiting planets, really. So that really implies that just from this one system alone ... it looks like one in three M-dwarfs has a rocky planet system," he said.
"Combine that with the fact that seven out of every ten stars in our galaxy are M-dwarfs, and ... [it seems] we're very lucky to live in a universe where rocky planets are so common."
New Planets Among Many Kepler Finds
Johnson and colleagues found the new planets by following up on data released to the public last February from NASA's Kepler spacecraft.
Kepler looks for planets by searching for dips in starlight as another object transits—or passes in front of—a star, as seen from Earth. So far the Kepler mission has confirmed 35 planets and uncovered another 2,326 planet candidates.
Kepler's main goal is to look for Earth-size planets around sunlike stars, which it does by essentially staring at a patch of sky near the constellation Cygnus.
Of the roughly 150,000 stars in the spacecraft's field of view, just 3,000 or so are M-dwarfs, Johnson said.
The team was inspired to try and confirm the planets around KOI-961 after learning of the star's close match to a known M-dwarf called Barnard's star a mere six light-years from Earth.
An amateur astronomer named Kevin Apps in the U.K. "tipped us off to the key to solving the whole problem," Johnson said, when Apps realized the close similarities between Barnard's star and KOI-961.
Sitting about 130 light-years away, KOI-961 is distant enough that the initial Kepler data underestimated the star's size.
But by taking the known properties of Barnard's star, such as its size and temperature, and gathering more detailed data on KOI-961 from ground-based observatories, the scientists could more accurately estimate the Kepler star's mass, size, and brightness.
With a better picture of KOI-961, the scientists could then look at the light dips caused by the suspected planets and better estimate their sizes.
The team calculates that the three worlds are 0.78, 0.73, and 0.57 times the radius of Earth.
Tiny Worlds Under Scrutiny
Looking at historical images of the stars around KOI-961 going back 60 years also helped the team determine that the transit dips are almost definitely caused by planets and not by other background stars crossing between us and the main star.
According to Johnson, one goal for future studies of the KOI-961 system is to try to find out more about the characteristics of the three tiny planets.
It's possible, for instance, that other studies would be able to tell whether the rocky worlds are actually the cores of gas giants that got stripped down over time by their host star's powerful radiation. (Related: "Stars Can Strip Gas Giants Naked.")
The new work could also inspire harder looks at the handful of M-dwarfs in Kepler's patch of sky to search for more such miniature planetary systems, he said.
"The longer Kepler goes, the more of these planets we will find."