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KOI-961: A Mini-Planetary System
01.11.2012
Artist's conception of KOI-961
Artist's concept of the KOI-961, with three planets all smaller than Earth: The smallest of the three planets, called KOI-961.03, is about the size of Mars and actually located the farthest from the star, but is pictured in the foreground. The planet in the upper right is KOI-961.01, 0.78 times the radius of Earth. The planet closest to the star is KOI-961.02, with a radius 0.73 times the Earth's. Image credit: NASA/JPL-Caltech
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Astronomers using data from NASA's Kepler mission have discovered the cutest planet system yet: three planets, the smallest yet found, orbiting a red dwarf, the system being comparable to the size of the Jupiter-moon system. The planets orbit a single star, called KOI-961, and are 0.78, 0.73 and 0.57 times the radius of Earth. The smallest is about the size of Mars. This is a remarkable demonstration of the exquisite planet-finding capability of the Kepler instrument.

All three planets are thought to be rocky like Earth but orbit close to their star, making them too hot to be in the habitable zone, which is the region where liquid water could exist. Of the more than 700 planets confirmed to orbit other stars, called exoplanets, only a handful are known to be rocky.

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

Koi-961 system--planet size comparison to Earth and Mars.
This chart compares the smallest known exoplanets, or planets orbiting outside the solar system, to our own planets Mars and Earth. The smallest, KOI-961.03, is about the size of Mars with a radius of only 0.57 times that of Earth. Image credit: NASA/JPL-Caltech

Kepler searches for planets by continuously monitoring more than 150,000 stars, looking for telltale dips in their brightness caused by crossing, or transiting, planets. At least three transits are required to verify a signal as a planet. Follow-up observations from ground-based telescopes also are needed to confirm the discoveries.

The latest discovery comes from a team led by astronomers at the California Institute of Technology in Pasadena. The team used data publicly released by the Kepler mission, along with follow-up observations from the Palomar Observatory, near San Diego, and the W.M. Keck Observatory atop Mauna Kea in Hawaii. Their measurements dramatically revised the sizes of the planets from what was originally estimated, revealing their small nature.

The three planets are very close to their star, taking less than two days to orbit around it. The KOI-961 star is a red dwarf with a diameter one-sixth that of our sun, making it just 70 percent bigger than Jupiter.

Diagram comparing the KOI-961 planet system with the system of Jupiter and the Galilean moons.
This artist's conception compares the KOI-961 planetary system to Jupiter and the largest four of its many moons. All three planets take less than two days to whip around their star. The planet and moon orbits are drawn to the same scale. The relative sizes of the stars, planets and moons have been increased for visibility. Image credit: Caltech

"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."

Red dwarfs are the most common kind of star in our Milky Way galaxy. The discovery of three rocky planets around one red dwarf suggests that the galaxy could be teeming with similar rocky planets.

"These types of systems could be ubiquitous in the universe," said Phil Muirhead, lead author of the new study from Caltech. "This is a really exciting time for planet hunters."

The discovery follows a string of recent milestones for the Kepler mission. In December 2011, scientists announced the mission's first confirmed planet in the habitable zone of a sun-like star: a planet 2.4 times the size of Earth called Kepler-22b. Later in the month, the team announced the discovery of the first Earth-size planets orbiting a sun-like star outside our solar system, called Kepler-20e and Kepler-20f.

Prior to these confirmed planets, only six other planets had been confirmed using the Kepler public data.

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