"Prior to the Kepler mission, we knew of perhaps 500 exoplanets across the whole sky," said Doug Hudgins, Kepler program scientist at NASA Headquarters in Washington.
Artist's concept of the KOI-961 star, a red dwarf with the smallest exoplanets yet discovered in its orbit (Image courtesy NASA/JPL-Caltech)
Of the more than 700 exoplanets now confirmed, only a handful are known to be rocky, like the Earth.
In size, 15 of the newly discovered planets are between Earth and Neptune, which is 17 times the mass of Earth.
The exoplanets orbit their host stars once every six to 143 days. All are closer to their host stars than Venus is to our Sun.
"Astronomers are just beginning to confirm thousands of planet candidates uncovered by Kepler so far," said Hudgins. "Finding one as small as Mars is amazing, and hints that there may be a bounty of rocky planets all around us."
Kepler identifies planet candidates by repeatedly measuring the change in brightness of more than 150,000 stars to detect when a planet passes in front of the star. That passage casts a small shadow toward Earth and the Kepler spacecraft.
Each of the new confirmed planetary systems contains two to five closely spaced transiting planets.
In tightly packed planetary systems, the gravitational pull of the planets on each other causes some planets to accelerate and some to decelerate along their orbits. The acceleration causes the orbital period of each planet to change.
The Kepler spacecraft and its field of view near by constellation Cygnus (Image courtesy NASA)
Planetary systems with TTVs can be verified without requiring extensive ground-based observations, accelerating confirmation of planet candidates. The technique also increases Kepler's ability to confirm planetary systems around faint, distant stars.
Five of the systems (Kepler-25, Kepler-27, Kepler-30, Kepler-31 and Kepler-33) contain a pair of planets where the inner planet orbits the star twice during each orbit of the outer planet.
Four of the systems (Kepler-23, Kepler-24, Kepler-28 and Kepler-32) contain a pairing where the outer planet circles the star twice for every three times the inner planet orbits its star.
"These configurations help to amplify the gravitational interactions between the planets, similar to how my sons kick their legs on a swing at the right time to go higher," said Jason Steffen, the Brinson postdoctoral fellow at Fermilab Center for Particle Astrophysics in Batavia, Illinois, and lead author of a paper confirming four of the systems.
Artist's concept of the Kepler-35 planetary system, in which a Saturn-size planet orbits a pair of stars. (Image by Lynette Cook courtesy NASA)
"The approach used to verify the Kepler-33 planets shows the overall reliability is quite high," said Jack Lissauer, planetary scientist at NASA Ames Research Center at Moffett Field, California, and lead author of the paper on Kepler-33. "This is a validation by multiplicity."
The decrease in the star's brightness and duration of a planet's transit in front of its star, combined with the properties of its host star, present a recognizable signature, the scientists say.
When astronomers detect planet candidates that exhibit similar signatures around the same star, the likelihood of any of these planet candidates being a false positive is very low.
At least three transits are required to verify a signal as a planet. Follow-up observations from ground-based telescopes confirm the discoveries.
These findings are published in four separate papers in the "Astrophysical Journal" and the "Monthly Notices of the Royal Astronomical Society."
Click here to view more than a dozen NASA videos of the Kepler spacecraft and its exoplanet discoveries.
In early December, NASA announced Kepler-22b, the mission�s first confirmed habitable zone planet and later in the month, announced Kepler-20e and Kepler-20f, the first Earth-size planets discovered around a Sun-like star.
NASA's Ames Research Center in Moffett Field manages Kepler's ground system development, mission operations and science data analysis. NASA's Jet Propulsion Laboratory, Pasadena, California, managed the Kepler mission's development.
Ball Aerospace and Technologies Corp. in Boulder, Colorado, developed the Kepler flight system and supports mission operations with the Laboratory for Atmospheric and Space Physics at the University of Colorado, Boulder.
The Space Telescope Science Institute in Baltimore archives, hosts and distributes Kepler science data. Kepler is NASA's 10th Discovery Mission and is funded by NASA's Science Mission Directorate at the agency's headquarters in Washington.