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Kepler  spacecraft has discovered 11 new planetary systems hosting 26 confirmed  planets, the National Aeronautics and Space Administration, NASA,  announced Thursday. These discoveries nearly double the number of  verified planets and triple the number of stars known to have more than  one planet that passes in front of its star.
These exoplanets, outside the Earth's solar system, orbit close to  their host stars and range in size from 1.5 times the radius of Earth to  larger than Jupiter, the largest planet in Earth's solar system.
"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.
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.
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.
Kepler detects this effect by measuring the changes, or so-called Transit Timing Variations, TTVs.
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 postdoctoral  fellow, and lead author of a paper confirming four of the systems.
Kepler-33, a star  that is older and more massive than our Sun, had  the most planets yet discovered. The system hosts five planets, ranging  in size from 1.5 to five times that of Earth.
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."