Amid a flurry of planetary discoveries from NASA's Kepler space telescope, scientists announced Thursday they have found 26 new worlds around 11 stars outside the solar system.

Artist's concept of the Kepler observatory in space. Credit: NASA
 
The newly-discovered planets nearly double the number of confirmed planets detected by Kepler since its launch in 2009. The discovery also triples the number of stars known to harbor more than one planet.

Kepler has now identified 61 worlds outside the solar system, according to NASA.

"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. "Now, in just two years staring at a patch of sky not much bigger than your fist, Kepler has discovered more than 60 planets and more than 2,300 planet candidates. This tells us that our galaxy is positively loaded with planets of all sizes and orbits."

The planets announced Thursday are all part of multi-world systems, ranging in size from 1.5 times bigger than Earth to larger than Jupiter. One of the stars, named Kepler-33, has five planets slightly larger than Earth orbiting closer than Mercury to the sun.

Ten of the 11 new planetary systems were identified with a new technique and without the aid of ground-based observatories, which were previously necessary to verify candidate planets were real and not false positives.

Kepler is stationed in an Earth-trailing solar orbit and aims its 3.1-foot telescope toward constellations Cygnus and Lyra, observing a 10-degree-wide field containing 4.5 million detectable stars. Kepler is focusing on approximately 156,000 stars for the purposes of its research.

The observatory monitors the stars for dips in brightness, an indication a planet could be passing in front of it.

The image shows an overhead view of orbital positions of the planets in systems with multiple transiting planets discovered by NASA's Kepler mission. Credit: NASA Ames/Dan Fabrycky, University of California, Santa Cruz
 
"Confirming that the small decrease in the star's brightness is due to a planet requires additional observations and time-consuming analysis," said Eric Ford, associate professor of astronomy at the University of Florida and lead author of the paper confirming Kepler-23 and Kepler-24. "We verified these planets using new techniques that dramatically accelerated their discovery."

In multi-world systems like those announced Thursday, Kepler can detect changes in the planets' orbits due to the gravitational pull of neighboring planets. The technique is called Transit Timing Variations, or TTVs.

"By precisely timing when each planet transits its star, Kepler detected the gravitational tug of the planets on each other, clinching the case for ten of the newly announced planetary systems," said Dan Fabrycky, Hubble Fellow at the University of California at Santa Cruz and lead author for a paper confirming Kepler-29, 30, 31 and 32.

The discoveries are published in the Astrophysical Journal and the Monthly Notices of the Royal Astronomical Society.