NASA’s Kepler mission has discovered 11 new planetary systems hosting 26 confirmed planets. These discoveries nearly double the number of verified planets and triple the number of stars known to have more than one planet that transits, or passes in front of, the star. The planets orbit close to their host stars and range in size from 1.5 times the radius of Earth to larger than Jupiter. Fifteen are between Earth and Neptune in size. Further observations will be required to determine which are rocky like Earth and which have thick gaseous atmospheres like Neptune. The planets orbit their host star once every six to 143 days. All are closer to their host star than Venus is to our sun.
Such systems will help astronomers better understand how planets form. 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. According to researchers each of the newly found planetary systems holds two to five closely spaced transiting planets. Since these systems are tightly packed, the planets exert gravitational forces on one another, speeding up or slowing down their orbits. The orbital period of each planet is altered in the process. By measuring the orbital changes, Kepler can identify potential planets in the system. This method, known as Transit Timing Variation, can be used to verify alien planets without extensive ground-based observations. Five of the systems (Kepler-25, -27, -30, -31 and -33) contain a pair of planets, the inner one circling its star twice in the time it takes the outer planet to make one lap. Four of the systems (Kepler-23, -24, -28 and -32) are home to a pair of planets where the outer one orbits the star twice for every three times the inner planet circles the parent star. The system with the most planets is Kepler-33. The star, which is older and more massive than the sun, hosts five planets that range in size from 1.5 to five times that of Earth. All of these planets orbit closer to their star than any planet circles our sun. Kepler has discovered more than 60 planets and more than 2,300 planet candidates.