By far the majority of the planets discovered to date were detected by NASA's Kepler satellite, which uses the transit technique. Kepler is a one-meter telescope that stares at one field containing 170,000 stars, looking for variations due to transits in all of them at once. Over a period of five years, images were taken every seven minutes (The scientific head of Kepler, NASA’s Bill Borucki, called it the most boring mission ever!). Even though transits are rare because the system has to be almost perfectly aligned, the spacecraft achieved success through good statistics and by gathering data for a long time. This highly successful mission has uncovered a bewildering variety of exotrasolar planet systems, from small to big, from hot to cold, and systems with many planets orbiting one star, and even planets orbiting multiple stars. One system discovered by Kepler has seven confirmed planets and two unconfirmed planets, making it as rich as our Solar System.
Kepler doesn't orbit the Earth itself, rather it was placed in an orbit around the Sun. It follows the Earth on a slightly slower 372.5 day orbit, so its orbit is called Earth-trailing. This orbit was chosen so that Kepler can stare at one region of the sky continuously for many years without being interrupted by the Earth Moon or Sun. The patch of sky chosen for Kepler is near the constellations Cygnus and Lyra. It was chosen because it is away from the ecliptic plane (the plane in which the Earth orbits the Sun) and because it has a high density of stars.
The mission's stated science goals from the NASA Kepler web site are to:
- Determine the abundance of terrestrial and larger planets in or near the habitable zone of a wide variety of stars.
- Determine the distribution of sizes and shapes of the orbits of these planets.
- Estimate how many planets there are in multiple-star systems.
- Determine the variety of orbit sizes and planet reflectivity, and the sizes, masses, and densities of short-period giant planets.
- Identify additional members of each discovered planetary system using other techniques.
- Determine the properties of those stars that harbor planetary systems.
Like all transit surveys Kepler works by staring at stars and waiting for them to get temporarily fainter. Since Kepler is in space, however, it can do this continuously. By contrast, a ground-based telescope could only observe at night, and only then if there were no clouds. This has allowed Kepler to achieve unprecedented sensitivity and has given it the ability to look for planets in the circumstellar habitable zone (CHZ). This is the region around a star where liquid water could exist on the surface of a planet. As listed above, searching for a planet like the Earth in the habitable zone is the number one goal of the Kepler mission.
As of mid 2015, the Kepler mission has identified over 4800 planet candidates. About 1000 have been officially confirmed, but due to the careful processing and selection that occurs, at least 90% of the planet candidates are expected to be real. These numbers are updated frequently as the mission data is analyzed. Kepler lost some of its core capability in 2013, so its most sensitive observations are now complete.
The Kepler team has not yet announced the discovery of an Earth-analogue: a 1 Earth-mass planet in the CHZ of its star, but they have found many planets slight larger than the Earth in the CHZ. The fist such planet discovered was Kepler-22b. Note that Kepler stars are named in the order in which they are discovered to have planets and planets are named in the order in which they are discovered around a star, starting with lower-case b. So Kepler-22 is the 22nd star found by Kepler to have a planet and b is the first planet discovered around the star. Kepler-22 is itself a Sun-like star. The planet Kepler-22b is 2.4 times the size of the Earth. We don't know its mass yet but it is probably no bigger than 36 times the mass of the Earth. This is a much bigger planet than Earth but the interesting thing about Kepler-22b is that its orbital period of 289 days places squarely in the middle of the CHZ, the first transiting planet discovered in this region.
One very interesting result from Kepler is the prevalence of multiple planet systems. These systems with more than one planet per star make up about one third of all Kepler planet candidates. One example is Kepler-20, a Sun-like star with at least 5 planets. The closest planet Kepler-20b is about 9 times bigger than the Earth and orbits very close to the star. The furthest one so far discovered, Kepler-20d, is even bigger.
Yet another intriguing result from Kepler is the discovery of planets orbiting binary stars. The first of these to be discovered, Kepler-16b, was dubbed the "Tatooine" planet due to its resemblance to the planet from the movie Star Wars. The next of these circumbinary planetary systems to be discovered, Kepler-47, is known to host two planets, both of which orbit both stars. The outermost planet, Kepler-47c, is yet another planet in the CHZ, though this time it is really the circumbinary habitable zone. These discoveries have dramatic implications for the formation and frequency of planets. It was long thought that the dynamics of binary star systems would make planet formation difficult at best. Furthermore most stars in our galaxy are in multiple star systems. These circumbinary systems hint that planets can form around binaries after all and that planets may be even more common than astronomers anticipated.