NASA has now released the third catalog featuring all the candidate planets discovered by the Kepler Space Telescope. This catalog contains 2321 candidates found during observations between May 2009 to September 2010. The histogram above summarizes the findings in the 27 February 2012 Kepler Planet Candidate catalog release by size.
The first 43 days of Kepler data was released in June 2010 and has 312 candidate planets (Borucki et al, 2010). The second release (Borucki et al, 2011) covered the first 13 months of data and was released in February 2011. It contains 1,235 candidate planets.
The third catalog contains 2,321 planet candidates identified during the first 16 months of observation. 46 planet candidates orbit within the habitable zone (liquid water) and ten of them are near Earth size.
Planets are identified by measuring the brightness of more than 150,000 stars. Planets that pass in front of a star (“transit”) cause the light to dim slightly. Candidates are considered possible if Kepler detects three transits.
The artist’s rendering below depicts the multiple planet systems discovered by NASA’s Kepler mission. Prior to the third catalog, scientists had verified six systems with multiple transiting planets (shown t the left in red).
Kepler’s observations have verified planets in 11 new planetary systems (shown to the right in green). Many of these systems contain additional planet candidates that are yet to be verified (shown here in dark purple).
For reference, the eight planets of the solar system are shown in blue (far left).
With the forthcoming publication in the journal Nature on 12 January, it is estimated that there are more than 100 billion planets in our Milky Way galaxy. That means more than one planet per star, and results show that there are more rocky small Earth-like planets than giant Jupiter-size gas planets.
The conclusions in the Nature article are based on micro-lensing studies.
Recent results from the Kepler Observatory have shown the existence of three small, rocky planets around the star KOI-961, a red dwarf. These three planets, named KOI-961.01, KOI-961.02 and KOI-961.03, are 0.78, 0.73 and 0.57 times the radius of Earth. The smallest is about the size of Mars (see below). Follow-up observations were made by the Palomar Observatory, near San Diego, and the Keck Observatory atop Mauna Kea in Hawaii.
Since it is now clear that rocky planets exist around millions, if not billions, of stars, the question arises as to whether there is life on them, and whether it may resemble life on Earth.
Whether a planet exists in the “Goldilocks” region around a star depends on many factors. Three factors include the type of star, how far away from the star the planet resides and the atmospheric pressure of the planet. A red dwarf, such as Gliese 581, means the planet has to be closer than the Earth to our Sun. A white hot star means the planet has to be farther away. And if the atmosphere is low, like Mars, or to high, like Venus, liquid water is not likely.
A fourth factor is axial tilt. If a planet has no axial tilt (the spin axis is perpendicular to the plane of its orbit around the star) then the polar regions freeze and the equatorial regions bake. There is little exchange between these regions due to atmospheric circulation. Axial tilt, such as the Earth has, allows distribution of heat between the equator and the poles.
Even if a planet has axial tilt, a recent study shows that interaction at a close distance (within the “Goldilocks” region) with red dwarf will eliminate axial tilt in less than 100 million years. Bacteria on Earth required 1,000 million years to evolve. Theoretically, a planet with no axial tilt could possess bands between the equator and the poles where liquid water would exist. But, it is quite possible the atmosphere would collapse, with gases being driven off into space at the very hot equator, and freezing solid on the ground at the poles. Such a possibility faces the planets around KOI 961.
Systems with stars like our Sun present better possibilities. The “Goldilocks” conditions exist much farther out, and axial tilt is eliminated much more slowly, as our Earth is witness. Systems such as Kepler-22b are good candidates.
The conclusion drawn from these studies is that systems similar to our Solar System present the best opportunities for life.