Kepler Planet Candidate Count – 2321

Kepler Planet Candidates
Histogram of Kepler Space Telescope Planet Candidates By Size
Image Credit: NASA Ames / Wendy Stenzel

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 summary:

  • Confirmed Planets: 61
  • Planet Candidates: 2,321
  • Eclipsing Binary Stars: 2,165

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).

Real Planets
Confirmed Planetary Systems Found from Kepler Data
Image Credit: NASA Ames / Jason Steffen, Fermilab Center for Particle Astrophysics

Exotic Watery World

GJ 1214b
GJ 1214b Orbiting a Star 42 Light Years from Earth
Image Credit: David A. Aguilar

New observations by the Wide Field Camera 3 aboard the Hubble Space Telescope have added new details to this previously studied exoplanet (also here). The study describing the planet’s properties will appear in the 1 March 2012 issue of The Astrophysical Journal.

The planet orbits a small star about 42 light years away, and is 6.5 times as heavy as the Earth. However, the density is very low, suggesting that the planet is mostly water, with a exotic solid water core, due to the high pressure.

More Planets than Stars – But Axial Tilt is the Key to Life

There is an average of more than one planet per star in the Milky Way
Image Credit: NASA / ESA / ESO

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.

Most recent discoveries have come from the Kepler Observatory using transit observations. Some of the earliest confirmation of gas giants came from radial velocity Doppler observations.

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.

Relative size of the three rocky planets around KOI-961
Image Credit: NASA / JPL-Caltech

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.

1,235 New Planets

Scientists working with the Kepler Observatory data announced Wednesday that they have 1,235 new exo-planet candidates and that they expect more than 90% to be confirmed. This is in addition to the 500 known exo-planets discovered during the past several decades.

Five of the planets are Earth sized and occupy the habitable zone of their star. The Kepler Observatory monitors a small section of space, which is 1/400 of the sky. If the entire sky were viewed, one might expect to find 2,000 Earth size inhabitable planets. Out of 54 planets in the habitable zone of their star, the remaining 49 habitable zone candidates range from super-Earth size (up to twice the size of Earth) to larger than Jupiter

Overall, the 1,235 planets break down as follows:

  • 68 are approximately Earth-size
  • 288 are super-Earth-size
  • 662 are Neptune-size
  • 165 are the size of Jupiter
  • 19 are larger than Jupiter

The image at the right is an artist's impression of the six planet system discovered around the star called Kepler-11. The system is tightly packed. All six planets would be inside the orbit of Venus.

The findings released yesterday are based on the results of observations conducted May 12 to Sept. 17, 2009, of more than 156,000 stars in Kepler's field of view

Artist impression of the six planets orbiting Kepler-11
Image Credit: Tim Pyle / NASA