Kepler is a space telescope which searches for planets around other stars by looking for the small dimming in a star’s brightness when a planet happens to pass in front of the star. The latest data released by the Kepler team adds 1091 new planet candidates, bringing the total number of planet candidates to 2331. Of the new planet candidates, 196 are Earth-size, 416 are super-Earth-size, 421 are Neptune-size and 41 are Jupiter-size. The remaining 17 planet candidates are much larger than Jupiter and a few of them are unlikely to be planet candidates as they are too large.
Figure 1: Radius versus orbital period for each of the planet candidates in the B10 (Borucki et al. 2011a) catalogue (blue points), the B11 (Borucki et al. 2011b) catalogue (red points) and for this latest contribution (yellow points). Horizontal lines denoting the radius of Jupiter, Neptune and Earth are included for reference. (Credit: Natalie Batalha et al. 2012)
The addition of 1091 new planet candidates results in a 197 percent increase in planet candidates smaller than 2 Earth diameters and a 52 percent increase in planet candidates larger than 2 Earth diameters. There is also a 123 percent increase in planet candidates with orbital periods longer than 50 days and an 85 percent increase in planet candidates with orbital periods shorter than 50 days. More than 91 percent of the new planet candidates are smaller than Neptune. Compared to previous data releases, the current addition of new planet candidates has a greater proportion of smaller planets at longer orbital periods. Hence, each successive catalogue shows a clear progress towards Earth-size planets in wider orbits that place them within the habitable zones of their parent stars.
The cumulative list of 2321 planet candidates also contains 1790 unique stars. 245 of which are 2-planet systems, 84 of which are 3-planet systems, 27 of which are 4-planet systems, 8 of which are 5-planet systems and one of which is a 6-planet system. These stars with multi-transiting planets show remarkable coplanarity where the spread in orbital inclinations for a typical system of planets around a star is around 1.0 to 2.3 degrees. For the planets in our Solar System, the spread in inclinations is 2.1 to 3.1 degrees and if Mercury is excluded, the spread in inclinations become 1.2 to 1.8 degrees.
Figure 2: Radius versus equilibrium temperature for each of the planet candidates in the B10 catalogue (blue points), the B11 catalogue (red points) and for this latest contribution (yellow points). Horizontal lines denoting the radius of Jupiter, Neptune and Earth are included for reference. Also included for reference are vertical lines marking the inner and outer edges of the Habitable Zone as defined by Kaltenegger & Sasselov (2011) as well as the equilibrium temperature for a present Earth-Sun analogue (middle line). (Credit: Natalie Batalha et al. 2012)
For a planet to be potentially habitable, it is assumed that the equilibrium temperature of the planet has to be between 180 to 310 degrees Kelvin. Within the cumulative list of 2321 planet candidates, 46 of them have equilibrium temperatures within this range. Amongst them, 9 are super-Earth-size and 1 is Earth-size. In comparison, the actual average temperature of the Earth is 288 degrees Kelvin and without the atmospheric greenhouse effect, the equilibrium temperature of the Earth is 255 degrees Kelvin. Each successive catalogue shows a clear trend toward Earth-size planets at Earth’s equilibrium temperature.
As of February 2012, over 60 planet candidates from the previous catalogues have been confirmed to be true planets. This is exclusive of the latest 1091 new planet candidates that were added. A list of Kepler’s milestone discoveries from previous catalogues include:
- Kepler-10 b (Batalha et al. 2011), the first rocky planet discovered by Kepler
- Kepler-11 (Lissauer et al. 2011a), a star with 6-transiting planets
- Kepler-16AB b (Doyle et al. 2011), the first circumbinary planet
- Kepler-22 b (Borucki et al. 2012), a 2.38 Earth-diameter planet in the habitable zone
- Kepler-20 e and f (Fressin et al. 2012), Kepler’s first Earth-size planets
1. Natalie Batalha, et al. (2012), “Planetary Candidates Observed by Kepler, III: Analysis of the First 16 Months of Data”, arXiv:1202.5852v1 [astro-ph.EP]
2. Daniel Fabrycky, et al. (2012), “Architecture of Kepler's Multi-transiting Systems - II. New investigations with twice as many candidates”, arXiv:1202.6328v1 [astro-ph.EP]