Ji Wang et al. (2013) report on the discovery of PH2 b, a Jupiter-size planet orbiting in the habitable zone of a Sun-like star catalogued as KIC 12735740. This discovery was made by the Planet Hunters project whereby human volunteers visually inspect the light curves collected by NASA’s Kepler space telescope to identify planetary transit signals which may be missed by computer programs.
Four transits of KIC 12735740 were detected by Planet Hunters. By noting the interval between 2 successive transits, the orbital period of PH2 b is 282.5 days. The best-fit model for the transit light curves suggests that PH2 b is a Jupiter-size planet with a radius of 10.12 ± 0.56 Earth radii. In addition, the temperature at the top of the planet’s atmosphere is estimated to be 281 ± 7 K. This places PH2 b within the habitable zone of its host star as defined by Batalha et al. (2012), where the temperature range defining the habitable zone is between 185 K and 303 K. Of course, PH2 b is unlikely to be habitable since it is a gas giant planet like Jupiter. Nevertheless, Ji Wang, a postdoctoral researcher at Yale University and lead author of the paper states, “any moon around this newly discovered, Jupiter-size planet might be habitable.”
Artist’s impression of a large moon orbiting a vastly larger gas giant planet.
The discovery of PH2 b is part of an effort which also found 42 planet candidates. Among these planet candidates, 20 appear to orbit within the habitable zones around their host stars. Most of these habitable zone planet candidates have sizes comparable to or larger than that of Neptune, although one planet candidate has a radius of 2.60 ± 0.08 Earth radii and may be a super-Earth or a mini-Neptune. For PH2 b, the confidence level that it is a bona fide planet is high enough for it to be deemed a planet rather than just a planet candidate.
- Ji Wang et al., “Planet Hunters. V. A Confirmed Jupiter-size Planet in the Habitable Zone and 42 Planet Candidates from the Kepler Archive Data”, 2013 ApJ 776: 10
- Natalie M. Batalha et al., “Planetary Candidates Observed by Kepler. III. Analysis of the First 16 Months of Data”, 2013 ApJS 204: 24