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.”
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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.
References:
- 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