The Tenth PlanetHunters Habitable Zone Candidate

Another potential exoplanet orbiting in the habitable zone of its host star has been identified during a review of Quarter 5 light curves on PlanetHunters, this time around a G type star. The Kepler Input Catalog lists a radius of .8x Sol and a Teff of 5586(K) for this 13.9 Kmag target, designated as 4820550. The uncorrected light curve exhibits a fair degree of variability but so far in the public data set a total of four transits are found in Quarters 3, 5, 7 and 9 respectively. The period of these events is 202 days.

Some preliminary investigation indicates no obvious offsets, and KIC 4820550 does not appear to be included in the current Kepler Object of Interest catalog, nor does it appear in the official False Positive list.

The light curve before and after detrending to remove the stellar variability and trending:

To confirm this exoplanet candidate an important host of confirmation checks would need to be performed, including advanced screening for close background / foreground binaries and spectroscopic follow-up of the host star, but initial characterization is encouraging and shows this target may indeed reside inside the habitable zone where liquid water states could exist. The potential radius would rank this exoplanet slightly above that of the Neptune, placing it well outside the Super-Earth regime. Atmospheric colors and distribution in the header graphic were chosen arbitrarily; it's interesting to speculate if this body originally formed further out as an Ice Giant similar to the pair in our own solar system, and has since migrated inwards to its current position.

"Based on the plot, I have a transit depth of 2500ppm and duration of 4.56 hr. This works out to a planet with radius 4.4x RE and a somewhat high impact parameter of 0.86. It orbits the primary at a distance of 0.62 to 0.65AU and has a Teq of 273 to 280K, putting it in the HZ!"  -Kianjin
Source

This candidate increases the PH list of potential habitable zone targets to total of ten!

A New Potential Habitable Zone Exo for Kepler

The preliminary characterization of another long period exoplanet within the Habitable Zone of its host star has become possible recently by way of a third transit spotted in public Quarter 7 data, around target KIC 9958387.

This star is listed in the KIC catalog as a main sequence spectral type F just slightly larger than our own Sun, with a temperature of 6159(K) and an apparent visual magnitude of 13.5. The transits for this candidate were noted during manual searches by PH users and display a period of  237 days (with Transits 1 and 2 occurring in Quarter 2 and 4, respectively), if the aforementioned KIC values are accurate then combining them with the observed depth and duration should correspond to a body of roughly 5 times Earth radius and a Teq of 325 to 332K!

Significant pixel offsets that might indicate a contaminating eclipsing binary do not seem readily apparent here and lend some more weight to this initial characterization, although further follow up is of course required.

With a possible radius of ~32 000km, this exoplanet certainly would not resemble Earth despite being at a comfortable distance from the F star; any rocky core is likely well swathed with a thick envelope of gases that could come in a wide range of compositions. But considering the potential HZ orbit, it's interesting to speculate if significant H20, organic molecules and other basics required by the sort of prokaryotes and extremophiles we recognize may be present there--or if there are also any accompanying exomoons!

This is a crude scale approximation to Earth below; the atmospheric colors and distribution are randomly generated.

Graphics courtesy of NEA, Sphere Builder & Kian Jek