Saturday, August 25, 2012

The Nine PlanetHunters Habitable Zone Candidates

There has been lots of exciting news about alien worlds recently with the announcement of confirmation studies for over 41 Kepler exoplanets across 20 star systems, and in addition the fourth circumbinary system in the form of Kepler-38b. It seems not a day goes by anymore without at least one interesting exoplanet related pre-print hitting it is several!

The Kepler deluge is perhaps a double edged sword in that the KOI list is ever expanding as the mission proceeds through each Quarter, greatly outpacing the expansion of the confirmed candidate family and surely making it an excruciating process for science teams to decide which targets to allot precious follow-up resources on.

Some of the most lucrative candidates include those where early characterization suggests the exoplanet may reside squarely inside or on the periphery of the Habitable Zone of its host star: these are the potential crown jewel discoveries in an extended Kepler mission and I believe around ~46 such targets are currently listed, with Kepler-22b being the first such confirmed.

A round up of targets from PH not included on the current KOI list which may also possess HZ orbits (based on analysis of their transits c/t predetermined host star parameters) could potentially add a further nine systems. Further spectroscopic investigation is needed to rule out the possibility of blends, but the group seems relatively free of offsets and all possess good looking transits. While none approach the size of a true Earth analog, everything we're learning about the possibilities for exotic life in the Universe still renders them very interesting candidates, especially if considering that many of these bodies may be prone to possession of small to medium size exomoons. No exomoons have been confirmed yet in any system, but currently thirty habitable candidates are predicted on the lists over at the Habitable Exoplanets Catalog!

Additional caveats applicable to the derived impact parameters for the list below: an assumed albedo of 0.3 was used in most cases and the exact planetary mass, eccentricity and atmospheric compositions are not known at this time (the colors and features shown in banner image were chosen rather arbitrarily). Further ambiguity in the accuracy of the KIC parameters for the host star, or dilution from nearby sources, could also dramatically throw off the assumed effective temperatures and radius values.

KIC 4552729

Of the nine targets this is the only one that has received any real follow up so far and was one of two exoplanets discussed in the second PlanetHunters paper by Lintott, et al from February, 2012. The orbital period is 97.46 days and a best fit seems to indicate a planetary radius of 4.65Re, however the presence of an incomplete Transit Timing Variation cycle and some variation in the depths precludes full confirmation at this time, leaving 4552729 in the candidate realm until a background eclipsing binary can be wholly ruled out.


KIC 6878240

Found around a type K dwarf star smaller than the Sun and appears in Q2, Q4 and Q5 with a period of 135.5 days. Object is thought to be around 4.1Re with a possible Teq of 246K.


KIC 7899070

Transits are seen throughout the public data set. Assuming the host star has a radius of .49x Sol, this could be a 3.0Re exoplanet orbiting at the equivalent of .43AU and possessing a Teq of 254K. The long transit duration could be explained by eccentricity or error in the listed KIC value for the host star radius.


KIC 9480535

With several transits visible and a 13.25 hour duration, this could be a 3.5Re exoplanet orbiting the type G host star at a distance of .57AU and a period of 160 days. The assumed Teq is 327K.


KIC 10360722

This is the smallest candidate on the list with a potential radius of 2.7Re, however there is some concern that a background binary could be interfering with the light curve and further follow up is needed. Assumed period is 163 days, at a distance of .43AU and a Teq of 296K; the host star is Type K.


KIC 12735740

Only two transits in the public data thus far, but they look clean and decidedly unlike an EB profile. Also by far the largest candidate on the list with an assumed radius of ~12.0Re (or 1.12Rj). The host star is a 12.6Kmag type G dwarf and the characterization suggest a 282.5 day orbit at .86AU and possible Teq of 292K!


KIC 3326377

This is an interesting potential multi-candidate system that seems to have a possible 'Super-Mercury' exoplanet at a short period of 13 days and then a second candidate much further out at .65AU and a period of 198 days. Uncertainties exist with the host star parameters, but the second candidate could exist in a minimum of the Super-Earth radius domain (2.84Re to 4.4Re) and may lie just outside the HZ possessing a Teq of 225K. *Edit: This co-discovery candidate is also described at the official PlanetHunters blog, found here.

KIC 9958387

The subject of my last blog post and a recent HZ addition to the PlanetHunters list, displaying three very nice transits, with the latter visible in public Quarter 7 data. This candidate has a period of 236.7 days and an assumed radius of 5.1Re with Teq of 325 - 332K.


KIC 2581316

This final entry in the list bears some significant uncertainty and was the subject of another blog post by Tom Jacobs after analysis was renewed on it several weeks ago. It appears to have been associated at one time with the official Kepler False Positive list, although it does not appear there now. The KIC still lists "Unknown" for the host star values of stellar type, temperature and solar radius, however the 217 day period transits are quite good for an 11.7 Kmag target and no significant offsets appear to be involved. If we assume that the host star is a Type G with Teff of ~5200K and solar radius equivalent to or just short of Sol, this could put the candidate inside the habitable zone at .68-.71AU and rough Teq of ~265K. The assumed radius is 10.5Re or .84Rj, making it a large exoplanet in any case and similar in domain to the candidate suspected at KIC 12735740.


Various further reading and links of interest:

The Habitable Exoplanets Catalog @ PHL
Kepler: The Long Road to Other Worlds
How to Find a Habitable Planet -  Kasting
How to Build a Habitable Planet - Broecker & Langmuir
IAU 293: Formation, Detection and Characterization of Extrasolar Habitable Planets (upcoming)
What We're Looking for in the Perfect Exoplanet

Transit plots courtesy of Kian Jek. Comparison banner graphic created using Sphere-Builder app.

In respectful memory of Neil Armstrong, who inspired us all to explore Strange New Worlds

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