The 13th and 14th PlanetHunters Habitable Zone Candidates

Follow up to some visual transit hunting done earlier this year has exposed two additional targets with possible habitable zone exoplanets on PH Talk: one candidate seems comparable with Neptune while the other is smaller and may be reside in either the mini-Neptune or super-Earth domain, depending on how it accreted. Neither appears in the current Kepler Object of Interest or False Positive lists on MAST.

KIC 6106282

The host star is listed in the KIC as a Type M with radius 0.8 compared to Sol at a Teff of 3912(K). The magnitude is 15.1 and the light curve is fairly variable even after pipeline corrections. Two transits appear to be lost in data gaps for the public time series, but the four remaining do not appear to exhibit clear pixel offsets which would indicate they are the result of a background eclipsing binary.

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 Transits display a period of ~101 days and initial characterization suggests the exoplanet may exist as a 2.58x RE orbiting at 0.385 to 0.409AU with an effective temperature of 237 to 245K. This is a little on the chilly side, but if accurate could still be offset by a variety of atmospheric conditions and provide an environment where liquid water states can exist.

KIC 9147029

The star is listed as a magnitude15.3 Type F with a stellar radius just short of Sol and a Teff of 6216(K). Two potential issues exist for this longer period candidate: there are currently only two transits visible in the public data set (one at BJD 419 in Q4 and another at BJD 816 in Q9) and a second point source in the form of KIC 9147033 lies right nearby in the FOV.

However the two transits look pretty good and appear to be free of APOs; if initial characterization is accurate this could be a 4.5x RE exoplanet orbiting in a 397 day period at a distance of 1.026AU. Similar to KIC 6106282, this candidate may be a bit questionable as a habitable zone contender with a slightly chilly potential Teq of 243 to 250K. But the suspected radius and potential for exomoons that receive additional heat generated by tidal forces tipped the scales for its inclusion on the PH HZ list. A third transit and spectroscopic follow up of the system will be required to lock this down further

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The search continues at PlanetHunters for smaller exoplanets that would truly rank as potentially habitable candidates, as opposed to just orbiting within the habitable zone of the host star.

The Twelfth PlanetHunters Habitable Zone Candidate

Sharp eyed volunteers at PlanetHunters have identified another set of transits that seem to reveal another exoplanet candidate in the habitable zone of its host star.

One of the transits visible in Quarter 6 PDC_SAP flux

The target is KIC 4947556; a magnitude 13.2 star interpreted in the catalog as a Type K dwarf of 4907(K) and a stellar radius of 0.9 compared to Sol. The public light curve exhibits significant variations throughout the time series, resulting in a group of transits that are not immediately obvious to the eye despite their ~141 day period.

Example of PDC_SAP time series for Quarters 4 and 5

The surrounding field of view is relatively clear and strong offsets are not present in the data correlating to the transit events, however there is a possible secondary source in the form of a very slight 'bulge' that will need to be evaluated for interference along with the usual checks for confusion with a faint eclipsing binary in the background.

Surrounding field sources

Preliminary fitting and characterization work done by PH Talk users Zoo3hans and Kianjin indicate this candidate may exist in the form of a 3.68xRe 'Warm Neptunian' orbiting at 0.52AU with an equilibrium temperature of 278 to 283K. 

                                          PH Talk  Discussion

This candidate follows close on the heels of the potential HZ exoplanets found earlier this month at KIC 9886255 and KIC 4820550.


Further recent Exoplanet reading:

New Mission Manager Update

Do two of the top 6 'Potential Habitable Planets' @ Gliese 581 actually exist?

Extremities of the Exoplanet Density Distribution  

PlanetHunters Blog @ Zooniverse: Sounding Good

The Nine PlanetHunters Habitable Zone Candidates

The Eleventh PlanetHunters Habitable Zone Candidate

Manual reviews of the light curve for Kepler target star KIC 9886255 have revealed the presence of some intriguing transits on a ~106 day period, visible in the public data for Quarters 3 through 9.

A transit in corrected flux for Quarter 3

The Kepler Input Catalog calls out 9886255 as a Type K star of 15.8 apparent visual magnitude, 5137(K) and an assumed radius of  0.6 compared to Sol.

A potential follow up issue exists with this target due to another source close in the field, identified as KIC 9886258 and seen with tag '#2" in the Skyview image below:

Currently this neighboring source is not a monitored target and no light curve exists on record for it. Preliminary investigation suggests no active pixel offset is occurring that might indicate the noted transits actually belong to this star as opposed to 9886255, however the close proximity and risk of blended flux is a cause of concern c/t analysis of this candidate.

If we take the data at face value, the depth and duration suggest a potential habitable zone exoplanet, albeit one that may be in a slightly eccentric orbit around its host star.

 Discussion Source

"Here's close-up of the transit, and although the flux values are spread quite widely and individual transits show a lot of variation, the averaged plot with a polynomial fit shows a U-shape and flat-bottom . The measured depth is 2500ppm with a duration of 7.37hr. This is at odds with a computed central transit of 5.6hr. If this is planetary this could be explained by an eccentric orbit. Using the stellar parameters provided in MAST, I estimate a 3.26x RE planet, at 0.4 AU and a Teq of 274K, putting this in the HZ." -Kianjin

Like most of the other habitable zone candidates we've seen at Planethunters, this exoplanet would weigh in far heavier than Earth; if the calculated radius is accurate it belongs to the Neptune or sub-Neptune domain as opposed to a Super-Earth, and likely has a thick, high pressure atmosphere. Additional Kepler data is going public soon so it will be interesting to see if any longer period transits show up for this system that may have been lost previously in time series data breaks and gaps.

Latest numbers have the official Kepler count at 2405 exoplanet candidates awaiting follow up and amongst confirmed systems 127 of the 834 known!

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

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 ArXiv.org--usually 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.

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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.

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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.

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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.

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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.

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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!

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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.

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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.

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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.

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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.

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

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