A Planet Hunters Review of the Kepler Transit Candidate List
The Kepler team uses automated routines, specifically the Transiting Planet Search (TPS) algorithm, to search for transit signals in the Kepler light curves. TPS triggers on many repeated transit-like features in the light curves dubbed Threshold Crossing Events or TCEs. TPS generates many many TCEs, much more than the number of real extrasolar planets. The majority are false detections, but a few are real transits due to orbiting exoplanets. A subset of the Kepler team examine the TCE list and whittles it down to make the KOI (Kepler Object of Interest) list. A handful of Kepler scientists review each TCE and data validation report, results from a series of checks and test to help rule out astrophysical false positives that might produce a transit-like signal such as blended background eclipsing binary. It takes many many months for this process. The current Kepler planet candidate list released in January was using Q1-8, but there are many more Quarters of Kepler data available.
The Kepler team has made all of their data products publicly available in the extended mission. In December, the Kepler team released the list of 18,406 TCEs found during a search of Quarters(Q) 1-12 data and the resulting reports produced by their data validation pipeline. These Q1-12 TCEs have yet to fully searched by the Kepler team, meaning there are likely discoveries waiting to be found.
For the past few months I’ve been working with Chris and Arfon to set up a Planet Hunters review of the TCE list. Today the review site is live, and we need your help to review these potential transit candidates and identify the ones that are likely due to real planets. We’re using a version of the round 2 review interface, we used before to vet planet candidates for my short period planets paper. For each TCE, you’ll be presented with a light curve (from the data validation report) that has been zoomed-in and folded on the period determined by TPS so that the repeat transits all line up on top of each other.
We are asking you to confirm that there is a visible transit in the light curve identified by TPS (“Is there a transit?”) and determine whether the red line matches the light curve (“Does the red line fit the data?”).
With the folded light curves we can see smaller planets, the rocky ones that are so hard for most of us to see in the regular light curves we show on the Planet Hunters website. There are other teams who are using the TCE list in their research and as targets for follow-up observations, but where I think we have an advantage is that we have the ability to review the entire TCE list, not just the rocky planet transits but also the Jupiter-sized and in between.
We’re not in a race with the Kepler team who am I sure are also vetting the current released list, but I believe what is unique to this project and Planet Hunters is the ability to review uniformly all the ~18,000 potential transit signals identified by TPS. The current versions of the Kepler KOI list currently has not gone back and reanalyzed all the previous planet candidates detected in previous TPS runs with the longer observational baseline. So we’ll have the first independent vetting of the Kepler Quarters 1-12 TCE catalog providing a uniform selected sample of planet candidates.
Each TCE will require 10 independent review before being retired. Once we’ve gotten through identifying what looks to be real transit candidates from the non-detections, I’ll apply some additional cuts based on the output from the Kepler validation pipeline (like how the consistent is the depth of the odd and even transits to rule out eclipsing binaries and pixel offsets in and out of transit that might indicate a blended background eclipsing binary is producing the signal) to come up with our very own Planet Hunters planet candidate list from the TCEs.
I think this project will result in a very interesting paper looking at the frequencies of super-Earth to Neptunes to Jupiter-sized planets in the Kepler field, and also serve as an efficiency estimate for the Kepler KOI vetting process. If it goes well, we may consider making this a more permanent fixture on Planet Hunters for future releases of the Kepler TCE list. My goal is to have the first results from the TCE review to show in a poster at Protostars and Planets VI conference in Heidelberg, Germany in July.
If you are interested in participating and helping out with this project, you can go to http://tcereview.planethunters.org/ where you can join in and characterize the TCEs. Please do read through the tutorial on the front page. It will guide you on what you should be doing, as well as show you some examples of non-detections and good TCE transit detections.
Thanks in advance,