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1,924 new KOIs

Below is an excerpt of an interview with Michael Hass, Kepler Science Office Director, by Michele Johnson. Read entire interview "New NASA Kepler Mission Data."

Excerpt: On May 28, 2013, NASA's Kepler mission delivered new data to the NASA Exoplanet Archive.
[MH=Michael Haas; MJ=Michele Johnson]

MJ: Mike, what information has the Kepler mission recently delivered to the NASA Exoplanet Archive?

MH: the NASA Exoplanet Archive [is] a majority of the Kepler Objects of Interest (KOIs) found by searching the data from Quarters 1 to 12 (May 2009 – March 2012) for transit-like signatures. ...the current delivery also includes 1,924 new KOIs.

flow diagram of Kepler data through the stages of becoming a planet candidate.
This diagram illustrates the flow of Kepler data through the stages of becoming a planet candidate. Source: NASA Ames/W. Stenzel
Photo of Michele Johnson
Michele Johnson,
Kepler Public Affairs Officer.

MJ: That's exciting! Does this mean that the Kepler mission has added 1,924 new planet candidates to the count?

MH: No. The 1,924 new KOIs have not been completely analyzed yet. The term KOI means exactly what the name implies – Kepler has declared these to be “objects of interest,” not planetary candidates. By promoting these transit-like signatures to KOI status, all we are saying is that their light curves contain interesting patterns of repetitive dips that might indicate the presence of a transiting planet.

However, there are several other ways to produce similar looking transit-like patterns. For example, the dips could be due to stellar variability, excess detector noise, other transient events associated with the spacecraft, or a background star occulting a second background star (i.e., a background eclipsing binary). We use the term “false positive” to describe those KOIs that are explainable by means other than the planetary hypothesis. We know that with further analysis, many of these new KOIs will become false positives.

MJ: If you haven’t finished the analysis, why are you releasing this information now? It seems rather preliminary.

MH: You are right, it is preliminary, but ...We started with the light curves of 192,313 stars that were observed for some or all of Quarters 1-12. ...When we began searching the Q1-Q12 data last fall, we identified 18,406 threshold-crossing events (TCEs) ... events that look transit-like. ...The criteria required to pass this first set of tests are intentionally lenient..., rather than to miss some really good events (i.e., small, Earth-size candidates in the habitable zone – the hardest candidates to find).

MJ: So, what happens next? Have you been analyzing the Q1-Q12 TCEs to figure out which are the most interesting?

photo of Michael Haas
Michael Haas,
Kepler Science Office Director.

MH: That is exactly right. We evaluated each TCE using objective criteria that are difficult to program into a computer. This exercise is called “triage” because it is a relatively quick assessment that eliminates the obvious false positives, while retaining anything that looks even remotely transit-like for further assessment. During this exercise, most of the events produced by spacecraft transients and stellar variability were discarded. This is process step 1 in the figure 'The Making of Kepler Planet Candidates.'

MJ: Is every TCE that passes triage automatically promoted to KOI status?

MH: No. If at least two scientists determine that a TCE looks transit-like, then the light curve is fit with a computer model of a transiting planet. If the model fit looks reasonable, then the TCE is promoted to KOI status....

...MJ: With previous Kepler data releases, the term 'KOI' was synonymous with planet candidate. Can you explain what has changed?

MH: This is a common misperception. Actually, the definition of KOI has not changed; but our reporting philosophy has. In the past, the Kepler mission published lists of KOIs that were deemed to be planet candidates; and separately posted the KOIs that were declared false positives at MAST (Mikulski Archive for Space Telescopes). This may have given some the mistaken impression that all KOIs are planet candidates, but this has never been the case. ...The reporting philosophy has been modified so that all KOIs can be archived in one place....

MJ: In the Q1-Q12 data set there are a surprising number of KOIs with orbital periods near one Earth-year. Do Earth-size planets tend to prefer Earth-like periods?

MH: Excellent question. Remember that the Kepler spacecraft orbits the sun every 371 days. Given its extremely stable environment, some noise sources associated with the local detector electronics exhibit repetitive behavior with this periodicity. Since these electronics read-out the charge-coupled devices (CCDs), this noise is intertwined with the astronomical signals in such a way that the two are almost impossible to disentangle. Hence, this repetitive noise can mimic the signature of a transiting planet....

...MJ: To summarize, the new 1,924 KOIs are not fully analyzed and not all the Q1-Q12 KOIs have been delivered yet. What guidance do you have for the scientific community about using the Q1-Q12 data now?

MH: The value of this delivery greatly depends on your scientific objectives. If you are looking for interesting KOIs to study or observe, then we have narrowed the search down from 192,313 light curves to 1,924. That’s a big help. If you are trying to understand the statistical population of small planets in the galaxy, this delivery isn’t going to hand you what you need right now. Stay tuned; good planets are hard to find. A team member once said that searching for planets is not like “looking for a needle in a haystack,” but more like looking for an aluminum needle made out of one aluminum alloy in a pile of needles made out of a different alloy....

See full interview.

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