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Exoplanet identification using Kepler data results in one third of false positives

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Posted January 8, 2014

The current exoplanet detection techniques are mainly based on the measurement of the planetary transit signal, which is received when a potential planet orbits across the face of a star. Probably the most important drawback of this method lies in its basic principle: the transit-like signal may be generated by other celestial bodies, including variable or eclipsing binary stars.

Possible scenarios of false positive planetary transit signals: a, A gas-giant planet blocks a small amount of starlight as it passes in front of its host star. The resulting drop in light is similar to that produced by other systems, as follows: b, an orbiting brown-dwarf or low-mass star, both of which have radii similar to gas-giant planets; c, blended stellar binaries in a triple-star system that have deep eclipses strongly diluted by a bright neighbouring star, mimicking the much shallower transits of a planet; d, grazing binary stars, in which the stars' disks overlap by only a tiny amount at each eclipse. Credit: Santerne, A. et al. Astron. Astrophys. 545, A76 (2012).

Possible scenarios of false positive planetary transit signals: a, A gas-giant planet blocks a small amount of starlight as it passes in front of its host star. The resulting drop in light is similar to that produced by other systems, as follows: b, an orbiting brown-dwarf or low-mass star, both of which have radii similar to gas-giant planets; c, blended stellar binaries in a triple-star system that have deep eclipses strongly diluted by a bright neighbouring star, mimicking the much shallower transits of a planet; d, grazing binary stars, in which the stars’ disks overlap by only a tiny amount at each eclipse. Credit: Santerne, A. et al. Astron. Astrophys. 545, A76 (2012).

A team of scientists from NASA and SETI Institute conducted a survey where they analyzed the so-called Kepler Objects of Interest (KOI), which are potential candidates to be validated as the full-fledged exoplanets. The goal of this study was to determine the number of false-positive identifications of exoplanets utilizing the data accumulated by Kepler mission over a three-year time span using the ephemeris matching method.

The Kepler space observatory observed over 170,000 stars during the mentioned period, resulting in almost 6,000 planetary transit-like signals, according to the scientists. Obviously, these measurements also yielded a particular rate of detection errors, as any other scientific experiment. These are the Type I errors, or false-positives, that could lead to incorrect determination of Earth-like planets in our galaxy or other inaccuracies related to planet population statistics.

The researchers performed the ephemeris-based matching among all the acquired planetary transit signals and known unique parent sources, such as eclipsing binaries and variable stars. They found that approximately 12 % of transit signals from the analyzed sample were false positives, i.e. those particular KOIs should be excluded from the list of the exoplanetary candidates.

Comparison of false positives detected by ephemeris matching and TCERT

Comparison of false positive planetary candidates detected by ephemeris matching and TCERT. Image courtesy of the researchers.

The scientists then compared their sample of detected false positive signals to corresponding rates estimated by Kepler Threshold Crossing Event Review Team (TCERT). The team stated that compared to the techniques used by TCERT, they were able to determine from 7.1 % to 27.9 % additional false positive KOI, subject to the transit depth of particular KOI. However, the team also points that the ephemeris matching method, if used alone, can successfully detect only approximately one third of all contamination events in Kepler photometric observations.

Also, as mentioned in the study, the total rate of false positives is estimated to reach 35 %. This statement supports the earlier claim made by French scientists back in September, 2012, who used a similar approach, although on considerably smaller sample.

The authors of the study hope to expand their research by acquiring additional 5 quarters of Kepler data (in total, 17 quarters are available currently). This data could extend the detectable transit period range by 40% and the transit depth by 19%, compared to using only 12 quarters of data. The scientists also suggest, that the ephemeris matching could be a complementary method to increase the precision of existing techniques used to detect the false positive Kepler Objects of Interest.

The paper presenting the results of the study is published at arXiv.org.

By Alius Noreika, Source: www.technology.org

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