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From an article on the Sky and Telescope website by Monica Young, originally posted on December 10, 2015.

More than half of the potential giant planets detected by NASA’s Kepler satellite might not be planets after all according to a study by Alexandre Santerne (University of Porto, Portugal, and Aix Marseille University, France). A press release following the Extreme Solar Systems III conference in Hawaii summarized the study with the headline: “Half of Kepler’s Giant Exoplanet Candidates are False Positives.”

Common-False-Positives-480

Credit: NASA/Ames Research Center

Santerne and colleagues followed up on Kepler’s long list of planet candidates during a five-year observing campaign. Between July 2010 and July 2015, the team spent 370 nights observing 129 planet candidates out of more than 4,000 that were identified by Kepler, and only 45 of these turned out to be bona fide planets. The rest fell into 3 different categories: 3 were brown dwarfs, 63 were multiple-star systems, and 18 were neither of these, but could not be confirmed as planets. Even if all of those 18 cases turned out to be planets, 51% of Kepler’s giant potential planets would still turn out not to be real.

Previous studies found a much lower ‘false-positive’ rate for Kepler’s planet candidates. However, according to experts on Kepler data, this seemingly surprising high false-positive rate is not surprising at all.

For more information on our Solar System, and others, see Chapter 9 of The Cosmos. Link to the original article here.

Credit: Rebekah Dawson, University of California, Berkeley

Credit: Rebekah Dawson, University of California, Berkeley

This new graph plots orbital period versus planetary radius for planetary candidates discovered by the Kepler Space Telescope (see also The Cosmos, Section 9.2d, pp. 240-243, and Figure 9-9, p. 239). The colored symbols represent how many planets orbit a given star, and the legend lists how many of this type of system are in the catalog. (For example, the catalog lists 2967 stars with only one planet orbiting them.) Planets with shorter orbital periods are overrepresented because geometric factors and frequent transits make them easier for Kepler to detect. The upward slope in the lower envelope of these points is caused by the difficulty in detecting small planets with long orbital periods, for which transits are shallow and few are observed.

Link: article in EOS; for more detailed information, see Lissauer, J. J., R. I. Dawson, and S. Tremaine (2014), Advances in exoplanet science from Kepler, Nature, 513, 336–344 (arXiv version available here).

Is Earth the only known world that can support life? In an effort to find life-habitable worlds outside our Solar System, stars similar to our Sun are being monitored for slight light decreases that indicate eclipsing, or transiting, planets (see section 9.2d, pp. 240-243). Many previously-unknown planets are being found, including over 700 worlds recently uncovered by NASA’s Kepler satellite.

Credit: Planetary Habitability Laboratory (UPR Arecibo)

Depicted above in artist’s illustrations are twelve extrasolar planets that orbit in the habitable zones of their parent stars. These exoplanets have the right temperature for water to be a liquid on their surfaces, and so water-based life on Earth might be able to survive on them. Although technology cannot yet detect resident life, finding habitable exoplanets is a step that helps humanity to better understand its place in the cosmos.

Links: APOD for full-size image; Kepler mission website.