Scientists from Princeton University and NASA have confirmed that 1,284 objects observed outside Earth’s solar system by NASA’s Kepler spacecraft are indeed planets. It is the largest single announcement of new planets to date and more than doubles the number of confirmed planets discovered by Kepler so far to more than 2,000. Kepler, which launched in 2009 and ended data collection for its primary mission in 2013, precisely measured the brightness of many stars simultaneously in order to find the dimming caused by planets as they cross in front of their home star. This photo shows a section of the Milky Way galaxy that includes the Kepler field of view. Credit: Photo by Carter Roberts, NASA
It is the largest single announcement of new planets to date and more than doubles the number of confirmed planets discovered by Kepler so far to more than 2,300. A new technique developed at Princeton allows scientists to efficiently analyze thousands of signals Kepler has identified to determine which are most likely to be caused by planets and which are caused by non-planetary objects such as stars. This automated technique – implemented in a publicly available custom software package called Vespa – computes the chances that the signal is in fact caused by a planet.
The researchers used Vespa to compute the reliability values for >7,000 signals identified in the latest Kepler catalog, and verified the 1,284 planets with 99% certainty. They also independently verified more than 700 additional planet signals that had already been confirmed as planets by other methods. In addition, the researchers identified 428 candidates as likely “false positives,” or signals generated by something other than a planet.
The researchers used an automated software developed at Princeton known as Vespa that allows scientists to efficiently determine if a Kepler signal is caused by a planet. Vespa computes the chances that a Kepler signal actually came from a certain type of planet. Automated software such as Vespa is necessary because of the sheer amount of Kepler data and the similarity that some planetary signals — especially those of larger planets — have to other objects such as stars that orbit each other. The graph above shows the type of planets newly verified by Vespa (orange) compared to the number of those planets previously confirmed (blue). Vespa more likely verified smaller planets because of their prevalence and unambiguous signal; signals thought to come from less common Jupiter-sized planets were more likely to actually emanate from stars. (Graph courtesy of NASA)
Timothy Morton, Princeton, associate research scholar of astrophysical sciences, developed Vespa because the vast amount of data Kepler has gathered since its 2009 launch has made the traditional method of confirming planets by direct ground-based follow-up observation untenable. Follow-up observations of Kepler data had confirmed a little more than a thousand planets prior to the Princeton-NASA announcement.
Kepler, which ended data collection for its primary mission in 2013, operated by precisely measuring the brightness of many stars simultaneously. The satellite looked for stars that exhibited subtle and regular dimming, which indicates that an orbiting planet is passing in front of, or transiting, that star. However, some scenarios can mimic the signature of a transiting planet, such as 2 stars that orbit each other, and provide a false positive signal. Distinguishing between true planets and false positives is one of the central challenges for any transiting planet survey, Morton said.
Efficient methods to confirm planets will become more crucial as NASA plans and launches more space telescopes eg Transiting Exoplanet Survey Satellite (TESS), which is expected to find 10s of 1000s of exoplanets.
The Vespa technique works by comparing the details of a transiting planet signal – specifically its duration, depth and shape – against simulated planetary and false positive signals to indicate the type of signal the candidate most likely is. At the same time, Vespa factors in the projected distribution and frequency of star types in the galaxy from which the signal originated to determine the chances that a planet with the characteristics being analyzed would exist.
Vespa was designed to supplement the internal vetting of Kepler data. More than 3,000 of the signals from the latest catalog were already identified as false positives well before Morton applied the recent Vespa analysis. Vespa is more likely to validate an internal ruling that a signal came from a small planet because of their frequency and unambiguous signal.
“If you have something that passes all those tests, then it’s likely to be a planet,” Morton said. “We know small planets are common, so if Kepler sees a small-looking planet candidate and it passes the strict internal vetting, it’s more likely to be a planet than a false positive because it’s hard to mimic that signal with anything else.”
On the other hand, if a planet candidate has the characteristics of a Jupiter-sized planet, Vespa is less likely to verify it as a planet. The signal could very well emanate from something else because of the relative rarity of Jupiter-sized planets. A number of planet candidates are 3-4 times larger than Jupiter, which means that Kepler most likely detected double-star systems in which one star was passing in front of the other.
http://www.princeton.edu/main/news/archive/S46/28/88S16/index.xml?section=topstories
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