An international team has reviewed all of the light curves from the CoRoT space telescope that were first published in 2016. Everything the satellite detected has been sorted and catalogued for use by the scientific community. During the course of its five-year mission that lasted from January 2007 to October 2012, CoRoT observed 160,000 stars, obtaining more than 170,000 brightness curves that have each been analysed for signs of an exoplanet partially eclipsing its star (an occurrence termed a ‘transit’). For example, what is initially thought to be a planetary transit could in fact be two very close stars revolving around the same centre of gravity and mutually eclipsing one another—what’s called an eclipse binary.
The authors of this analysis obtained 4,123 detections to be interpreted. The difficulty lies in distinguishing between these two cases to avoid false positives. For now, 37 planets and brown dwarves have been formally discovered by CoRoT, from 557 candidate planets. Another difficulty is the time needed to collect sufficient data: the further a planet is from its star, the longer it takes to complete a full orbit. This may take years, and several transits of a planet across its star are necessary to confirm its existence with confidence.
“It’s possible to affirm an exoplanet’s existence with only a single transit across its star, but to do that you need to have studied the star in detail,” says Magali Deleuil. “One transit doesn’t give us its precise period of revolution, so we have to confirm that using another method, typically the radial velocity method, which consists in measuring the variations in the star’s velocity caused by an orbiting planet. That’s because the planet induces a periodic motion in the star that reflects the planet’s motion about their common centre of gravity.” Complementing CoRoT data with radial velocity measurements also yields information about the planet’s mass, which when combined with the radius of the transit allows its mean density to be estimated and its nature—rocky or gaseous—to be identified.
Hunting out planets
Certain ‘planets’ are therefore eliminated and identified as eclipse binaries or false positives. Others can neither be confirmed nor completely ruled out. Distinguishing between the two requires close examination of each detection, calculation of the transit parameters (depth, duration, etc.) and analysis of all results, including complementary observations by ground-based instruments. The catalogue gives the scientific community access to all detections processed in a uniform manner, providing a definitive record of the CoRoT mission.
“All the planets have already been published elsewhere, with more detailed analyses performed with different software. In this case, we used the same software to compile the final catalogue. Before, they were published individually, but now we have a complete record and detections have been classified into false positives of stellar origin or due to instrument errors, clearly identified planets and brown dwarves, and candidate planets, which can’t be confirmed by current instruments that aren’t precise enough to measure very faint stars. Such confirmation is essential, as false detections are always a danger. That’s why the catalogue also gives a list of false detections, where we’ve been able to identify them. For example, in figure 8 of the article (see below), the orange star is a periodic variable whose light is contaminating that from the other stars seen by CoRoT’s detector. Its photometric signature is imprinted in all the stars plotted in red.”
Credits: M. Deleuil et al.: Planets, candidates and binaries from the CoRoT/Exoplanet programme, Astronomy & Astrophysics
Half of the candidate planets still haven’t been observed with ground-based telescopes. And the exact nature of those that have remains unsure. Better instruments capable of measuring smaller-amplitude signals will be needed to find that out. But the results from CoRoT are already providing a yardstick for new-generation space telescopes like the U.S. TESS satellite launched in April 2018 and the European PLATO mission scheduled to launch in 2024.
The catalogue is set to be posted online in the Strasbourg astronomy data centre (CDS).
Planets, candidates, and binaries from the CoRoT/Exoplanet programme M. Deleuil et al. Astronomy and Astrophysics, Volume 619, November 2018
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