Launched in 2009, the Kepler Space Telescope revolutionized astronomy by discovering thousands of exoplanets in over 150,000 star systems. Kepler was specifically designed to detect Earth-sized planets by monitoring stars for periodic dips in brightness, which may result from planets passing in front of their star relative to the observer. Known as the Transit Method (or Transit Photometry), this technique has allowed astronomers to identify the majority of the more than 6,000 exoplanets in the current census. However, the method is not perfect and produces some false positives (initially as high as 5%–10%), which can sometimes be caused by other celestial objects.
According to new research by a team of astronomers from the Chinese Academy of Sciences (CAS), this was the case with KOI-1755, a star located 982 light-years from Earth that periodically dims. As they indicated in their study, recently published in The Astrophysical Journal, the transit signal was a false positive caused by an eclipsing binary. The research team was led by Ph.D. candidate Wang Haozhi under the supervision of Prof. Ali Esamdin at the CAS’s Xinjiang Astronomical Observatory (XAO).
Kepler first detected a transit-like event from KOI-1755 in 2014 and again in 2021, both of which corresponded to a period of about 25 days. On the second occasion, the signal was interpreted as a possible indication of a planet with 5.9 Earth radii (or 0.5 Jupiter radii), making it comparable in size to a Neptune-sized gas giant. Despite this periodic dimming, the true source of the signals has remained a mystery. To determine the true source of the dimming, the CAS team analyzed data from Kepler’s Target Pixel Files (TPFs).
They subjected this data to pixel-level flux modeling. centroid shift measurements, and cross-matching with the Gaia mission’s third data release (DR3). This modeling strategy significantly improved the quality of the Kepler light curves, and the DR3 data allowed them to avoid contamination from nearby stars. This allowed the team to isolate the uncontaminated light curve from the background star, confirming that it was caused by an eclipsing binary.
In short, their analysis revealed that the system is not a single star, but instead composed of two dwarf stars with an orbital period of about 6.14 days. In addition to eclipses, their analysis showed the binary system exhibits periodic modulations caused by starspots and differential rotation. This study not only clarified the true source of KOI-1755’s signals but also demonstrated the effectiveness of the pixel-level photometric modeling method developed by Wang and his colleagues. It also demonstrates that there could still be an invaluable amount of information on stellar dynamics embedded in Kepler and K2 mission data.
The method could also be very promising for investigating other transit-like signals in the Kepler archives and other missions that rely on the Transit Method to detect exoplanets, such as the Transiting Exoplanet Survey Satellite (TESS), and demonstrates how retired missions can experience a second life through improved analysis techniques.
Further Reading: CAS, The Astronomical Journal