The Transit Exoplanetary Survey Satellite (TESS) was launched by NASA on April 18, 2018. It scans the whole sky, looking for planets around bright stars by detecting small light loss as a planet transits in front of the host star. Thousands of transiting exo-planets have already been discovered from space by the Kepler mission, and also from the ground. The strength TESS, however, is in the uniform survey of bright stars which are most interesting for detailed study of the exo-planets.
Binary stars present a problem for the transits because (i) the light of the companion dilutes the depth of the observed transits, causing under-estimation of the planet's radius, and (ii) if the faint companion is an eclipsing binary, it can mimic exo-planet transits around the main star and give a false-positive detection. Screening all exo-host candidates for binary companions is therefore an essential complement to their observations from space. High angular resolution is needed to reveal close binary companions that escape detection in the standard, seeing-limited images.
TESS began its survey by scanning the southern sky. The
High-Resolution speckle camera at the 4.1-m SOAR telescope was the natural choice for screening hundreds of TESS objects of interest (TOIs) for close companions. The team led by C. Ziegler from the University of Toronto and including NOAO astronomers A. Tokovinin and C. Briceno started the observations in September 2018, soon after the very first TOIs were announced. These observations are highly automated, covering up to 300 objects per night. The data were processed within a week and posted promptly on the TESS follow-up web site. Several papers on TESS exo-planets already used these observations. It turns out that about 1/5 of all 542 TOIs observed at SOAR to date have companions within 3 arcseconds, and only a fraction of those companions were known previously. The image on the left shows two new companions to TOI-612: a close 0.17" pair at 2" from the main target. High-resolution follow-up of TOIs is also conducted at the Gemini 8-m telescopes, at a much slower pace.
The results of the TESS follow-up campaign at SOAR are submitted to the Astronomical Journal (see the preprint here). Apart from the purely technical utility (correction of transit depth and false positives), the large and uniformly observed sample of TESS exo-hosts revealed interesting statistical trends. The frequency of binary companions with separations below ~50 au is reduced by a factor of 4, compared to similar stars that do not host transiting planets. The same trend was found earlier for the Kepler exoplanet-hosts by A. Kraus et al. (2016). Apparently, the existence of a close binary companion suppresses (albeit not completely) formation of planets. Even more intriguingly, the SOAR survey revealed the excess of wide binaries among TESS exo-hosts. However, the surplus of wide companions is found almost exclusively for stars hosting "hot Jupiters" (massive planets on tight orbits, like the one orbiting 51 Pegasi). This finding points to the relation between binary systems and the properties of their planets. It is currently believed that hot Jupiters were formed at large distances from their stars (like the Jupiter in our Solar system) and later migrated to their actual tight orbits. Wide binary companions could have played a critical role in promoting the migration of hot Jupiters, e.g., by de-stabilizing other planets that could have thrown the hot Jupiters onto close orbits. In support of this hypothesis, researchers noted that hot Jupiters seem to have a reduced chance of being accompanied by other, smaller planets.
The SOAR/TESS team plans to continue observations of more TOIs accessible to SOAR and to follow the orbital motion of some newly discovered binaries to learn about their orbits.
Updated: Aug 30, 2019 A. Tokovinin