In a study to be published in the Astronomical Journal, astronomers found that many exoplanet-hosting stars identified by NASA’s Transiting Exoplanet Survey Satellite (TESS) are actually binary systems, where the planets orbit one of the stars in the pair; and that exoplanets twice the size of Earth or smaller could not be detected using the transit method when observing these binary systems.
This illustration depicts an exoplanet partially hidden in the glare of its host star and a nearby companion star. Image credit: Gemini Observatory / NOIRLab / NSF / AURA / J. da Silva.
Physical pairs of stars that are close together can be mistaken for single stars unless they are observed at extremely high resolution.
So Dr. Katie Lester of NASA’s Ames Research Center and colleagues turned to the Gemini North and South telescopes in Chile and Hawai’i to inspect a sample of exoplanet host stars in painstaking detail.
Using a technique called speckle imaging, the astronomers set out to see whether they could spot undiscovered stellar companions.
Using the ‘Alopeke and Zorro instruments on the Gemini telescopes, respectively, they observed 517 stars that TESS had identified as potential exoplanet hosts.
The sample consisted mainly of bright F-, G-, and K-type stars at distances of less than 500 parsecs (1,631 light-years).
They discovered that 73 of these stars are really binary systems that had appeared as single points of light until observed at higher resolution with Gemini.
“With the Gemini Observatory’s 8.1-m telescopes, we obtained extremely high-resolution images of exoplanet host stars and detected stellar companions at very small separations,” Dr. Lester said.
The researchers also studied an additional 18 binary stars previously found among the TESS exoplanet hosts using the NN-EXPLORE Exoplanet and Stellar Speckle Imager (NESSI) instrument on the WIYN 3.5-m Telescope at Kitt Peak National Observatory.
After identifying the binary stars, they compared the sizes of the detected planets in the binary systems to those in single-star systems.
They realized that TESS found both large and small exoplanets orbiting single stars, but only large planets in binary systems.
These results imply that a population of Earth-sized planets could be lurking in binary systems and going undetected using the transit method employed by TESS and many other planet-hunting telescopes.
“Since roughly 50% of stars are in binary systems, we could be missing the discovery of — and the chance to study — a lot of Earth-like planets,” Dr. Lester said.
The authors also analyzed how far apart the stars are in the binary systems where TESS had detected large planets.
They found that the stars in the exoplanet-hosting pairs were typically farther apart than binary stars not known to have planets.
This could suggest that planets do not form around stars that have close stellar companions.
“This is a major finding in exoplanet work,” said Dr. Steve Howell, an astronomer at NASA’s Ames Research Center.
“The results will help theorists create their models for how planets form and evolve in double-star systems.”
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Kathryn V. Lester et al. 2021. Speckle Observations of TESS Exoplanet Host Stars. II. Stellar Companions at 1-1000 AU and Implications for Small Planet Detection. AJ, in press; arXiv: 2106.13354
