Nightfall would be a rare event on exoplanets with two suns -- one to orbit and another one just to loom brightly in the sky. Now, astronomers say that exoplanets with host stars that are binaries are much more common than we ever thought.
About a thousand exoplanets have been confirmed using NASA’s Kepler space telescope, along with thousands of stars that could possibly host planets. The latter are called Kepler Objects of Interest, or KOI. Turns out, binary stars are surprisingly common: About half of the stars in the sky are thought to be comprised of two stars orbiting each other. So, are these planet-hosting stars just as likely to have a companion star?
A team led by Elliott Horch from Southern Connecticut State University investigated using the WIYN telescope located on Kitt Peak in southern Arizona and the Gemini North telescope located on Mauna Kea in Hawaii. They used a technique called speckle imaging, which consists of collecting digital images taken 15 to 25 times a second of a small portion of the sky surrounding a particular star of interest.
These images are combined to yield a final, high-resolution image that allowed the team to detect companion stars that are up to 125 times fainter than the targeted star. The distance between the target and the companion stars can be up to 100 times the distance between Earth and the sun. The team also performed a model simulation to separate true-bound companions from line-of-sight companions that are unconnected with the system.
Pictured to the right is the surface plot of Kepler-14, one of the first Kepler stars shown to host a planet and have a stellar companion using speckle imaging.
From these images, the team calculated occurrence rate and found that stars with exoplanets are just as likely to have a stellar companion. Up to 50 percent of host stars are actually binary stars.
“An interesting consequence of this finding is that in the half of the exoplanet host stars that are binary we can not, in general, say which star in the system the planet actually orbits,” says study coauthor Steve Howell of NASA Ames Research Center in a news release. The work will be published in the Astrophysical Journal.
Planets that orbit both stars in very close binary systems are called circumbinary planets, and Kepler has already discovered a number of these. There also exist exoplanets that orbit one of the stars in very wide binary systems, where the farther star probably appears as just another star in the sky. In the case of Tatooine in Star Wars, two stars are very close to each other and the planet is far away. “Somewhere there will be a transition between these two scenarios,” Howell says,” but we are far from knowing where.”