An exoplanet in a polar circumbinary disc surrounding two stars is a planet that orbits two stars in a binary system, and whose orbit is in a polar configuration. The first such exoplanet was announced in 2019 around the star HD 98800, which is a quadruple star containing a pair of binary stars.
A circumbinary planet typically orbits farther from the center of the binary system than either of the two stars. In contrast, circumstellar planets in a binary system have stable orbits around one of the two stars, closer in than the orbital distance of the other star.
As of October 1, 2023, there are 5,506 confirmed exoplanets in 4,065 planetary systems, with 878 systems having more than one planet. The James Webb Space Telescope is expected to discover more exoplanets.
AC Herculis (AC Her) is a binary star about 4200 light-years away. The primary star is well-studied, while its partner is invisible. It has a polar circumbinary disk, which is unusual but not unheard of. In a new paper, a team of researchers presents evidence for the polar circumbinary exoplanet.
Here are some other exoplanets that orbit two stars:
- Kepler-16b This Saturn-mass exoplanet orbits two stars that are also orbiting each other. It’s located in the constellation Cygnus and is about 200 light-years from Earth. It orbits its binary stars with a period of 228.8 days.
- Kepler-47 This solar system has two stars, one similar in size to the sun but only 84 percent as bright, and the other one-third the size of the sun and less than one percent as bright.
- BEBOP-1c This exoplanet confirms the second-ever known planetary system orbiting twin stars.
- TOI 1338 b This exoplanet was discovered by NASA’s TESS mission.
As of June 12, 2023, there are 12 circumbinary planet systems. Only one of these systems, Kepler-47, hosts multiple circumbinary planets:
- Kepler-47 b: Has an orbital period of 49.5 days
- Kepler-47 d: Has an orbital period of 187.4 days
- Kepler-47 c: Has an orbital period of 303.2 days and is within the system’s habitable zone
Kepler-1647b is the largest known circumbinary Kepler planet. It’s a gas giant similar in size to Jupiter, with an orbital period of 1107 days
The first circumbinary planet discovered by NASA’s TESS mission was TOI-1338 b. TOI-1338 is a low-mass eclipsing binary system that consists of a 1.13 solar mass (M⊙) F8-type primary star and a 0.31 M⊙ M-dwarf companion. TOI-1338 b is around 6.9 times larger than Earth, or between the sizes of Neptune and Saturn.
The first circumbinary planet around a main sequence star was found in 2005 in the system HD 202206. HD 202206 is a Sun-like star orbited by two objects, one of 17 MJ and one of 2.4 MJ.
The first circumbinary planet detected with radial velocities was Kepler-16 (AB) b. Kepler-16b is a giant circumbinary exoplanet similar to Saturn in both size and mass. It orbits its two parent stars, Kepler-16A and Kepler-16B, every 229 days at a distance of 65 million miles.
Circumbinary systems have been found in the following locations:
- PSR B1620-26: Located in the globular cluster M4, this system contains a millisecond pulsar and a white dwarf.
- TOI-1338: Located in the constellation Pictor, this system contains two stars and is 1,320 light-years distant.
- Kepler-47: This system is believed to have three orbiting exoplanets.
- HD 202206: This system contains a Sun-like star orbited by two objects.
- TOI-733 b: This super Earth exoplanet orbits a G-type star.
The first extra-solar planet wasn’t detected until the early 1990s. The first actual circumbinary planet was detected in 2005.
Kepler-1647b is the largest known circumbinary planet. It’s a gas giant with a radius similar to Jupiter’s and an orbital period of 3.0 years, the longest of any confirmed transiting planet. It’s located in the constellation Cygnus.
Kepler-1647b is also the largest in both size and orbit of any discovered circumbinary planet. It’s substantially larger than any of the previously known circumbinary planets.
Circumbinary planets are also known as “Tatooine” planets, named after Luke Skywalker’s home world in Star Wars. The fictional world of Tatooine was once the most famous example of a circumbinary planet.
The discovery of Kepler-16b paved the way for the detection of 14 transiting planets in 12 binary systems by NASA’s Kepler/K2 and TESS missions.
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