Circumbinary Exoplanets: What Can We Expect From ESA’s Gaia Mission?

In a recently published paper researchers outline Gaia’s ability to locate and study circumbinary exoplanets up to a distance of 200 parsecs. Due to its additional astrometric instruments the team predict a bumper harvest of new discoveries if statistical models, constructed from NASA’s already obtained datasets, prove correct (Kepler Space Telescope).

To date, Kepler has found a number of circumbinary systems which, as a sample, include: Kepler-16, Kepler-47 and Kepler-34. Statistical analysis from these discoveries has placed the number of expected planet/binary-star systems at roughly 10%—this is likely to be an underestimate due to Kepler’s limited technique capabilities.

Currently, there are five operational techniques for locating exoplanets: transit photometry, eclipse-timing (ETV) or transit-timing variations, radial velocimetry, direct imaging and astrometry. Due to its single photometer Kepler is restricted to methods that rely solely on light variations. This means only coplanar systems that exhibit transitory/eclipsing behaviour can be detected accurately.

Where Gaia is expected to improve on this situation is through its ability to monitor astrometric displacements. Astrometry is where physical movements of stars—or gravitational wobbles—can be observed/monitored, and provides an additional complimentary technique to photonic based measurements. Due to the fact astrometry does not rely on electromagnetic sampling planets external to the binary star’s orbital plane are expected to be discovered. Researchers believe that circumbinary planets coalesce and evolve on a range of mutual inclinations; if proved correct statistical likelihood is predicted to increase: depending on protoplanetary to binary star orbital plane orientations, ‘a Gaussian distribution with standard deviation of 5 degrees corresponds to an abundance of 20%’.

Gaia satellite (Credit: ESA)

Gaia satellite (Credit: ESA)

To emphasise this, using ETV techniques, Kepler-413 and its two companion planets have mutual inclinations of 4.1°, 10° and 19° respectively. This suggests that current estimates will need to be positively adjusted: ‘We expect that Gaia will discover hundreds of giant planets around binaries with FGK dwarf primaries within 200 pc of the Sun, if we assume that the giant planet mass distribution and abundance are similar around binaries and single stars’, said ESA team member, Dr Johannes Sahlmann.

The team estimate ‘500’ new circumbinary exoplanets are expected for discovery within the five year mission window—first detections are expected ‘on the fourth data release, i.e. not before 2018’.

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Paul Hattle

As a strong advocate for science and learning, I am a passionate supporter of the 'Campaign for Science and Engineering' (CaSE) Fellow of the 'Royal Astronomical Society' (RAS) Associate Member of the 'Institute of Physics' (IOP) & 'The Institute of Scientific and Technical Communicators' (ISTC)

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