A new look at the planet with two suns

At a distance of 245 light-years from Earth, the exoplanet Kepler-16 (AB) b orbits two stars that orbit each other. They form an eclipsing binary system, as astronomers call it. Kepler-16 b with its two Suns inevitably resembles Tatooine, the planet of Luke Skywalker in the saga of “Star Wars”. The comparison stops there, because this planet is a world unsuitable for Life.

Noticed over 10 years ago

Kepler-16 b was identified by NASA’s Kepler Space Telescope (which completed its mission in 2018) in 2011 using the transit method (see box). It is a large, cold, gaseous planet about the size of Saturn. It revolves around the binary system in 229 days, and the two stars themselves revolve around each other in 41 days. It was one of the first discovered near-Earth planets.its presence actually confirmed that planets could form and orbit around these binary star systems. Today we know much more“, points out Alexandre Santern of the Marseille Astrophysical Laboratory (LAM/AMU), who “discovered” Kepler-16 b from the ground using a different method, the radial velocity method (see box).

Representation of the binary system and planet Kepler-16 b. Credit: NASA/JPL-Caltech/T. Pyle

The observations were carried out at the Observatory of Haute Provence using the SOPHIE spectrograph mounted on a 193 cm telescope. This is the first time that a circular binary planet has been detected using the radial velocity method from Earth, and this is an achievement that is the subject of a publication in Royal Astronomical Society Monthly Notices, bodes well for many other discoveries. The radial velocity method may turn out to be even more efficient than the transit method.”because sometimes, due to interactions in these three-body systems, we can no longer see planets transiting in front of the binary system.”, explains Alexander Santern. So this is Kepler-16 b, which was unobservable in 2020 with NASA’s TESS space telescope (which also uses this transit method) and will not be detectable until 2042 due to the precession of its orbit. .

Simulator to understand how we detect planets

Two indirect methods are mainly used to detect exoplanets: the transit method and the radial velocity method. The first is the detection of tiny periodic changes in light intensity that occur when a planet passes in front of its star. This also allows us to determine its size. The second analyzes the light spectrum of a star: the presence of a planet in orbit causes the regular movements of the latter. For an earthly observer, the star moves away and approaches. These changes are accompanied by a shift towards red as it recedes and towards blue as it approaches. These very subtle shifts can be observed with spectrographs such as SOPHIE. This method also provides information about the exoplanet’s mass.

The simulator run by LAM provides an overview of the curves that astronomers need to analyze for these two methods. You can vary several parameters and thus realize their influence on the tools they use. The star in this simulation is similar to the Sun.

Lots of exoplanets to discover

Now that we know that the radial velocity method is suitable for detecting near-Earth planets, we will look for this type of star around unexplored binary systems and be able to discover new exoplanets.”, Alexander Santern rejoices. Thus, an extensive survey is already underway with the SOPHIE instrument for the northern hemisphere and the HARPS spectrograph for the southern hemisphere. Each of them will study 40 to 50 binary systems, and the first results already show possible planetary candidates. The discovery and characterization of these exoplanets will allow a better understanding of the mechanism of their formation, which is not yet entirely clear.

Illustration of the detection of Kepler-16b at the Observatory of Haute-Provence. Credit: University of Birmingham/Amanda Smith..

Indeed, in binary systems, two stars are surrounded by a protoplanetary disk (a vast structure of matter and gas). It is within the latter that future planets develop. This model of formation is well known for single star systems, but for binaries it is more complicated: the presence of a second star disrupts dust accretion, a mechanism that promotes the formation of larger and larger bodies. In addition, the more circumbinary objects known to astronomers, the more data they will have to decipher the processes occurring in these systems, which are very numerous in the Milky Way.

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