Gas giant GJ 896Ab orbits one of the stars in a close pair of red dwarfs, moving in the opposite direction.
Of the thousands of exoplanets known today, only a few have been found using astrometry, an ultra-precise change in the position of their parent stars. The attraction of a sufficiently massive body orbiting a star causes it to wobble slightly, although the deviations are so small that it is hard to call the method productive. Recently, however, it was astrometry that helped not only spot a new exoplanet but also, for the first time, determine the three-dimensional structure of its orbit.
Scientists from Mexico, Germany and the US observed the GJ 896AB double system, which is just 20 light years away. It comprises a pair of small, faint red dwarfs with masses of 44 percent and 17 percent of the Sun's mass. They are separated by about the same distance as Neptune is from the Sun, making a complete revolution around the centre of the star in 229 Earth years. Astronomers used an array of VLBA radio telescopes for a new study of the system, as well as archived data from observations in the optical range. Faint fluctuations in the motion of the larger star in this tandem showed that an exoplanet orbits around it.
Scientists estimate that it is a gas giant 2.3 times heavier than Jupiter, making an annual rotation in 284 of our days. The orbit of GJ 896Ab is slightly smaller than that of Venus, and VLBA has been able to determine it in all three dimensions. It turns out that the orbital plane of this exoplanet is tilted by 148 deg with respect to the orbit of the binary system itself: in fact, it moves around its star in the opposite direction to the rotating further 'younger' star. Nothing like this occurs in the solar system.
The authors point out that red dwarfs are the most common type of star in the Galaxy, most of them are in binary systems. Therefore, our knowledge of them, in its own way, is even more significant than anything that can be found within the solar system. They allow us to refine existing ideas about the formation and evolution of most planets, which clearly need to be clarified. "In particular, the models predict that such a large planet is unlikely to be the neighbour of such a small star. So perhaps these models should be reconsidered," summed up (Joel Sanchez-Bermudez), one of the authors of the study.
