Artistic representation of an exoplanet on a retrograde orbit: Six planets have now been discovered that are not rotating in the same direction as their central star. Picture: ESO / L. Calçada
Reading aloud The direction of motion of a planet in its orbit does not necessarily coincide with the direction of rotation of the star it is orbiting. This has been discovered by an international team of astronomers who studied 27 so-called transit planets in alien suns. Transit planets pass in front of their star as seen from Earth, slightly obscuring it. However, some of the planets observed rotate in the opposite direction as their parent star, others have a tilted orbit. However, both are not likely to be common in the evolutionary model: according to this planets are born in a disk of gas and dust, which rotates together with their central star around one and the same axis of rotation. The reason for the now discovered irregularities could be a tug-of-war between stars around the planets. The astronomers team originally wanted to confirm only the discovery of nine new extrasolar planets using a telescope from the European Southern Observatory (ESO) at the La Silla Observatory in Chile. In this context, they investigated the properties of these nine and 18 other known transit planets. Surprisingly, when combining the observation data, astronomers found that more than half of the planets studied were? so-called hot Jupiter? the orbit is tilted against the axis of rotation of the star. Six of the planets even show a retrograde motion: they are orbiting their star in the wrong direction.

Planets form into slices of gas and dust surrounding young stars that have just emerged. Such a protoplanetary disk and its central star rotate together around one and the same axis, which is perpendicular to the disk. Accordingly, the astronomers assumed that the planets in the disk also run around the star in the disk plane? in the same direction as the star. This is indeed the case with the planets in our solar system, but not with some of the Hot Jupiter. These planets, first discovered 15 years ago, have a mass at least as large as that of Jupiter, the largest planet in our solar system. Previously, it was assumed that the Hot Jupiter would rise far from their star and then move toward it until they reach their final orbit? a migration caused by gravitational interactions with the dust disk. Also, the orbit of the hot Jupiter would have to align itself so along the direction of rotation of the star around itself.

The scientists are now considering an alternative mechanism: Considered a process lasting several hundred million years, in which a kind of tug-of-war with distant planets or companion stars of the parent star takes place. After such disturbances have steered a large exoplanet onto an inclined and elongated orbit, would it lose energy every time it encounters the star? and finally, the new Hot Jupiter would end up near the star on an orbital, but arbitrarily inclined orbit, opposite the main plane of the planetary system.

Andrew Cameron (University of St Andrews) et al: Lecture at the National Astronomy Meeting ddp / science.de? Rochus Rademacher advertisement

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