A strange exoplanet that breaks all orbital rules

In our solar system, the orbits of all planets have a similar orientation. Their orbital planes differ by a few degrees, but almost all planets rotate in the same direction. This fixed plane, as it is known, also has an orientation within a few degrees of the plane of the sun’s rotation. Most planetary systems have a similar arrangement, with the orbits of the planets and the rotations of the stars roughly aligned, but few exoplanets defy this trend, and we’re not quite sure why.

joint orientation within planetary system It makes sense given how planetary systems form. The primary stellar cloud that makes up the star and its planets usually has some inherent rotational momentum. As the star begins to coalesce, a protoplanetary disk forms around the star. Since the planets formed within this disk, they all end up with similar orbits. Things can be more complicated with binary or multi-star systems, but you expect one star planetary systems We have a fixed plane similar to an airplane. However, this is not true of the planetary system known as WASP-131, as a recent study shows.

WASP-131 is known to have at least one planet, 131b. It’s hot gas planet With a mass slightly less than Saturn it orbits 131 every five days. Previous studies of 131b found the planet unusual because of its dense atmosphere. Although it has only a quarter of the mass of Jupiter, its diameter is about 20% larger than that of Jupiter. 131b has a low density for a gaseous planet known as a supernovae.

The planet was discovered transit methodWhich means that it passes in front of its star from our viewpoint. It’s an effective way to find exoplanets, but it can also be used to check the rotational motion of a star. Because of the stellar rotation, the light coming from the region of the star that rotates toward us turns slightly blue, and the light coming from the region that rotates away from us shifts slightly red.

This means that the spectral lines from the star are slightly blurred. The effect is known as Doppler expansion. When the planet passes in front of the star, it occludes part of the regions, which, in turn, change color to blue and blue-red. This causes spectral lines For the star to turn a little bit. This Rossiter-McLaughlin effect allows astronomers to measure the direction of rotation of stars.

When the team analyzed WASP-131’s rotation, they found that it did not resemble the rotation of its planet. the orbit of 131b is tilted about 160 degrees from the star’s rotational plane, which means that it is in a nearly high polar orbit. Of course, this raises the question of how the planet got such a strange orbit.

One idea is a process known as the Kozai effect. Dynamic interactions between a planet, its star, and other planets in the system can cause the orbit to shift away from the stationary planet. We see this in our own country Solar System With Pluto and Neptune, which have changed Pluto’s orbit over time. The Kozai effect is more pronounced with a smaller effect planetsHowever, the planet-star interaction alone is not sufficient to explain such an inclined orbit. Another possibility is that there was a magnetic interaction between the planet and the protoplanetary disk early in its formation period.

Although the mechanism behind the single orbit is not clear, it follows a pattern seen with many hot gaseous exoplanets. About a quarter of them have noticeably tilted orbits. It seems that those planets Sometimes get out of line.

The study has been published on arXiv Prepress server.