In our solar system, the planetary orbits all have a similar orientation. Their orbital planes vary by a few degrees, but pretty much the planets all orbit in the same direction. This invariant plane, as it is called, also has an orientation within a few degrees of the Sun’s plane of rotation. Most planetary systems have a similar arrangement, where planetary orbits and stellar rotation are roughly aligned, but a few exoplanets defy this pattern, and we don’t know exactly why.
A common orientation within a planetary system makes sense given the formation of planetary systems. The protostellar cloud from which a star and its planets form usually has some inherent rotational momentum. When a star begins to merge, a protoplanetary disk forms around the star. Since the planets form in this disk, they all end up with similar orbits. Things can be more complicated with binary or multiple star systems, but you would expect single star planetary systems to have an invariant plane similar to ours. However, this is not true for a planetary system known as WASP-131, as a recent study shows.
WASP-131 is known to have at least one planet, 131b. It is a hot gas planet with a mass a little less than Saturn that orbits around 131 every five days. Previous studies of 131b have found the planet unusual due to the thickness of its atmosphere. Although its mass is only a quarter of that of Jupiter, its diameter is 20% larger than that of Jupiter. 131b has such a low density for a gaseous planet that it is known as a super puff planet.
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The planet was discovered by the method of transits, which means that it passes in front of its star from our point of view. This is an effective way to find exoplanets, but it can also be used to check the rotational motion of the star. Due to stellar rotation, light from the region of the star rotating towards us is slightly blue-shifted, and light from the region rotating away from us is slightly red-shifted. This means that the star’s spectral lines are a bit blurry. The effect is known as Doppler broadening. As the planet passes in front of the star, it alternately blocks part of the blue-shifted and red-shifted regions. This causes a slight shift in the star’s spectral lines. This Rossiter-McLaughlin effect, as it is known, allows astronomers to measure the orientation of stellar rotation.
When the team analyzed WASP-131’s rotation, they found that it was not similar to that of its planet. 131b’s orbit is inclined about 160 degrees to the star’s plane of rotation, meaning it is in a high, near-polar retrograde orbit. Of course, this raises the question of how the planet could have had such a strange orbit.
An idea is a process known as the Kozai effect. Dynamic interactions between the planet, its star, and other planets in the system can cause the invariant planet’s orbit to move away. We see it in our own solar system with Pluto and Neptune, which has tilted Pluto’s orbit over time. The Kozai effect is more pronounced with smaller planets, however, and the interaction between the planet and the star alone is not enough to explain such an inclined orbit. Another possibility is a magnetic interaction between the planet and the protoplanetary disk early in its formative period.
Although the mechanism behind the odd orbit is unclear, it follows a pattern seen with many hot gas exoplanets. About a quarter of them have considerably inclined orbits. It seems that these planets sometimes go out of line.
Reference: Doyle, L., et al. “WASP-131 b with ESPRESSO I: A puffy sub-Saturn in a polar orbit around a differentially rotating sun-like star.” arXiv preprint arXiv:2304.12163 (2023).
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