In our solar system, the planetary orbits all have a similar orientation. Their orbital planes vary by a few degrees, but the planets all orbit in roughly the same direction. This immutable plane, as it is known, also has an orientation within a few degrees of the Sun’s plane of rotation. Most planetary systems share a similar arrangement, with planetary orbits and stellar rotation roughly aligned, but some exoplanets buck this trend, and we’re not entirely sure why.
A common alignment within a planetary system makes sense given how planetary systems form. The protostellar cloud from which a star and its planets form usually has some intrinsic rotational momentum. When a star begins to merge, a protoplanetary disk forms around the star. Because the planets form within this disk, they all have similar orbits. Things can be more complicated with binary or multiple star systems, but you would expect one-star planetary systems to have an invariant plane similar to ours. However, this is not true for a planetary system called WASP-131, a recent study shows.
WASP-131 is known to have at least one planet, 131b. It’s a hot gaseous planet, slightly less massive than Saturn, orbiting 131 every five days. Previous studies of 131b found the planet unusual due to its thick atmosphere. Although its mass is only a quarter that of Jupiter, its diameter is 20% larger than Jupiter’s. 131b is so sparsely dense for a gas planet that it is known as a super-puff planet.
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The planet was discovered in transit, which means that it passes in front of its star from our point of view. It’s an effective way to find exoplanets, but it can also be used to check the star’s rotational motion. Because of stellar rotation, light coming from the region of the star that is spinning toward us is slightly blue-shifted, and light from the region that is spinning away from us is slightly red-shifted. That means the star’s spectral lines are a little blurry. The effect is known as Doppler broadening. As the planet passes in front of the star, it alternately blocks part of the blueshift and redshift regions. This shifts the spectral lines of the star slightly. This so-called Rossiter-McLaughlin effect allows astronomers to measure the orientation of the star’s rotation.
When the team analyzed WASP-131’s rotation, they found that it was unlike that of their planet. 131b’s orbit is tilted about 160 degrees from the star’s plane of rotation, meaning it is in a high, almost polar retrograde orbit. This, of course, begs the question of how the planet could have gotten 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 orbit to shift away from the immutable planet. We see this in our own solar system with Pluto and Neptune, which has tilted Pluto’s orbit over time. However, the Kozai effect is more pronounced in smaller planets, and planet-star interaction alone is insufficient to explain such an inclined orbit. Another possibility is a magnetic interaction between the planet and the protoplanetary disk in the early stages of its formation.
Although the mechanism behind the odd orbit is not clear, it follows a pattern seen in many hot-gas exoplanets. About a quarter of them have significantly inclined orbits. It seems that these planets sometimes get way out of line.
Reference: Doyle, L., et al. “WASP-131 b with ESPRESSO I: A bloated sub-Saturn in polar orbit around a differentially rotating Sun-like star.” arXiv form arXiv:2304.12163 (2023).
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