The unusual orientation of WASP-131: A puzzling exoplanetary system defying conventional patterns

In our solar system, the planetary orbits exhibit a remarkable consistency in their orientation. While there may be slight variations in the orbital planes of different planets, they generally orbit in the same direction. Furthermore, the plane of their orbits aligns closely with the rotational plane of the Sun. This phenomenon, known as the invariable plane, is commonly observed in many planetary systems. However, there are a few exoplanets that defy this trend, and scientists are still trying to understand the reasons behind this.

The prevalence of a common orientation within a planetary system can be attributed to the process of planetary formation. When a star and its planets are born from a protostellar cloud, the cloud possesses a certain amount of rotational momentum. As the star begins to form, a protoplanetary disk is created around it. The planets within this disk subsequently develop orbits that are similar to each other. While binary or multiple-star systems can introduce complexities, it is generally expected that single-star planetary systems would exhibit an invariable plane akin to our own. However, recent studies of the planetary system known as WASP-131 have challenged this assumption.

WASP-131, which is known to host at least one planet called 131b, is an intriguing system. 131b is a hot gas planet slightly smaller than Saturn, with an orbital period of five days around its host star. Previous investigations of 131b have highlighted its unusual characteristics, particularly its remarkably thick atmosphere. Despite having only a quarter of Jupiter’s mass, 131b boasts a diameter that is 20% larger than Jupiter’s. This unique gas giant is often referred to as a “super-puff” planet due to its low density.

The discovery of 131b was made using the transit method, which involves observing the planet as it passes in front of its star from our vantage point. This technique not only allows for the detection of exoplanets but also provides insights into the rotational motion of the star. Due to the star’s rotation, the light emitted from the region rotating toward us exhibits a slight blue shift, while the light from the region rotating away from us displays a slight red shift.

Consequently, the spectral lines of the star become slightly blurred, a phenomenon known as Doppler broadening. When the planet transits the star, it sequentially blocks parts of the blueshifted and redshifted regions, causing a shift in the star’s spectral lines. This effect, known as the Rossiter-McLaughlin effect, enables astronomers to measure the orientation of the star’s rotation.

Upon analyzing the rotational dynamics of WASP-131, the research team made a surprising discovery: the planet’s orbit did not align with the rotational plane of its star. In fact, the orbit of 131b exhibited a remarkable tilt of approximately 160 degrees, suggesting a highly inclined retrograde orbit that almost approached a polar orientation. Naturally, this finding raises the intriguing question of how such an unusual orbit could have been acquired by the planet.

One potential explanation for this peculiar orbit is a phenomenon known as the Kozai effect. The Kozai effect involves intricate interactions among the planet, its host star, and other planets within the system, leading to the shifting of the planet’s orbit away from the invariable plane. This effect can be observed within our own solar system, where the interactions between Pluto and Neptune have gradually tilted Pluto’s orbit over time. However, the Kozai effect is more pronounced in the case of smaller planets. Nonetheless, interactions solely between the planet and its star cannot fully account for the highly inclined orbit observed in this system. Another possibility that scientists have considered is a magnetic interaction between the planet and the protoplanetary disk during the early stages of its formation.

Although the exact mechanism responsible for the odd orbit remains unclear, it does follow a discernible pattern observed in many exoplanets of the hot gas variety. Around a quarter of such planets exhibit significantly tilted orbits, indicating that these celestial bodies can occasionally deviate significantly from the expected alignment. This intriguing study detailing the findings has been published on the arXiv preprint server.

Source: Universe Today

Leave a Comment