Which of the exoplanets are having retrograde rotation?

Question

By exoplanet, I mean planets outside of our solar system(i.e., planets in other solar systems). If we talk about our solar system, then Venus, Uranus, and Pluto have retrograde rotation. Most of the planets in our solar system rotate counterclockwise, but Venus, Uranus, and Pluto rotate clockwise, which is called retrograde rotation. I want to know which planets or moons in other solar systems also have retrograde rotation. I tried to find the answer on google but didn't get anything. Can anyone provide me with this information and also the references?

Answer 1

We know the rotation direction of nearly no exoplanet, but there is hope for the nearby future. Here is why:

Exoplanets as planets around other stars are mostly detected indirectly: one observes the star and infers the planets existence from periodic variations in the spectrum (radial velocity method) or from periodic partial obscurance of this light (transit method). Neither information allows any spatially resolved treatment. It is already considered an achievement to obtain spectra of the atmospheres of the exoplanets, albeit also indirectly: taking spectra of the star's light without the planet in front of it, and then taking spectra while the planet transists in front of the star and attributing the observed differences to the influence of the planet's atmosphere.

There are a few direct imaging observations of nearby exoplanets, and getting spectra spatially resolved with the very small diameter needs interferometric observations. The first system the spin-orbit alignment of planets and stars was measured on is Beta Pictoris in 2020 by Krauss et al - and even in this very read-worthy and nicely-illustrated paper they "only" present measurements which confirm that the debris disk around that young star rotates in the same sense as the star itself.

They note in their conclusion

At present, spin-orbit align- ment measurements with VLTI are limited to nearby stars with large apparent diameters and to pressure- broadened lines of relatively fast-rotating stars, which strongly limits the numbers of stars that are accessible with this technique. A dedicated high-spectral resolu- tion (R = 25, 000), short-wavelength instrument oper- ating in the J-band (1-1.4μm) and optimised for preci- sion phase measurements, such as the proposed VLTI visitor instrument BIFROST (Kraus 2019), will be able to mitigate these limitations and enable spin-orbit mea- surements for hundreds of systems in the planet samples that are expected from direct-imaging facilities (JWST and ELTs) and the GAIA astrometry mission.

We might thus hope to get these kind of information in the forthcoming months or years.