Astronomers have been studying a star system discovered in 2008 by the W. M. Keck Observatory and Gemini Observatory from Hawaii. The star system is called HR 8799 and is 129 light-years away from Earth. Within that star system is a quartet of planets, all known as super-Jupiters.
As the name suggests, super-Jupiter planets are more massive than Jupiter in our solar system. HR 8799 is notable because it’s one of the first planetary systems to be imaged directly by a telescope. One big mystery about the star system was the rotation periods or spin rates of the planets within it. Knowing the rotation rates allows astronomers to know the length of a day on the planet.
That type of data has been measured for only a few of the thousands of exoplanets that have been discovered so far. A group of astronomers has developed an instrument called the Keck Planet Imager and Characterizer commissioned between 2018 and 2020. The instrument can observe exoplanets with extremely high spectral resolution. The instrument can provide a resolution high enough to decipher how fast planets are spinning.
The study has shown that the minimum rotation speeds for two of the planets in the HR 8799 star system, known as HR 8799 d and HR 8799 e, spin at 10.1 kilometers per second and 15 kilometers per second respectively. That data means the length of a day can be as short as three hours or as long as 24 hours on Earth, depending on how the planets are tilted. Unfortunately, the tilt of the planets is undetermined at this time.
To compare, Jupiter spins at 12.7 kilometers per second, and a day on the planet is nearly 10 hours. The team was able to constrain the spin of the third planet, HR 8799 c, at an upper limit of less than 14 kilometers per second. Unfortunately, researchers were unable to determine the rotation rate of the fourth planet in the system, HR 8799 b.