One Day on a Distant Planet Offers Clues to Its Origins

University of Virginia astronomer is leading a team taking the first direct measurement of the length of a day on a young planet known as a “super-Jupiter” in a distant solar system. Using NASA's James Webb Space Telescope, the study has uncovered new evidence that will help astronomers understand how the planet was formed. 

The findings, published in The Astronomical Journal, show that Beta Pictoris b — a planet more massive than Jupiter located 63 light-years from Earth — completes one rotation every nine hours, about the same as Jupiter. More importantly, the measurements reveal that the planet's rotation is aligned with its orbit, its host star's rotation and the surrounding disk of dust and debris, evidence that it formed gradually within that disk rather than as a result of a more chaotic process. 

Yifan Zhou, an assistant professor in the University of Virginia's Department of Astronomy, led an international team that observed Beta Pictoris b continuously for 16 hours with the Webb telescope. Using a device to block the star's glare, the researchers achieved an unprecedented level of imaging precision and detected subtle changes in the planet's brightness as clouds and other atmospheric features rotated in and out of view. 

The repeating patterns of brightness observed by the team allowed them to determine the planet's rotation period; using measurements of its speed of rotation, the team was also able to calculate the tilt of the planet’s axis of rotation. Like Jupiter and unlike Earth, Beta Pictoris b most likely spins upright. 

The alignment of the planet's spin with its orbital system offers an important clue to its history. Giant planets can form gradually as gas and dust accumulate within a protoplanetary disk that gives rise to a planetary body, or they can emerge through the rapid gravitational collapse of a cloud of material. The latter process is expected to produce planets with randomly tilted spin axes. Instead, Beta Pictoris b's orderly alignment points to a less tumultuous origin within its star's planetary matrix. 

The study marks the first time astronomers have measured the rotation period of a young planet that was directly observed, and it gives researchers a unique new way to test competing theories of how these planetary giants are formed. 

Zhou and his collaborators plan to apply the same technique to additional exoplanets, which could lead to new insights into the evolution and characteristics of planets.

“The measurement demonstrates a precision not previously achieved for a directly imaged planet,” Zhou said. “This opens a new and powerful way to use the James Webb Space Telescope to study exoplanet atmospheres, trace planet formation and potentially detect extrasolar moons through their eclipses.”