First measurement of the alignment of the rotating orbit on the planet Beta Pictoris b

Astronomers have made the first measurement of rotary orbit alignment for a distant ‘super-Jupiter’ planet, demonstrating a technique that could allow advances in the quest to understand how exoplanet systems form and evolve.

An international team of scientists, led by Professor Stefan Kraus of the University of Exeter, has carried out measurements of the exoplanet Beta Pictoris b, located 63 light-years from Earth.

The planet, which is in the constellation Pictor, has a mass about 11 times that of Jupiter and orbits a young star in a Saturn-like orbit in our solar system.

The study, published today (June 29, 2020) in the Astrophysical charts, marks the first time scientists have measured the alignment of the rotating orbit for a planetary system with direct images.

Crucially, the results provide a new insight to improve our understanding of the history of the formation and evolution of the planetary system.

Professor Kraus said: “The degree to which a star and a planetary orbit are aligned with each other tells us a lot about how a planet was formed and whether various planets in the system dynamically interacted after its formation.”

Some of the earliest theories of the planet formation process were put forward by prominent eighteenth-century astronomers Kant and Laplace. They observed that the orbits of the planets in the solar system are aligned with each other and with the axis of rotation of the Sun, and concluded that the solar system was formed from a rotating and flattened protoplanetary disk.

“It was a big surprise when more than a third of all nearby exoplanets were discovered to orbit their host star in orbits that are misaligned with respect to the stellar equator,” said Professor Kraus.

“Some exoplanets were even found to orbit in the opposite direction of the star’s direction of rotation. These observations challenge the perception of planet formation as an orderly and orderly process that takes place in a geometrically thin, coplanar disk.”

For the study, the researchers devised an innovative method that measures the small spatial displacement of less than one billionth of a degree caused by the rotation of Beta Pictoris.

The team used the GRAVITY instrument on the VLTI, which combines light from telescopes 140 meters apart, to take the measurements. They discovered that the axis of stellar rotation is aligned with the orbital axes of the planet Beta Pictoris by its extended debris disk.

“Absorption of gas in the stellar atmosphere causes a small spatial shift in spectral lines that can be used to determine the orientation of the stellar axis of rotation,” said Dr. Jean-Baptiste LeBouquin, astronomer at the University of Grenoble in France and a team member

“The challenge is that this spatial shift is extremely small: about 1/100 of the star’s apparent diameter, or the equivalent of the size of a human step on the moon as seen from Earth.”

The results show that the Beta Pictoris system is as well aligned as our own solar system. This finding favors planet-planet dispersion as the cause of the orbital obliquities seen in more exotic systems with hot Jupiter.

However, observations on a large sample of planetary systems will be required to answer this question conclusively. The team proposes a new interferometric instrument that will allow them to obtain these measurements in many more planetary systems that are yet to be discovered.

“A dedicated high-spectral resolution instrument in VLTI could measure rotational orbit alignment for hundreds of planets, including those in long-period orbits,” said Professor Kraus, “This will help us answer the question of what form dynamic processes form the architecture of planetary systems. ”