Over the last three decades, astronomers have discovered over 4,000 exoplanets in the Milky Way and there are pressing questions about how they form and how they evolve. To answer those, we need to know as much as possible about these distant worlds. Newly released research gives us a new technique to study an important property: the spin-orbit alignment.
Stars spin on their axis and planets orbit around them. The simplest scenario has the plane of the orbit perpendicular to the star spin but in the universe, deviations are the norm rather than the exception. The Solar System has a 7-degree misalignment; other systems are much more extreme.
In The Astrophysical Journal Letters, researchers report the first measurement of the spin-orbit alignment of a wide-separation directly-imaged planetary system. The system in question is Beta Pictoris, where two planets both several times more massive than Jupiter – "super-Jupiters" – orbit a star less than 30 million years old. The system also possesses a large debris disk.
The team collected precise measurements of the star rotation and when compared to the orbit of the larger and more distant planet, Beta Pictoris b, it showed a good alignment between the two. They are as well-aligned as our Solar System.
“The degree to that a star and a planetary orbit are aligned with each other tells us a lot about how a planet formed and whether multiple planets in the system interacted dynamically after their formation,” lead author Professor Stefan Krauss, from the University of Exeter, explained in a statement.
Planets are believed to form in a thin disk surrounding their star. Given the stars' size compared to planets, stars dominate the angular momentum of the system, so the disk is usually aligned to the equator of the star.
“It was a major surprise when it was found that more than a third of all close-in exoplanets orbit their host star on orbits that are misaligned with respect to the stellar equator,” Prof Krauss said. “A few exoplanets were even found to orbit in the opposite direction than the rotation direction of the star. These observations challenge the perception of planet formation as a neat and well-ordered process taking place in a geometrically thin and co-planar disk.”
For planetary orbit to shift, sizable interactions have to occur. Astronomers believe that planetary migration might be one of the mechanisms that create more outlandish configurations. To test this, researchers will need to measure the spin-orbit alignment for many systems, and better understand the commonality between the system and the planets in them.
Beta Pictoris b is a super-Jupiter, weighing about 12 times our Solar System's local gas giant. It is as far from its star as Saturn is from the Sun, located in the constellation of Pictoris (The Easel), 63 light-years from Earth.