Rare stellar system rotates against its host star

Artistic hypothetical diagram of the K2-290 star system.

Scientists have discovered a new star system in which two planets orbit the star in a direction that is surprisingly opposite to the direction of the star’s rotation.

This is a very unusual phenomenon. This is so rare,” said Rebekah Dawson of Pennsylvania State University, one of the researchers. Because all planets are born in primordial planetary disks around stars, which are a whole disk of dense gas dust that surrounds a star after it is born and rotates along with the star, a phase that can last millions of years, after which planets are born in these disks. So, in general, the direction in which the born planets orbit the star should be the same as the direction of the star’s rotation.”

The star K2-290 system observed in this study is about 897 light-years away, and its host star, K2-290A, is 1.2 times the mass and 1.5 times the radius of the Sun.

The planet in its inner orbit, K2-290Ab, orbits its host star in 9.2 days and has a radius three times that of Earth, like a hot version of Neptune; the planet in its outer orbit, K2-290Ac, orbits its host star in an average of 48.4 days and has a radius 11.3 times that of Earth, like a warm version of Jupiter.

The two planets orbit in the same plane, but at such a large angle of 124 degrees to the plane of the main star’s rotation. One of the researchers, John Zannazzi of the University of Toronto in Canada, explains, “It’s the same thing as a gyroscope spinning, when the gyroscope slows down, its spin axis starts to dip and draws a cone-shaped orbit.”

The most peculiar aspect of this system is the direction in which the planets orbit their primary star. All the planets in our solar system orbit the Sun in the same direction as its rotation, but these two planets orbit K2-290A in the opposite direction of K2-290A’s rotation.

A theory first proposed by some scientists in 2012 speculates that the presence of another star in the “vicinity” of a star could affect the orbits of the planets in that star system. In astronomical terms, two stars are said to be in each other’s “vicinity” if they are within one light year of each other.

In this system, another star, K2-290B, does exist in the vicinity of K2-290A, and the gravitational force from K2-290B is likely to have an effect on the orbits of the two planets in the K2-290A system.

The researchers say the K2-290 system is the first and best example that can be used to test this theory, and that the next step is to understand how the nearby star actually affects the orbits of the two planets, and that finding this system has been like “finding a treasure.

The study was published Feb. 23 in the Proceedings of the National Academy of Sciences of the United States of America (PNAS).