A tail facing backwards, with two coplanar planets in a multi-star system

In a recent paper in the Proceedings of the National Academy of Sciences a group of researchers led by Maria Hjorth and Simon Albrecht of the Stellar Astrophysics Center, Aarhus University, have published the discovery of a special exoplanetary system in which two exoplanets revolve around their star. This surprising orbital architecture was caused by the protoplanetary disk in which the two planets were formed by the second star tilted in this system.

The study is titled “A Tail Turning Backwards, with Two Planetary Planets.”

Maria Hjorth says: “We have found a very intriguing planetary system. There are two planets orbiting the star in the opposite direction, while the star revolves around its own axis. It is different from our own solar system, where all the planets turn in the same direction as the sun’s rotation. ”

Joshua Winn of Princeton University says: “This is not the first known case of a ‘backward’ planetary system – the first was seen more than ten years ago. But it’s a rare case in which we think we know which is the drastic maladaptation, and the explanation is different from what researchers assumed could have happened in the other systems. ‘

Co-author Rebekah Dawson of Pennsylvania State University, USA, says: ‘In any planetary system, the planets are thought to form in a rotating, circular disk material that revolves around a young star for several million years, after the star self-born, the so-called protoplanetary disk. Usually the disk and the star rotate in the same way. However, if there is a neighboring star (where ‘neighboring’ means in astronomy within a light year or so), the gravitational force of the neighbor can tilt the disk. ‘

John Zannazzi of the University of Toronto, Canada, continues: “The underlying physics is linked to the behavior that exhibits a rotation-top, when the rotation slows down and the axis itself begins to rotate in a cone.”

The scenario was first theorized in 2012, and now this research team has found the first system in which this process played out. Teruyuki Hirano of the Tokyo Institute of Technology says: “After discovering the K2-290 system, we realized that this system is ideal for testing this theory, as it not only orbits two planets, but also two stars. “So logically, the next step would be to study the system more closely, and we did hit the jackpot.”

Ph.D. student Emil Knudstrup from the University of Aarhus, says: “The idea that planets move on a very wrong orbit fascinated me during my postgraduate studies. It is one thing to predict the existence of these crazy orbits, so many unlike what we see in the solar system .. it’s quite a thing to do to really find it! Also fascinating is the idea that a structure as enormous as a protoplanetary disk by similar physics as a rotating top is controlled. “

One implication of the discovery is that astronomers can no longer assume that the initial states of planet formation show an alignment between galaxies and planetary orbits. Importantly, other theories aimed at explaining misinterpretations in exoplanet systems tend to work best on large, Jupiter-like planets in short period orbits, but the disk tilt mechanism applies to planets of any size. For example, there may be another Earth-like world moving across the north and south poles of its home star.

“I find our results encouraging because it means we have found another aspect of systems architecture where planetary systems show a fascinating variety of configurations,” concluded Simon Albrecht of the Stellar Astrophysics Center, Aarhus. ‘How would astronomy have developed here on earth if the situation here had been similar to K2-290 – then Galileo would have seen sunspots moving in the opposite direction of the earth around the sun. One only wonders what his statement would have been about it? ”