Astronomers have found a planet moving in the opposite direction to its host star's spin, and the usual explanation for such behaviour appears to be missing, according to Martin Cid Magazine.
The world orbits a red dwarf, a small, cool class of star that is the most common type in the galaxy. Most planets travel the same way their star rotates, because both form from the same spinning disk of gas and dust. A planet that goes the other way, known as a retrograde orbit, is therefore unusual.
The standard account for retrograde planets points to a giant planet nearby. Through gravitational interactions, a large companion can tilt or flip a smaller planet's path over time. But in this system, Martin Cid Magazine reports, there is no giant in the neighbourhood that could have done the job. That leaves the observed backward motion without an obvious cause.
Red dwarfs are of particular interest to researchers because they are so numerous and because several are known to host small planets. Their planetary systems also tend to differ from our own, which makes them useful for testing ideas about how worlds form and settle into their orbits.
The finding adds to a small but growing set of planets whose paths do not match the simple picture of formation inside a single, orderly disk. Each case puts pressure on existing models, since any explanation has to fit the star, the planet and the absence of a massive perturber.
What to watch next: follow-up observations of the system, and modelling work testing whether some process other than a giant companion can produce a retrograde orbit around a red dwarf.
