In our Solar System alle the planets are orbiting the Sun in a nice and orderly way, the same way around and with orbits well behaved in approximately the same plane coinciding with the equator of the Sun. In the world of exoplanets this is not always so. Quite a few exoplanets have their orbits inclined strongly compared to the orbital direction of their star; some of them even in perpendicular orbits.

On Wednesday 30 June 2021 Marcus Marcussen defended his master's thesis and at the same time he is one of the main authors of a paper published in the esteemed Astrophysical Journal Letters. The paper has to a high degree been based on the thesis work by Marcus on inclined planetary orbits, and it is a fine example of the efforts at Aarhus University to involve students in real hands-on science from a very early point in their studies.

Marcus and his colleagues at the Aarhus University Stellar Astrophysics Centre and the universities in Pronceton and Pennsylvania, USA have studied 57 exoplanets with unusual orbits. It seems that exoplanets prefer either to be orbiting nice and regularly as we know it from our own Solar System or to go all-in and orbit almost perpendicularly to the normal. Not many exoplanets have orbits that are just slightly inclined. This tendency has been known for some years as one of the riddles of astrophysics.

Trying to solv the riddle, the research team tried - in vain - to find som common parameters of the stars or the unbehaved exoplanets. Using statistical methods they also secured that it was not just a selection effect of some sort.

The riddle has not yet been solved, but in the paper the astronomers are suggesting four possible ways that can cause an exoplanet to end up in a perpendicular orbit. Currents in the surface gasses of the star may wring the planetary orbit out of shape, exoplanets in extremely oblong orbits around flattened stars may feel a stronger wringing, other planets in the system may disturb an exoplanetary orbit via tidal effects or magnetic fields may early on in the history of the planetary system turn the protoplanetary disc of dust and gas that later on condense into planets. None of these suggestions fit very well to what is observed, though, so the hunt for an explanation continues.

The title of the paper in Astrophysical Journal Letters is, with a bit of a letter rhyme "A Preponderance of Perpendicular Planets", og and the paper is available here.

Main author of the paper is Simon Albrecht, SAC albrecht@phys.au.dk

Marcus Marcussen marcuslmarcussen@gmail.com

The American Astronomical Society has honoured the paper with special mention on several social media and in a Highlight-note.

For his thesis Marcus has produced a string of short and informative videos, available at the following YouTube links: