Computer simulations have revealed the reason why some exoplanets are inclined at large angles and why this might lead to habitable planets being evicted from planetary systems.

Planets are thought to form within giant discs of gas and dust which surround young protostars. Clumps of material start to stick together and eventually collide with each other to create a protoplanet, which then sweeps up even more material from the disc. This protoplanetary disc rotates in the same direction as the protostar and should result in planets orbiting in the same plane as the equator of the star. However, there are many exoplanets which have broken the rules and orbit at large angles from the equatorial plane. What makes these systems so different?

A team of German and British astronomers have been running computer simulations in order to solve this mystery. They have discovered that if a protoplanetary disc, which they dub the “bullet”, collides with another disc known as the “target” then the bullet disc will steal material from the target disc. This extra gas and dust will result in the bullet disc moving from the equatorial plane of the star to a plane which is at a large angle from the stars equator. The chance of such collisions between discs rises if the discs exist within a star cluster. “Encounters between stars and/or protostellar objects are expected to be highly likely or even unavoidable in dense clusters like the Trapezium Cluster in the Orion Nebula,” Ingo Thies from the University of Bonn tells Astronomy Now. “However, other kinds of ‘target clouds’ than a huge disc are possible, like dense filaments, clumps etc. This makes encounters even more likely.”

The exoplanets that have been discovered so far exhibit other unusual properties, such as orbiting very close to their parent star or orbiting in highly elliptical orbits. “However, while the first two can indeed be explained by unstable planetary systems, e.g with too small radial separations between the orbits, or with one or two very massive planets, the strongly inclined orbits require an initial misalignment in such a system,” explains Thies.

The extra material in the disc not only results in inclined planets, but it can go so far as to cause the protoplanetary disc to rotate in the opposite direction to the star. This results in planets which orbit in the opposite direction to which the star is spinning, known as retrograde motion. If it turns out that some of these highly inclined planets do not orbit in the same plane as each other, then the system can be come very unstable. The increased likelihood of these inclined planets in star clusters implies that the number of habitable terrestrial planets is probably less than currently expected, because unstable systems will eject smaller planets.

“Unstable and thus uninhabitable planetary systems may also result from unusually dense circumstellar discs, since more massive giant planets may form and perturb the orbits of other planets,” adds Thies. However, this bleak outcome will not always be the case. “If the initial disc and the ring of new material have enough time to align, and no planets have yet formed therein, there will probably only be a moderate mutual tilt (a few degrees, like the 2-3 degrees mutual inclination of the planets in the Solar System).”

Thies remarks that the planetary systems discovered to date are unlike our own Solar System as they orbit much closer to their parent stars. “Future detections of transiting planets on wider orbits will hopefully help to answer the question whether misaligned orbits are similarly common in Solar-like planetary systems.”