Low-mass planets make good neighbours for debris discs

Astronomers have detected massive debris discs around 61 Virginis and Gilese 581, two nearby stars that are known to host “super-Earth” planets - so-called because their mass is between that of Earth and Neptune. Debris discs are belts of comets and asteroids orbiting the star.

The study, which was carried out using the European Space Agency’s Herschel Space Observatory, also reveals that debris discs are preferentially found in planetary systems with low-mass planets than in those hosting high-mass planets. This suggests that debris discs may survive more easily in the absence of planets with a very high mass, and highlights the importance of debris discs in the study of planet formation.

The formation of planets, around a newly-born star, is a dynamic process than can last hundreds of millions of years. Debris discs are a by-product of the process. They consist of everything orbiting a star that is not a planet: asteroids, comets, planetesimals and the dust that derives from them. In our own Solar System, the debris disc is mainly concentrated in two belts - the asteroid belt (between the orbits of Mars and Jupiter) and the Kuiper Belt beyond the orbit of Neptune.

Debris discs were first detected in other systems during the 1980s. Several hundred are now known. Astronomers are currently using the Herschel Observatory to search for discs around a variety of stars in our Galaxy - the Milky Way - deeper and more thoroughly than was previously possible. By exploiting the telescope’s unprecedented sensitivity and resolution, it is possible to detect very faint discs and image them in great detail.

One survey using Herschel, known as DEBRIS (an abbreviation of Disc Emission via a Bias-free Reconaissance in the Infrared/Submillimetre) has now produced two studies which detected discs around a handful of nearby stars, known to host planets. These all appear to be systems with super-Earths - planets with a mass between that of our own and Neptune, which means that their mass is relatively low.

The results hint that the presence of debris discs which are bright enough to be detected with current observatories could be related to whether their parent star has low-mass planets in orbit around it.

“One of the debris discs surrounds the star 61 Virginis, which is very similar to our Sun in terms of its mass, temperature and age,” Mark Wyatt, from the University of Cambridge’s Institute of Astronomy and leader of the analysis of G-type stars in the DEBRIS survey, said. G-type stars are of the same spectral type as the Sun.

61 Virginis is also known to host at least two planets. These have masses equivalent to about five and 18 times the mass of Earth and orbit their parent star in positions much closer than Mercury is to the Sun.

“The debris disc extends well beyond the orbits of the system’s known planets,” Wyatt said. “Since planets and debris discs occupy such different scales, one would not necessarily expect a correlation between their properties. However, material in the debris disc is also a fossil from the epoch of planet formation so it may carry information about the processes that contributed to build up the planetary system.”

Wyatt and his collaborators took a closer look at the 60 G-type stars that are nearest to the Sun. From this sample, they found 11 with planets. Five host high-mass planets, the remaining six host low-mass planets. Of the latter group, four showed debris discs, whereas this was true of none of the high-mass planet systems. This suggests that the presence of high-mass planets may hinder the survival of debris discs.

A similar result has also emerged from a second study based on M-type stars in the DEBRIS survey. These are stars with very low masses and temperatures and are the most abundant kind in the Milky Way. Until now, only one M-type star was known to possess a debris disc - the very young star AU Mic, which is about 12 million years old.

Given the lower surface temperature of these stars, astronomers expect them to retain debris discs more easily than hotter stars, where the radiation pressure may drive the debris away. However, M-type stars have a different internal structure from their higher-mass counterparts, which creates very intense magnetic fields and leads them to radiate plenty of X-rays. It is possible that both effects may disperse a debris disc.

The study found a new debris disc around an M-type star, known as Gilese 581. The star is more than two billion years old, suggesting that debris discs can actually survive for a long time around M-type stars. Gilese 581 also hosts at least four planets - all with low masses at a “super-Earth” level.