The ring of a dwarf planet questions a theory that has been in force since 1850

Artist's conception of the Quaoar and its ring / IAA

Science | Space

It is located beyond the Roche limit, that is, the set point for the formation of these objects.

Elena Martin Lopez

As we were taught in school, planets can have rings, that is, disks containing many small pieces of ice and other material that orbit a larger object. Saturn is the most famous ringed planet, but others like Jupiter, Uranus or Neptune also have this characteristic. Until a decade ago, these giant planets were the only ones known to have rings around them, but more recently these same discs have been observed around some non-planetary bodies in the outer Solar System, such as Chariklo and Haume. This Wednesday, a new ringed object, called Quaoar, which is located beyond the orbit of Neptune, has been added to the list, the difference is that its ring is located beyond the Roche limit, something that had not been seen never. The finding has been made by an international team of researchers, in which scientists from the Institute of Astrophysics of Andalusia (IAA-CSIC) have participated, and has been published in the journal

The Roche limit is a theory developed by Edouard Roche around 1850 that marks the closest proximity to which two bodies can be with gravity so that one does not disintegrate the other. For example, the Roche limit between the Earth and the Moon is 9,500 kilometers, or what is the same, if the Moon came closer than 9,5000 kilometers from the Earth, the gravity of our planet would tear it apart and it would become a ring of fragments that would stay spinning around us. This same theory applies to stars, planets, satellites or asteroids.

On the contrary, outside the Roche limit the particles are expected to aggregate and form a satellite in a short time. Until now, at least, that is what has always been observed. Known ringed planets had their rings inside the Roche limit. In contrast, the trans-Neptunian object Quaoar found in this new study, which is about half the size of Pluto, has a ring that is 4,100 kilometers from its center, while its Roche limit (also measured from its center) is of 1,780 km. That is, the ring occupies an orbit where a moon should have formed, which defies the 1850 theory.

international effort

This ring is too small and narrow to be detected directly, even with large telescopes like the James Webb, so the authors used multiple telescopes. «When we first saw the possible existence of a ring outside the Roche limit in the excellent data obtained with the Gran Telescopio Canarias, we realized that it could take many years to conclusively prove this circumstance, but we finally got it in few years thanks to the international effort", highlights José Luis Ortiz, a researcher at the IAA-CSIC who is participating in the work.

Among the explanations given by scientists as to why this ring has not become a satellite, Bruno Morgado, a researcher at the Federal University of Rio de Janeiro and lead author of the study, argues that the ring is perhaps made of debris that recently released and hasn't had enough time to re-stack, but this seems like the least likely option.

Another scenario is that the ring material is more elastic than commonly assumed, making the particles more likely to bounce off each other than stick together. And a third option is that the particles in the ring are subject to gravitational forces from other bodies, such as Quaoar's moon (called Weywot), which are preventing the particles from aggregating. Thus, the finding not only challenges current models of planetary rings; it also reveals that solid particles do not always accumulate into larger bodies as quickly as might be expected.