Until very recently we thought that only planets, and particularly giant planets, could have rings. In the case of our solar system, it is well known that Saturn exhibits and spectacular them. However, it is much less known that also Jupiteruranus and Neptune They have their own, although they are much weaker and thinner than Saturn’s.
Everything we thought we knew about rings changed in the year 2013, when, using a very precise technique called stellar occultation, a ring around Chariclo was discovereda small icy body of the solar system barely 250 kilometers in size.
Chariclo belongs to a family of objects known as centaurs, small icy bodies with detected orbits between that of Jupiter and Neptune. They have a double nature: sometimes they behave like inactive bodies and other times like active comets. Hence its name, inspired by mythological creatures half horse half human.
The discovery of this ring around a non-planetary body like Chariclo was a surprise that unleashed more questions than certainties: how had it been formed?, how was it found confined?, how old was it?, how long could it last before disappear?
The revolution of the rings
But the surprise did not end there. Shortly after the evidence, also obtained using the stellar occultation technique, seemed to indicate that another centaur called Chiron (about 200 kilometers in size) she might as well have her own ring. The revolution of the rings around non-planetary bodies had only just begun.
But before advancing on the new discoveries, after the detections in the centaurs Cariclo and Chiron, we are going to describe what the stellar occultation technique which is giving such spectacular results.
In essence, it is a very basic method, since it consists of predicting and observing when a body in the solar system passes in front of a star, blocking its light. By observing the occultation from various locations on the Earth’s surface, we will be able to reconstruct the shape of the body with great precision and obtain information comparable to what would be achieved by a space mission to such a body.
The problem, as often happens, is in the details, since predicting a stellar occultation is not an easy task.
To begin with, the apparent size in the sky of these distant objects in the Solar System is very small: on the order of ten or a few tens of milliarc seconds (1 milliarc second equals 1/3600,000 degrees). To give us an idea, an object with a size in the sky of 30 milliarc seconds is equivalent to a seen one euro coin… 140 kilometers!
But not only the small dimensions of these objects complicate the predictions. It is also made difficult by the great uncertainty in their orbits, which means that we do not know with great precision where they are going to be in the sky at any given moment.
And to complicate matters further, the position of the stars themselves in the sky must be known with exquisite precision, something that has only been possible in recent times thanks to measurements of the ESA’s Gaia satellite.
Despite all the difficulties, we have managed to predict and detect so far about 140 occultations produced by about 50 of these distant bodies.
Unfortunately, only about twenty of them have been observed from more than two places, which is the necessary condition to obtain precise measurements of the shape and size of the object.
More ringed surprises
It was the prediction and detection of one of these stellar occultations by the dwarf planet Haumea, on January 21, 2017, which made it possible to measure with great precision the three-dimensional shape, size, and density of this object of about 1,600 kilometers in diameter. medium And, oh surprise, it was also surrounded by a ring!
This dwarf planet (a category to which Pluto now belongs) is part of the family of transneptunian objects; that is, they orbit beyond Neptune, in the icy confines of our solar system.
After the discovery of a new ring around a non-planetary body it became clear that not only centaurs, but also trans-Neptunian objects could have had them. These ring structures appear to be composed of water ice, along with particles of dust and other ice.
And that’s how things were until, gathering data on stellar occultations between 2018 and 2021 from the trans-Neptunian object Quaoar (which is about 1,100 kilometers in size), another ring was discovered around this dwarf planet. We’d only be talking about one more detection (and that’s four!) if it weren’t for the fact that this ring is where it shouldn’t be.
After the discovery of a new ring around a non-planetary body it became clear that not only centaurs, but also trans-Neptunian objects could have had them. These ring structures appear to be composed of water ice, along with particles of dust and other ice.
And that’s how things were until, gathering data on stellar occultations between 2018 and 2021 from the trans-Neptunian object Quaoar (which is about 1,100 kilometers in size), another ring was discovered around this dwarf planet. We’d only be talking about one more detection (and that’s four!) if it weren’t for the fact that this ring is where it shouldn’t be.
Until now, all discovered rings have been located fairly close to their parent bodies, within the so-called Roche limit, where tidal forces prevent material with reasonable densities from aggregating to form a satellite. Well, Quaoar’s ring is outside the mentioned limit. We are, therefore, seeing something theoretically impossible: a ring where there should actually be a moon.
One possible explanation is that elastic collisions occur between the particles of the Quaoar ring that prevent them from «merging» to form a satellite. Furthermore, the new discovery found that rings around such bodies appear to be much more common than we thought: perhaps around 20-25% of trans-Neptunian and centaurian objects could have rings.
unknown land
All the rings discovered are, curiously, very close to the so-called 3:1 spin-orbit resonance.. This means that the bodies rotate around themselves three times in the same time that a ring particle makes a complete orbit around the object. This ring relationship seems to be of great importance in the confinement and stability of said rings.
We still do not know their origin or age, although we suspect that they could be formed from material torn from the very surface of the objects they surround by collisions or by very fast rotation. Their longevity is for now a mystery, but they must be relatively stable and have long lifetimes for us to have been able to detect at least four.
Related news
In the coming years we will see the discovery of many more rings around centaurs and trans-Neptunian objects, shedding light on these and other unsolved mysteries. And so, once again, the current paradigm on the physics of our solar system and other planetary systems will change.
This article was originally published on The conversation. read the original.