This small celestial body at the edge of our solar system has a ring far too large for its size.
The planet Saturn is known for its majestic rings. But the latter are not the prerogative of large celestial bodies: there are also some around smaller stars, such as those found beyond Neptune, the last planet in our solar system. Quaoar, discovered in 2002, is one of these objects. It orbits very far from us: the distance that separates it from the Sun is 43 times greater than that between the Earth and our star. This dwarf planet, which is about 1 100 km in diameter – half the size of Pluto – also has a ring. But above all, it is in a completely new area: it is far too large compared to what is expected and documented elsewhere. Astronomers, including French, announced this discovery in an article published in the scientific journal Nature (article in English)Wednesday, February 8.
For this study, several instruments were mobilized between 2018 and 2021, in the Canaries, in Namibia, and in space with the Cheops space telescope, of the European Space Agency (ESA). This one shared an illustration image of Quaoar’s ring.
Quaoar has a radius of only 550 km, but that of its ring is 7.4 times larger and reaches 4 100 km. What constitute an anomaly both for the founding work in this field, carried out in 1850 by the mathematician and astronomer Edouard Roche, and for the observations made since. “It’s the first time we’ve seen this”comment to franceinfo Josselin Desmars, co-author of the article and researcher at the Paris Observatory.
“The only example that does not obey this famous limit”
Edouard Roche’s calculations gave rise to a notion called the Roche limit. “If a satellite around a planet enters the Roche limit, then it is dislocated by tidal forces”, explains Josselin Desmars. In other words, if a satellite is too close to a celestial body, it disintegrates and the residue forms rings. Conversely, if the satellite exceeds the famous limit of Roche, then it is no longer subjected to excessive forces and can be maintained in one block, abounds the astronomer. “Overall, the Roche limit is at a distance between two and three times the radius of the central body. But it can vary depending on the mass of that body.”
“While all the rings of planets or small bodies lie within the limit of Roche, that of Quaoar is very far from it”adds on Twitter the Space Studies and Astrophysical Instrumentation Laboratory (Lesia). Quaoar “is the only example that does not obey this famous Roche limit”he continues, with an infographic showing how far his intriguing ring is from the limit.
As shown in the left image above, Saturn’s rings lie within the Roche boundary. This is also the case for the three other giant planets of our solar system which are rings: Jupiter, Uranus and Neptune. Scientists have also seen it for Chariklo, in 2014, and Hauméa, in 2017. Like Quaoar, these two objects are located in the Kuiper belt, this zone which encircles our solar system.
Chariklo, an asteroid only 250 km in diameter, has two thin, dense rings. They were observed again at the end of January by the James Webb telescope, NASA reported (in English) January 25. Chariklo is the smallest body we know to have one, as explained in this video from the European Southern Observatory (in English).
As for the dwarf planet Haumea, elongated in shape and 1,600 km in diameter, it too is accompanied by a ring. And this one is in the famous limit.
The Roche limit has everything “validated theory”, insists Josselin Desmars to underline the surprise of the astronomers who observed the great distance between Quaoar and its ring. So should we throw the stone at Roche? Does this discovery undermine his entire reasoning? The teacher-researcher argues that models show that rings are finally possible beyond the limit, under certain conditions, that is to say by integrating new parameters. More than a total questioning, it is therefore more a refinement of Roche’s theory that is taking shape.
Further observations needed
Additional observations are expected to improve the state of knowledge and better characterize the Quaoar ring, which remains poorly understood. “We know that it lets some light through. In this, it looks more like pieces of a ring than a ring as such”specifies Josselin Desmars, noting that the whole is made up of “particles greater than 100 micrometers”SO “rather big”.
To know if the ring of Quaoar is the only exception to the Roche theory, it is necessary to increase the number of scrutinized objects. Josselin Desmars reports that between 100 and 200 targets have already been identified within the Kuiper Belt as part of the European Lucky Star program, which has not been funded since 2021. The researcher hopes that the recent discovery will attract interest. attention and new funds to continue the observations.