An international group of researchers with several Brazilian participants discovered that Quaoar, one of the large objects orbiting beyond Neptune, has a ring. The finding suggests that these structures must be much more common than previously imagined and calls into question the understanding of the conditions that allow them to form.
Quaoar is one of the known large objects that reside in the so-called Kuiper belt, where Pluto, the largest of them, is also located. At an estimated 1,110 km in diameter, it is half the size of its famous “cousin”.
It is probably also a dwarf planet, but the International Astronomical Union (the “Fifa” of astronomy) does not classify it that way, because one of the conditions to receive the name is to have reached hydrostatic equilibrium (translating from scientificese, to be approximately spherical ). The scientists’ work began precisely with this objective: to try to determine the shape of Quaoar.
Observations of light reflected from the object are not very useful for this. Because it’s relatively small and distant, even our best telescopes don’t reveal details of its shape. Quaoar only appears as a few lit pixels, even in the best image. To get around the difficulty, astronomers exploit phenomena known as occultations.
The thing happens when the orbit of the object makes it transit in front of a more distant star, temporarily blocking its light. The blocking time indicates the size of the star. And if that blockage is measured from several points on Earth, it’s like measuring several parallel sections, each with its own length. Together, they can allow the creation of an approximate model of the form.
Occultations, however, are rare events and difficult to observe. For the current work, published in this week’s issue of Nature, the researchers relied on data from four different occultations produced by Quaoar, which occurred on September 2, 2018, June 5, 2019, June 11, 2020, and August 27, 2019. 2021.
Cross-sections have not yet determined the shape (work that is still in progress), but they have revealed something even more interesting: the presence of a ring around the star, orbiting about 3,545 km from the surface — closer than the only known moon from Quaoar, Weywot, which orbits further away, 12,700 km from the soil of the potential dwarf planet.
The ring becomes detectable by itself producing rapid occultations of the background star, and from the cross-sections it was possible to model it. Interestingly, it reveals an irregular pattern, with denser and less dense regions, and the researchers expect a composition of stones of various sizes, ranging from micrometers to a kilometer in diameter.
Same but different
The idea is already consolidating that rings are common structures in the outer Solar System, province of the giant gaseous planets. Jupiter, Saturn, Uranus and Neptune have them, and recently occultations have revealed that the Chariclus object has two rings. It is a star with about 250 km in diameter that orbits between Saturn and Uranus and is a member of a population known as centaurs (they are essentially objects originating from the Kuiper belt, located beyond Neptune, that ended up thrown into the solar system). and relocated in the region of the giant planets). By the way, the discovery, announced in 2014, was made by the same group as the current find, also with Brazilian leadership, and the two rings were provisionally named Oiapoque and Chuí. In 2017, the dwarf planet Haumea was discovered to also have a ring.
The big difference between these and the new finding is that until then practically all the rings already seen in the Solar System, in the giant planets or in other smaller objects, were close to or within an imaginary line from which tidal gravitational effects prevent preservation. of large objects —and for that very reason become a favorable environment for the formation of rings.
Quaoar’s is well beyond that boundary, known as the Roche limit, which, in short, means that, by astronomers’ expectations, it should coalesce to eventually form moons. The ring is 4,100 km from the center of the system, and the imaginary boundary would be 1,780 km away.
“The main result of this work is the discovery of a dense ring beyond the known Roche limit”, says Bruno Morgado, a researcher at the Federal University of Rio de Janeiro (UFRJ) and first author of the study, which has 59 authors from 14 countries , including researchers from the National Observatory (ON), the Interinstitutional Laboratory of e-Astronomy (LIneA), the Federal Technological University of Paraná (UTFPR), the São Paulo State University (Unesp) and the Federal University of Uberlândia (UFU).
“For the first time we found a counterpoint to this theory that has been used since 1850. This discovery reveals to us that the process of satellite formation must be more complex than previously imagined and that we need to understand how Quaoar’s ring is stable so far away from it , which effects allow it not to be becoming a moon.”
One possibility is that it is indeed forming a natural satellite, that is, that we are facing a ring that was only recently formed by some collision event and that will still coalesce to form a moon.
“However, this hypothesis is very improbable”, explains Morgado. “A ring beyond the Roche limit would become a satellite in the order of a few tens of years. Which makes it extremely unlikely, taking into account the age of the Solar System. [4,6 bilhões de anos]that we are at the right time and in the right place to observe this event.”
Of course, improbable is not impossible. Therefore, it will be important to continue to observe it, with future occultations, to see how it evolves. “If it is indeed true that we got incredibly lucky, the changes will be clear [em novas observações]”, says Morgado. “The ring will disappear, and what will remain will be just a moon of a few kilometers in radius.”
It will be even more intriguing if the ring doesn’t disappear —as the researchers are betting. New measurements will help to map it more precisely and perhaps help to clarify one of its mysteries: it is quite irregular, with denser and thicker regions and others that are narrower and more discreet.
“Probably the same dynamic mechanisms that make the ring stable outside the Roche limit must be influencing the shape and structure of this ring. Only with more dynamic models and more observations will we be able to understand this.”
Fact is, the find, pointing the existence of rings to where they were not expected, suggests that there are many more of them out there waiting to be found. For Morgado and his colleagues, it’s a full plate. “Quaoar is the third example of a ring around a small body in our Solar System, and this is unlikely to be the final number. With better equipment, new cameras and better telescopes, we will surely be able to discover other rings. And now we must look not only in the expected region, inside the Roche limit, but also outside that region.”
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