An international group of 59 astronomers, led by Brazilian scientists, have found a dense ring, formed by small blocks of ice and rock, revolving at a remove of 4,100 kilometers (km) around a distant object in the Solar System. The far-flung location of the circular structure orbiting Quaoar, as the icy object has been named, is completely at odds with a fundamental equation in celestial mechanics. According to the Roche limit, a formula used since the mid-nineteenth century to calculate the expected position of structures such as rings and natural satellites (moons) orbiting larger space objects, the ring around Quaoar should be located a maximum distance of 1,780 kilometers from its center.
“A dense ring has never been observed orbiting an object outside the Roche limit,” explains astrophysicist Bruno Morgado, from the Valongo Observatory of the Federal University of Rio de Janeiro (OV-UFRJ). Morgado is lead author of the discovery, described in an article published February 8 in the journal Nature. “At a distance of more than 4,000 kilometers from Quaoar, what should have formed is a moon, not a ring.” The finding is expected to lead scientists to refine the theories that predict the formation of rings and satellites around planets and other celestial objects.
Discovered in 2002, Quaoar is a small, icy world, 1,121 km in diameter, approximately one-twelfth the size of Earth. Its name is a reference to the god of creation in the mythology of the Tongva, an aboriginal American people who live in what is now southern California in the United States. Quaoar is generically classified as a trans-Neptunian object. It is located in the Kuiper belt, which begins just beyond the orbit of Neptune, the eighth planet in the Solar System (Pluto, which is also inside the belt, was considered the ninth planet until 2006 when it was downgraded to dwarf planet status). The Kuiper belt is made up of trillions of pieces of material (rocks and ice) left over from the formation of the Solar System, 4.6 billion years ago. Its most prominent objects can be classified as comets or dwarf planets. Quaoar, which is half the diameter of Pluto, is a candidate for official recognition as a dwarf planet.
There are no images of Quaoar’s ring, only indirect evidence of its presence. Using telescopes in various locations around the Earth, researchers observed reductions in the brightness of stars whenever Quaoar and its ring passed in front of them. “It’s a phenomenon similar to an eclipse,” says astrophysicist Felipe Braga Ribas, from the Federal Technological University of Paraná (UTFPR), one of the study’s coauthors. “We recorded the typical signature of an occultation caused by the passage of an object with a ring.”
The signature of an occultation caused by an object with a ring is characterized by three consecutive reductions in the brightness of the star over the course of one minute: one smaller diminution, caused by the passage of one side of the ring; a larger one, caused by the crossing of the object itself (in this case, Quaoar); and a second smaller one, produced by the other section of the ring. The two smaller dips in stellar luminosity caused by the Quaoar ring differed slightly in intensity. “The ring is not completely symmetrical,” says astrophysicist Rafael Sfair, from the School of Engineering and Sciences at São Paulo State University (UNESP), another of the study’s coauthors. “One side is wider than the other.” Sfair’s research, which studies the characteristics and orbital dynamics of rings, is partially funded by a FAPESP project.
The Roche limit value is calculated from various parameters, mainly the radius and density of the central object and the mass of the structures that orbit it (moons or rings). Roughly speaking, within the Roche limit, the influence of the gravitational force of the larger body prevents the smaller pieces of material in its orbit from coming together and forming a moon. (There are only one or two known exceptions to this rule). In such cases, the dominant pattern is that a ring forms around the larger body. Outside the Roche limit, the loose material is able to aggregate and form moons. Quaoar itself has a natural satellite, Weywot, which obeys this formula.
When a satellite or comet crosses the Roche boundary of a planet, it tends to disintegrate due to the gravity of the larger object. In July 1992, the comet Shoemaker–Levy 9 broke into more than 20 pieces as it entered Jupiter’s Roche boundary. Two years later, these remnants crashed into the planet.
Saturn, the second largest planet in the Solar System, has the system’s largest and most spectacular ring system, although Uranus, Jupiter, and Neptune also have ring structures encircling them. In this century, rings have been found around other types of objects, such as exoplanets, moons, stars, and other celestial bodies. In 2013, Ribas was the lead author of a study that discovered the first ring around an asteroid, which is located between the orbits of Saturn and Uranus, an object 250 kilometers in diameter called Chariklo. “But that ring is within the Roche limit,” observes the UTFPR astrophysicist.
For now, astrophysicists have no explanation for the remote orbit of Quaoar’s ring. One possibility is that there is some kind of gravitational influence on the system that prevents the pieces of the ring from coming together and forming a natural satellite, as would normally be expected. This could be caused by some anomaly on Quaoar, its moon Weywot, or a small unknown satellite.
“If it is a transient structure, the ring should gradually disappear as its material aggregates,” writes astrophysicist Matthew Hedman, from the University of Idaho, in the United States, in a commentary article also published on February 8 in Nature. “However, if it is long-lasting, the opacity variations along its length could be tracked over time to narrow in on exactly how fast the ring material is orbiting around Quaoar.”
Brazilian researchers Morgado, Ribas, and Sfair argue that ring fragments, like the ones found orbiting this icy body in the Kuiper belt, would theoretically coalesce and form a moon in five or ten years. The exact age of Quaoar is not known, but it is reasonable to assume that it has existed for millions or even billions of years. “It would be almost impossible for us to have been lucky enough to find the ring just before its fragments come together and form into a satellite,” comments the UFRJ astronomer.Republish