With a diameter of just under 1100 kilometres, the dwarf planet Quaoar is about half the size of Pluto and orbits beyond Neptune, at the edge of our solar system. A research team led by Bruno Morgado from the Universidade Federal do Rio de Janeiro in Brazil has now discovered that Quaoar is surrounded by a ring system that, according to current theories, should not exist. The scientists used the Gran Telescopio Canarias on La Palma. In the specialist journal "Nature" they now put forward several hypotheses as to how the puzzling ring could be explained.
Ring systems are not unusual in themselves, the best known being those of Saturn and Neptune. Other small bodies in the solar system, such as the asteroid Chariklo or the dwarf planet Haumea, are also surrounded by such accumulations of material. Quaoar's rings, however, lie beyond the so-called Roche boundary and are thus significantly further away from the central body than all previously known ring systems. This is surprising, because according to current theories, rings should only exist within the Roche boundary, where the material cannot come together due to the stronger gravitational pull of the main body. Beyond this boundary, the rocks and chunks of ice that form the rings clump together and form moons.
Because
the rings are not the same as the main body. Because Quaoar, discovered in 2002, orbits the Sun so far from Earth - at a distance of nearly 6.5 billion kilometres - it is little more than a tiny speck in images taken by large telescopes. Extremely faint ring systems cannot be imaged directly. But they can be detected indirectly. When Quaoar passes by a star in the sky as seen from Earth and covers it briefly, the light curve of this encounter can reveal quite a bit. In Quaoar's case, there was not just one dip in the star's brightness, but three: a strong one in the middle, surrounded by two faint ones. The two faint dimmings of the star, the researchers concluded, come from Quaoar's ring system, which surrounds it at a distance of 4100 kilometres. . Why didn't a moon form from the matter in the rings? The researchers suspect that Weywot, Quaoar's 170-kilometre moon, interferes with the formation of other moons with its gravity. Encounters with it, or smaller previously undiscovered moons, could repeatedly accelerate the chunks of rock and ice. As a result, their velocity would never drop far enough to stay together after collisions. Another explanation for its unusual rings could be a moon that was recently destroyed and whose debris now orbits Quaoar. However, this scenario is rather unlikely, the researchers write, as the debris should already have reassembled into a moon within a few decades. Which of these explanations for the wide ring is correct must now be shown by further observations of Quaoar and its surroundings.
Spectrum of Science