Mysterious rings discovered around dwarf planet Quaoar
Several bodies in our solar system have rings of ice and rock. However, according to current theories, the rings around the dwarf planet Quaoar should not exist.
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 scientific journal "Nature", they now put forward several hypotheses as to how the mysterious ring could be explained.
Ring systems are nothing 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. However, Quaoar's rings lie beyond the so-called Roche limit and are therefore 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 limit, 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 Quaoar, which was discovered in 2002, orbits so far away from the Earth - at a distance of almost 6.5 billion kilometres - it is little more than a small speck in images from large telescopes. Extremely faint ring systems cannot be imaged directly. However, they can be detected indirectly. When Quaoar passes by a star in the sky as seen from Earth and briefly covers it, the light curve of this encounter can reveal a lot. In the case of Quaoar, there was not just one dimming of the star, but three: a strong one in the centre, 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, the 170-kilometre moon of Quaoar, interferes with the formation of other moons with its gravity. Encounters with it, or with smaller, previously undiscovered moons, could repeatedly accelerate the chunks of rock and ice. As a result, their speed 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 is now orbiting Quaoar. However, this scenario is rather unlikely, the researchers write, as the debris should form a moon again within a few decades. Further observations of Quaoar and its surroundings must now show which of these explanations for the wide ring is correct.
Spectrum of Science
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Image: ESA / QUAOAR / CC BY-SA 3.0 IGO CC BY-SA IGO
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