A tiny moon, too small to see from Earth, could be playing cosmic sheep by shaping the thin rings around the distant minor planet Chariklo, according to new computer simulations.
Chariklo a Centaur – a breed of small body that orbits the sun on a path sandwiched somewhere between Jupiter and Neptune. Centaurs are thought to come in from the Oort cloudwhich is a huge halo of trillions of frozen cometary bodies that extends perhaps a light year from the sun. Over millions of years, they flow in, but also to be detached from Solar system at the gravity of the giant planets, or to move into Jupiter-family Comets — Comets with orbital periods of less than 20 years subject to the gravity of mighty Jupiter.
At about 155 miles (250 kilometers) in diameter, Chariklo is the largest known Centaur. In 2013, Chariklo was seen passing in front of a distant star in our time, causing the starlight to dim briefly. Observations showed that Chariklo’s solid body not only blocked the light, but also formed a pair of thin rings around the Centaur. Stellar occultations have shown that another centaur, Chiron, also has rings, although they are not similar completely stable.
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“Rings around minor planets have only recently been discovered, and only a small number of such systems are currently known,” said Amanda Sickafoose, senior scientist at the Planetary Science Institute in Arizona, in a. press release.
The two rings of Chariklo are located approximately 243 miles and 252 miles (391 and 405 km), respectively, from the center of Chariklo, although measurements can vary slightly. The rings themselves are narrow, just 4.3 miles and 1.9 miles (7 and 3 km) wide.
These rings are not like the rings of the huge planet, who are held back by the gravity of those vast lives. Computer models show that the Chariklo rings should be short-lived, and that they should expand and then spread. So what is holding them in place?
Sickafoose and Mark Lewis, a computer scientist at Trinity University in San Antonio, performed N-body computer simulations of the rings made up of millions of particles, and showed that the influence of a small moon could keep the rings narrow and prevent them from spreading. . .
“We have shown that one of the possibilities for the existence of thin rings around small bodies is that they are being sculpted by a small man satellite,” said Sickafoose.
Shepherd Moons is nothing new; all the gas giant planets have them, especially Saturnwhere the tiny moons Daphnis, Epimetheus, Janus, Pan and Prometheus carve gaps in The rings of Saturn and then herd the ring material to keep it tidy.
Chariklo’s shepherd moon, if it exists, would be about 1.9 miles (3 km) across and have a mass of about 10 trillion kilograms (about 22 trillion pounds here on Earth). Although it is too small to look directly from World in the distance of Chariklo – the elongated orbit of the Centaur gets as close to the sun as 1.2 billion miles (1.9 billion km) and as far away as 1.7 billion miles (2.8 billion km) – its presence could be noticed during stellar occulations .
There is one more explanation for the staying power of the rings. If we think there is a gravitational anomaly within Chariklo — a denser part of the rock, for example — then a gravitational resonance between the rings and the anomaly could help bind the rings. The rings orbit almost 1:3 with the Centaur, meaning that Chariklo rotates three times (once every seven hours) for every one rotation of the rings. However, the rings are also placed suspiciously close to the Roche limit. This is the distance from a central body where gravity prevents particles from entering the Moon. At the edge of this limit, the ring particles should be able to start sticking together to build a new moon. A shepherd’s moon would be able to intervene to prevent this.
“A satellite in this case can interfere with the ring material and prevent it from accreting,” Sickafoose said.
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“We think that the ring particles are made mainly of water ice, like those of the gas giants,” said Sickafoose. Indeed, the James Webb Space Telescope water-ice sensor there in 2023. However, things we don’t know include how “hard” or “soft” the particles are when they collide, or the size distribution of the particles in the rings.
It is also not known how the rings were formed in the first place. Are they obliterated by cryovolcanism on the surface of Chariklo, or are they the remains of a small body that got too close to Chariklo, inside the Roche boundary, and was torn apart?
The results were published on 6 February i The Planetary Science Journal.