[meteorite-list] First Ring System Discovered Around Asteroid (Chariklo)

[Ron Baalke]

http://www.eso.org/public/news/eso1410/

First Ring System Around Asteroid

Chariklo found to have two rings
European Southern Observatory
26 March 2014

[Image]
Observations at many sites in South America, including ESO's La Silla 
Observatory, have made the surprise discovery that the remote asteroid 
Chariklo is surrounded by two dense and narrow rings. This is the smallest 
object by far found to have rings and only the fifth body in the Solar 
System - after the much larger planets Jupiter, Saturn, Uranus and Neptune 
- to have this feature. The origin of these rings remains a mystery, but 
they may be the result of a collision that created a disc of debris. The 
new results are published online in the journal Nature on 26 March 2014.

The rings of Saturn are one of the most spectacular sights in the sky, 
and less prominent rings have also been found around the other giant planets. 
Despite many careful searches, no rings had been found around smaller 
objects orbiting the Sun in the Solar System. Now observations of the 
distant minor planet [1] (10199) Chariklo [2] as it passed in front of 
a star have shown that this object too is surrounded by two fine rings.

"We weren't looking for a ring and didn't think small bodies like Chariklo 
had them at all, so the discovery - and the amazing amount of detail we 
saw in the system - came as a complete surprise!" says Felipe Braga-Ribas 
(Observatorio Nacional/MCTI, Rio de Janeiro, Brazil) who planned the observation 
campaign and is lead author on the new paper.

Chariklo is the largest member of a class known as the Centaurs [3] and 
it orbits between Saturn and Uranus in the outer Solar System. Predictions 
had shown that it would pass in front of the star UCAC4 248-108672 on 
3 June 2013, as seen from South America [4]. Astronomers using telescopes 
at seven different locations, including the 1.54-metre Danish and TRAPPIST 
telescopes at ESO's La Silla Observatory in Chile [5], were able to watch 
the star apparently vanish for a few seconds as its light was blocked 
by Chariklo - an occultation [6].

But they found much more than they were expecting. A few seconds before, 
and again a few seconds after the main occultation there were two further 
very short dips in the star's apparent brightness [7]. Something around 
Chariklo was blocking the light! By comparing what was seen from different 
sites the team could reconstruct not only the shape and size of the object 
itself but also the shape, width, orientation and other properties of 
the newly discovered rings.

The team found that the ring system consists of two sharply confined rings 
only seven and three kilometres wide, separated by a clear gap of nine 
kilometres - around a small 250-kilometre diameter object orbiting beyond 
Saturn.

"For me, it was quite amazing to realise that we were able not only to 
detect a ring system, but also pinpoint that it consists of two clearly 
distinct rings," adds Uffe Grae Jorgensen (Niels Bohr Institute, University 
of Copenhagen, Denmark), one of the team. "I try to imagine how it would 
be to stand on the surface of this icy object - small enough that a fast 
sports car could reach escape velocity and drive off into space - and 
stare up at a 20-kilometre wide ring system 1000 times closer than the 
Moon." [8]

Although many questions remain unanswered, astronomers think that this 
sort of ring is likely to be formed from debris left over after a collision. 
It must be confined into the two narrow rings by the presence of small 
putative satellites.

"So, as well as the rings, it's likely that Chariklo has at least one 
small moon still waiting to be discovered," adds Felipe Braga Ribas.

The rings may prove to be a phenomenon that might in turn later lead to 
the formation of a small moon. Such a sequence of events, on a much larger 
scale, may explain the birth of our own Moon in the early days of the 
Solar System, as well as the origin of many other satellites around planets 
and asteroids.

The leaders of this project are provisionally calling the rings by the 
nicknames Oiapoque and Chui, two rivers near the northern and southern 
extremes of Brazil [9].

Notes

[1] All objects that orbit the Sun, which are too small (not massive enough) 
for their own gravity to pull them into a nearly spherical shape are now 
defined by the IAU as being small solar system bodies. This class currently 
includes most of the Solar System asteroids, near-Earth objects (NEOs), 
Mars and Jupiter Trojan asteroids, most Centaurs, most Trans-Neptunian 
objects (TNOs), and comets. In informal usage the words asteroid and minor 
planet are often used to mean the same thing. 

[2] The IAU Minor Planet Center is the nerve centre for the detection 
of small bodies in the Solar System. The names assigned are in two parts, 
a number - originally the order of discovery but now the order in which 
orbits are well-determined - and a name.

[3] Centaurs are small bodies with unstable orbits in the outer Solar 
System that cross the orbits of the giant planets. Because their orbits 
are frequently perturbed they are expected to only remain in such orbits 
for millions of years. Centaurs are distinct from the much more numerous 
main belt asteroids between the orbits of Mars and Jupiter and may have 
come from the Kuiper Belt region. They got their name because — like the 
mythical centaurs - they share some characteristics of two different things, 
in this case comets and asteroids. Chariklo itself seems to be more like 
an asteroid and has not been found to display cometary activity.

[4] The event was predicted following a systematic search conducted with 
the MPG/ESO 2.2-metre telescope at ESO's La Silla Observatory and recently 
published.

[5] Besides the Danish 1.54-metre and TRAPPIST telescopes at ESO's La 
Silla Observatory, event observations were also performed by the following 
observatories: Universidad Catolica Observatory (UCO) Santa Martina operated 
by the Pontificia Universidad Catolica de Chile (PUC); PROMPT telescopes, 
owned and operated by the University of North Carolina at Chapel Hill; 
Pico dos Dias Observatory from the National Laboratory of Astrophysics 
(OPD/LNA) - Brazil; Southern Astrophysical Research (SOAR) telescope; 
Caisey Harlingten's 20-inch Planewave telescope, which is part of the 
Searchlight Observatory Network; R. Sandness's telescope at San Pedro 
de Atacama Celestial Explorations; Universidade Estadual de Ponta Grossa 
Observatory; Observatorio Astronomico Los Molinos (OALM) - Uruguay; Observatorio 
Astronomico, Estacion Astrofisica de Bosque Alegre, Universidad Nacional 
de Cordoba, Argentina; Polo Astronômico Casimiro Montenegro Filho Observatory 
and Observatorio El Catalejo, Santa Rosa, La Pampa, Argentina.

[6] This is the only way to pin down the precise size and shape of such 
a remote body - Chariklo is only about 250 kilometres in diameter and 
is more than a billion kilometres from Earth. Even in the best telescopic 
views such a small and distant object just appears as a faint point of 
light.

[7] The rings of Uranus, and the ring arcs around Neptune, were found 
in a similar way during occultations in 1977 and 1984, respectively. ESO 
telescopes were also involved with the Neptune ring discovery.

[8] Strictly speaking the car would have to be rather fast - something 
like a Bugatti Veyron 16.4 or McLaren F1 - as the escape velocity is around 
350 km/hour!

[9] These names are only for informal use, the official names will be 
allocated later by the IAU, following pre-established rules.

More information

This research was presented in a paper entitled "A ring system detected 
around the Centaur (10199) Chariklo", by F. Braga-Ribas et al., to appear 
online in the journal Nature on 26 March 2014.