Asteroid rings discovered

Last week astronomers announced the surprise discovery of an asteroid with its own ring system. Until now, rings have only been detected around the giant planets in our Solar System – Jupiter, Saturn, Uranus and Neptune – and it was thought that only massive planets could host rings.  The discovery of two narrow rings around the minor planet Chariklo, a Centaur in the outer Solar System, has forces astronomers to revise their ideas of rings.

Artist’s impression of the rings surrounding the minor planet Chariklo, the first non-planetary body in our Solar System discovered to have its own ring system. (Credit: Lucie Maque)

Chariklo is the largest of the Centaurs and orbits between Saturn and Uranus with an average distance of nearly 16 AU (16 times further from the Sun than the Earth).  Centaurs are small, rocky-icy minor planets – or asteroids – on unstable orbits in the outer Solar System. Because their orbits carry them close to the giant planets, their orbits cannot be stable for more than a few million years.  The name Centaur – which is a half-man-half-horse from Greek mythology – comes from the fact that these minor planets share properties with both asteroids and comets, including cometary coma-like activity.

Chariklo was discovered in 1997 and had a very surprising property: both its brightness and composition were seen to change over time.  After its discovery, its brightness dropped dramatically and the water-ice features initially seen in the spectra of Chariklo fading away. Equally mysteriously, this trend has reversed since 2008, with the brightness increasing and the water-ice features returning.

To learn more about this mysterious object, Felipe Braga-Ribas, of the Observatório Nacional in Brazil, devised an experiment to study the size and shape of Chariklo during a stellar occultation.  From a vantage point in South America, Chariklo was predicted to pass in front of a star on 3 June 2013. Using seven different telescope across Brazil, Argentina, Uruguay and Chile, Braga-Ribas and collaborators watched as Chariklo blocked the starlight, but were surprised to see a dip in the star’s light a few second before and after the main occultation.  Using data from the 7 different telescopes revealed that these extra dips in stellar brightness were caused by two thin rings around Chariklo, just 7 and 3 km wide. The rings on Uranus were discovered in the same way during stellar occultation in the 1977.

The light curve of UCAC4 248-108672 during Chariklo’s occultation on 3 June 2013. The light curve shows the stellar light as a function of time, with the central dip caused by Chariklo transiting in front of the background star, and the two dips either side due to Chariklo’s two rings blocking some of the stellar light. (Credit: Braga-Ribas et al. 2014, Nature)

The ring system nicely explains the mystery of the changing brightness and composition of Chariklo. The reflective, icy rings would add to the brightness of the asteroid when viewed face-on, but the brightness and the water-ice signal would drop when the rings are seen edge-on.

What remains a mystery is how the rings formed, and remain stable, around such a small object. Chariklo is just 250 km in diameter. The most likely explanation is that the rings formed from debris left over from a collision, either from a small body catastrophically colliding with Chariklo and ripping off some of its outer icy layer, or perhaps the collision of pre-existing icy satellites.  Given 5% of Centaurs are known to have small companions, the idea of colliding satellites is not unreasonable. For the rings to be stable, any collision that caused them must have occurred at very low speeds to ensure that the ring particles remained within the gravitational attraction of the asteroid’s tiny mass.  And to have two thin rings, the ring material must be confined by tiny km-sized ‘shepherding’ satellites.

While Chariklo is the first satellite found to host rings, it is unlikely to be unique in our Solar System.

For more information, see

[Sarah Maddison]

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