Ring spotted around Chiron

2015-04-29 06:00

ASTRONOMERS at the South African Astronomical Observatory (SAAO) and Massachusetts Institute of Technology (MIT), America, have recently detected what may be rings around a minor planet in our solar system named Chiron.

While the planet Saturn is famous for its spectacular ring system, all the gassy giant planets (Jupiter, Saturn, Uranus and Neptune) have rings. There is also another object, a minor planet called Chariklo, which has rings.

Chiron and Chariklo belong to a class of minor planets called centaurs: small rocky bodies that possess properties of both asteroids and comets. These objects are found in a band between the orbits of Jupiter and Pluto.

Prior to the discovery of Chariklo’s rings, scientists had thought that centaurs were relatively dormant objects. The latest discovery of a possible ring system around another centaur, Chiron, if confirmed, suggests that ring systems are far more common in our solar system than previously thought.

“Until Chariklo’s rings were found, it was commonly believed that these smaller bodies did not have a ring system. The likelihood that Chiron also has a ring system prompts fundamental questions about how planetary rings form and evolve,” says Dr Amanda Gulbis (SAAO), co-investigator in the study.

Gulbis at the South African Astronomical Observatory and her collaborators were first alerted to the possibility that Chiron has a ring system following observations conducted in November 2011.

Chiron was observed passing in front of a bright star, an event known as a stellar occultation. Using Nasa’s Infrared Telescope Facility on Mauna Kea, Hawaii, and the Las Cumbres Observatory Global Telescope Network at Haleakala, Hawaii, astronomers monitored the brightness of the star’s light as it was occulted by Chiron.

In addition to seeing the expected drop in brightness as Chiron passed in front of the star, they identified features in the data indicative of a disk of debris around the centaur. The features could be explained by either a ring system around the centaur, a circular shell of gas and dust around the centaur, or symmetric jets of material shooting out from the centaur’s surface.

In order to observe the occultation event, astronomers had to work out if and when the centaur would pass in front of a bright star. They began charting the orbits of Chiron and nearby stars in 2010 in order to pinpoint exactly when the centaur might occult a bright star.

They calculated that a stellar occultation would occur on 29 November 2011, and reserved time on two large telescopes in hopes of observing the event. They knew they would have to be fortunate to catch the occultation which would last just a few minutes.

Dr Amanda Bosh, a leader on the project and a lecturer in MIT’s Department of Earth, Atmospheric and Planetary Sciences, says: “There’s an aspect of serendipity to these observations. We need a certain amount of luck, waiting for Chiron to pass in front of a star that is bright enough. Chiron itself is small enough that the event is very short; if you blink, you might miss it.”

The group analysed the light received from the star during the occultation event and detected something unexpected. A simple body, with no surrounding material, would create a straightforward pattern, blocking the star’s light entirely. However, astronomers were surprised to find that a small fraction of light from the star was blocked just before and just after Chiron passed in front of it.

There were actually two dips in brightness before and after the occultation suggesting that there could be two separate rings of material around the centaur. Judging from the dips in intensity of the star’s light, astronomers have calculated that the two possible rings are located about 300 km from the centre of the centaur and are around 3 km and 7 km wide respectively and separated by a distance of around 7,5 km.

Astronomers are, however, uncertain as to exactly what form this obscuring material actually takes: It could be rings, a shell of gas and dust or symmetrical jets of gas and dust shooting out from the centaur’s surface.

“This result is particularly interesting because Chiron is a centaur – a relatively small, cold object that has an unstable orbit in the region of the giant planets. How does a body that is only a few hundred kilometers in diameter, and who’s orbit is only stable for a few million years, have a ring system?” asked Gulbis.

Several explanations have been put forward to explain how the centaur Chiron could have gained rings. One possibility is that another minor body broke up and the resulting debris was captured gravitationally around Chiron. Alternatively, the rings may have formed from leftover material from the formation of Chiron itself.

“Another possibility involves the history of Chiron’s distance from the sun. Centaurs may have started further out in the solar system and, through gravitational interactions with giant planets, have had their orbits perturbed closer in to the sun,” says Jessica Ruprecht, a student at the MIT’s Lincoln Laboratory, who was the lead author on the manuscript.

“The frozen material that would have been stable out past Pluto is becoming less stable closer in, and can turn into gasses that spray dust and material off the surface of a body.”

An independent group has since combined the data obtained from this study with other data, and has concluded that the features around Chiron most likely represented a ring system.

However, researchers will have to observe more stellar occultations of Chiron to truly determine which interpretation: rings, shell or jets, is the correct one.

“If we want to make a strong case for rings around Chiron, we’ll need observations by multiple observers, distributed over a few hundred kilometers, so that we can map the ring geometry,” says Ruprecht. “But that alone doesn’t tell us if the rings are a temporary feature of Chiron, or a more permanent one. There’s a lot of work that needs to be done.”

The results from this study have been published in the journal Icarus.

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Dr Nicola Loaring is an astronomer at the South African Astronomical Observatory (SAAO)

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