Two groups of astronomers have independently detected a possible ring system around an outer Solar System body called Chiron.
This false-color image shows Chiron (April 28, 1998). Image credit: Adrian Silva / Sergio Cellone, doi: 10.1016/S0032-0633(01)00052-6.
Chiron, officially designated 2060 Chiron and 96P/Chiron (the former designation being asteroidal and the second is cometary), was discovered on 1 November 1977 by astronomer Charles Kowal.
This object was the first of four bodies discovered so far with similar orbits and properties. These bodies have been named Centaurs, after the race of half-man/half-horse beings from Greek mythology, in recognition of their dual comet/asteroid nature.
Centaurs are a set of Solar System objects whose orbits are confined between those of Jupiter and Neptune. The current belief is that they are objects scattered from the Kuiper Belt that may eventually end up in the inner Solar System as short-period comets.
Named after the wisest of the Centaurs, the tutor of Achilles and Hercules, Chiron is unusual because it has a detectable coma, indicating that it’s a cometary body, but it is over 50,000 times the characteristic volume of a comet, a size more commensurate with a large asteroid.
The diameter of the Chiron’s coma has been measured to reach almost 2 million km in diameter on occasion, and the brightness can fluctuate by a factor of four over a period of a few hours.
In addition, a gravitationally bound ‘dust atmosphere’ appears to be suspended in the inner 1,200 km of the coma, and this dust displays evidence of structure, indicating the possibility of particle plumes emanating from the nucleus.
Chiron, also known as 1977 UB, is the third Centaur in which water ice has been detected. This trend suggests that water ice is common on the surface of Centaurs.
Estimates of Chiron’s diameter range from 148 to 208 km. It orbits the Sun every 50.39 years and has a rotation period of 6 hr.
A team of astronomers from South Africa and the United States led by Jessica Ruprecht of Massachusetts Institute of Technology has obtained precise observations of Chiron, using two large telescopes in Hawaii: NASA’s Infrared Telescope Facility and the Las Cumbres Observatory Global Telescope Network.
In 2010, the scientists started to chart the orbits of Chiron and nearby stars in order to pinpoint exactly when the centaur might pass across a star bright enough to detect. They determined that such a stellar occultation would occur on November 29, 2011, and reserved time on the two large telescopes in hopes of catching Chiron’s shadow.
They observed the stellar occultation remotely; the entire event lasted just a few minutes, and the telescopes recorded the fading light as Chiron cast its shadow over the telescopes.
The team then analyzed the resulting light, and detected something unexpected. A simple body, with no surrounding material, would create a straightforward pattern, blocking the star’s light entirely.
But the scientists observed two symmetrical features, each about 300 km from the Chiron’s center, near the start and end of the stellar occultation – a sign that material such as dust might be blocking a fraction of the starlight. Judging from the optical data, the features are 3 and 7 km wide, respectively.
The findings were published online February 4, 2015 in the journal Icarus.
Ruprecht and her colleagues said that Chiron may still possess symmetrical jets of gas and dust. However, other interpretations may be equally valid, including the intriguing possibility of a shell or ring of gas and dust.
Another team of scientists, led by Dr Jose Luis Ortiz from the Institute of Astrophysics of Andalusia in Granada, Spain, has reported the detection of a ring material around Chiron.
In a paper published in the journal Astronomy & Astrophysics (arXiv.org preprint), they present several possible formation scenarios for the Chiron’s rings from qualitative points of view and speculate on why rings might be common in most centaurs.
Dr Ortiz and his colleagues also speculate on whether the known color distribution of the centaurs could be due to centaurs with rings and centaurs without rings.
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Jessica D. Ruprecht et al. 2015. 29 November 2011 stellar occultation by 2060 Chiron: Symmetric jet-like features. Icarus, vol. 252, pp. 271–276; doi: 10.1016/j.icarus.2015.01.015
J. L. Ortiz et al. 2015. Possible ring material around centaur (2060) Chiron. A&A, vol. 576, A18; doi: 10.1051/0004-6361/201424461
