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| Figure 1: (left) Visualization of Chariklo and its rings, provided by
the European Southern Observatory
(http://www.eso.org/public/images/eso1410b/). Credit: ESO/L. Calçada/M.
Kornmesser/Nick Risinger (skysurvey.org). (right) Visualization of the rings as seen from Chariklo’s surface, provided by the European Southern Observatory (https://www.eso.org/public/images/eso1410a/) Credit: ESO/L. Calçada/Nick Risinger (skysurvey.org). |
A team of researchers has clarified the origin of the rings recently
discovered around two minor planets known as centaurs, and their results
suggest the existence of rings around other centaurs. These findings
were published on August 29 in Astrophysical Journal Letters, and
introduced in AAS Nova, a website for research highlights from the
journals of the American Astronomical Society.
The lead author of the paper is HYODO Ryuki
(Kobe University Department of Planetology, Graduate School of Science),
and co-authors are Professor Sébastien Charnoz (Institute de Physique
du Globe/Université Paris Diderot), Project Associate Professor GENDA
Hidenori (Earth-Life Science Institute, Tokyo Institute of Technology),
and Professor OHTSUKI Keiji (Kobe University Department of Planetology,
Graduate School of Science).
Centaurs are minor planets that orbit between
Jupiter and Neptune, their current or past orbits crossing those of the
giant planets. It is estimated that there are around 44,000 centaurs
with diameters larger than one kilometer.
Until recently it was thought that the four
giants such as Saturn and Jupiter were the only ringed celestial bodies
within our solar system. However, in 2014 observations of stellar
occultation (an event that occurs when light from a star is blocked from
the observer by a celestial body) by multiple telescopes revealed that
rings exist around the centaur Chariklo (see Figure 1). Soon after this,
scientists discovered that rings likely exist around another centaur,
Chiron, but the origin of the rings around these minor planets remained a
mystery.
The team began by estimating the probability
that these centaurs passed close enough to the giant planets to be
destroyed by their tidal pull. Their results showed that approximately
10% of centaurs would experience that level of close encounter. Next,
they used computer simulations to investigate the disruption caused by
tidal pull when the centaurs passed close by the giant planets. The
outcome of such encounters was found to vary depending on parameters
such as the initial spin of the passing centaur, the size of its core,
and the distance of its closest approach to a giant planet (Figure 2).
They found that if the passing centaur is differentiated and has a
silicate core covered by an icy mantle, fragments of the
partially-destroyed centaur will often spread out around the largest
remnant body in a disc shape, from which rings are expected to form.
The results of their simulations suggest that
the existence of rings around centaurs would be much more common than
previously thought. It is highly likely that other centaurs with rings
and/or small moons exist, awaiting discovery by future observations.
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| Figure 2: Examples of snapshots of simulations with different initial conditions. The center of each panel shows a centaur after being partially destroyed, and the fragments around it distributed in a disc shape, from which observed rings are expected to form (from Hyodo et al. 2016, Astrophysical Journal Letter 828, L8) |
Technical terms
- 1. Centaur:
- small celestial bodies that orbit between Jupiter and Neptune. Their current or past orbits repeatedly cross those of the giant planets, and sometimes pass very close by the giant planets themselves.
- 2. Chariklo:
- a centaur with a radius of approximately 250 kilometres. In 2014 it was clarified by stellar occultations that this centaur has rings.
- 3. Chiron:
- a centaur with a radius of approximately 220 kilometres. Like Chariklo, it is thought to possess rings based on data from multiple observations.
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