[meteorite-list] Chiron May Possess Saturn-Like Rings

[Ron Baalke]

http://newsoffice.mit.edu/2015/planet-chiron-saturn-like-rings-0317

A second minor planet may possess Saturn-like rings

Researchers detect features around Chiron that may signal rings, jets, 
or a shell of dust.

Jennifer Chu 
MIT News Office 
March 17, 2015

There are only five bodies in our solar system that are known to bear 
rings. The most obvious is the planet Saturn; to a lesser extent, rings 
of gas and dust also encircle Jupiter, Uranus, and Neptune. The fifth 
member of this haloed group is Chariklo, one of a class of minor planets 
called centaurs: small, rocky bodies that possess qualities of both asteroids 
and comets.

Scientists only recently detected Chariklo's ring system - a surprising 
finding, as it had been thought that centaurs are relatively dormant. 
Now scientists at MIT and elsewhere have detected a possible ring system 
around a second centaur, Chiron.

In November 2011, the group observed a stellar occultation in which Chiron 
passed in front of a bright star, briefly blocking its light. The researchers 
analyzed the star's light emissions, and the momentary shadow created 
by Chiron, and identified optical features that suggest the centaur may 
possess a circulating disk of debris. The team believes the features may 
signify a ring system, a circular shell of gas and dust, or symmetric 
jets of material shooting out from the centaur's surface.

"It's interesting, because Chiron is a centaur - part of that middle section 
of the solar system, between Jupiter and Pluto, where we originally weren't 
thinking things would be active, but it's turning out things are quite 
active," says Amanda Bosh, a lecturer in MIT's Department of Earth, Atmospheric 
and Planetary Sciences.

Bosh and her colleagues at MIT - Jessica Ruprecht, Michael Person, and 
Amanda Gulbis - have published their results in the journal Icarus.

Catching a shadow

Chiron, discovered in 1977, was the first planetary body categorized as 
a centaur, after the mythological Greek creature - a hybrid of man and 
beast. Like their mythological counterparts, centaurs are hybrids, embodying 
traits of both asteroids and comets. Today, scientists estimate there 
are more than 44,000 centaurs in the solar system, concentrated mainly 
in a band between the orbits of Jupiter and Pluto.

While most centaurs are thought to be dormant, scientists have seen glimmers 
of activity from Chiron. Starting in the late 1980s, astronomers observed 
patterns of brightening from the centaur, as well as activity similar 
to that of a streaking comet.

In 1993 and 1994, James Elliot, then a professor of planetary astronomy 
and physics at MIT, observed a stellar occultation of Chiron and made 
the first estimates of its size. Elliot also observed features in the 
optical data that looked like jets of water and dust spewing from the 
centaur's surface.

Now MIT researchers - some of them former members of Elliot's group - 
have obtained more precise observations of Chiron, using two large telescopes 
in Hawaii: NASA's Infrared Telescope Facility, on Mauna Kea, and the Las 
Cumbres Observatory Global Telescope Network, at Haleakala.

In 2010, the team 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. The researchers determined that such a stellar 
occultation would occur on Nov. 29, 2011, and reserved time on the two 
large telescopes in hopes of catching Chiron's shadow.

"There's an aspect of serendipity to these observations," Bosh says. "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 team observed the stellar occultation remotely, from MIT's Building 
54. The entire event lasted just a few minutes, and the telescopes recorded 
the fading light as Chiron cast its shadow over the telescopes.

Rings around a theory

The group 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 researchers observed 
symmetrical, sharp features near the start and end of the stellar occultation 
- a sign that material such as dust might be blocking a fraction of the 
starlight.

The researchers observed two such features, each about 300 kilometers 
from the center of the centaur. Judging from the optical data, the features 
are 3 and 7 kilometers wide, respectively.  The features are similar to 
what Elliot observed in the 1990s.

In light of these new observations, the researchers say that Chiron may 
still possess symmetrical jets of gas and dust, as Elliot first proposed. 
However, other interpretations may be equally valid, including the "intriguing 
possibility," Bosh says, of a shell or ring of gas and dust.

Ruprecht, who is a researcher at MIT's Lincoln Laboratory, says it is 
possible to imagine a scenario in which centaurs may form rings: For example, 
when a body breaks up, the resulting debris can be captured gravitationally 
around another body, such as Chiron. Rings can also be leftover material 
from the formation of Chiron itself.

"Another possibility involves the history of Chiron's distance from the 
sun," Ruprecht says. "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. The frozen material that 
would have been stable out past Pluto is becoming less stable closer in, 
and can turn into gases that spray dust and material off the surface of 
a body."   

An independent group has since combined the MIT group's occultation data 
with other light data, and has concluded that the features around Chiron 
most likely represent a ring system. However, Ruprecht says that 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," Ruprecht says. "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."

Nevertheless, Bosh says the possibility of a second ringed centaur in 
the solar system is an enticing one.

"Until Chariklo's rings were found, it was commonly believed that these 
smaller bodies don't have ring systems," Bosh says. "If Chiron has a ring 
system, it will show it's more common than previously thought."

Matthew Knight, an astronomer at the Lowell Observatory in Flagstaff, 
Arizona, says the possibility that Chiron possesses a ring system "makes 
the solar system feel a bit more intimate."

"We have a pretty good feel for what most of the inner solar system is 
like from spacecraft missions, but the small, icy worlds of the outer 
solar system are still mysterious," says Knight, who was not involved 
in the research. "At least to me, being able to picture a centaur having 
a ring around it makes it seem more tangible."

This research was funded in part by NASA and the National Research Foundation 
of South Africa.