Is Chiron an Asteroid, a Comet, or a Trapped Kuiper Belt Object?

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Discovery

Charles Kowal didn’t go out looking to rock the astronomical world but that is what he did when Chiron was found. While at Palomer in 1977, he looked closer at photographic plates of his from October 18 and 19th and saw an 18th magnitude object tentatively titled 1977 UB which was designated a minor planet at the time. This was because it was shown to have less than 3 seconds of total travel between plates and therefore wasn’t a faraway object. After some more observations with the 122-cm Schmidt telescope that Palomar processed and looking at plates of the past, it was given the official designation of 2060 Chiron, an asteroid. But time would show unusual characteristics that demanded Chiron to be reclassified (Stern 28, Kowal 245).

Centaur?
Centaur? | Source

The Battle: Asteroid vs. Comet

For starters, Chiron has a 51 year orbit that places it between Saturn and Uranus, well far from the Asteroid Belt. Though this was odd, some have been found in populations outside that region. But Chiron (a 6th absolute magnitude object) is also very bright, reflecting about 10% of the light that hits it. That, folks, is squarely with predictions of a comet and not an asteroid. After much measurements of this brightness, Chiron was found to be a little over 200 kilometers which is way larger than the typical 3-10 kilometers for a comet. At this juncture, Chiron was considered too small to be a planet, too bright to be an asteroid, and too big to be a comet. So a new possibility was raised: maybe it came from the Kuiper Belt (Stern 28, Koval 248-9).

At the time, the Kuiper Belt was a hypothetical region of the solar system beyond Neptune that had many icy remnants from the early days of the solar system. It was first hypothesized by Gerald Kuiper in 1951 when he noticed how the solar system suddenly stops at about 30 AU. He figured that is a ring of objects were past Neptune, they would pull objects towards it and cause the tapering off witnessed. No hard evidence for its existence had been found at the time of Chiron’s discovery, so scientists knew if Chiron was indeed from there then it would be a chance to learn what to look for and gain a better insight into our solar system’s history (Stern 31).

Comet?
Comet? | Source

But more evidence was needed to be considered. For one, Chiron’s orbit seemed to be unstable, with a possible 1:2 or 3:5 resonance with Saturn, implying it was a recent entry and more than likely in a short-lived orbit. This could be because of gravitational tugs from the gas giants or a possible collision with another minor planet. Chiron also completes one rotation in 5.92 hours. And the high-brightness levels mentioned earlier changes over the years. In 1970 the magnitude was 5.5-5, and it increased to a min between 7-6.5 in 1985 before beginning to grow in the 1990’s as perihelion approached. But a random fluctuation in brightness in 1988 by Dave Tholen (University of Hawaii) with Bill Hartmann, Karen Meech, and Dale Cruikshank, saw Chiron increase its brightness by nearly double. Was it a coma? An impact? A geyser? Chiron was keeping us guessing! (Stern 28-9, Koval 249)

Enter Alan Stern, everyone’s favorite planetary scientists who helped spearhead New Horizons aka the first mission to Pluto. He began his look into Chiron in 1988 by looking into the coma theory. He did this by developing a computer program which would look at temperature rates as well as any sublimation that may happen. If what had been seen was a coma, then it was too far away to be made of water ice (the most common material of a coma). It could be possible that carbon monoxide, carbon dioxide, methane, or nitrogen could sublimate at that distance (Stern 29).

Kuiper Belt Object?

But some quick thinking led to a problem. It was realized that based on the proximity Chiron has to the Sun at perihelion, anything worth sublimating should have done it a long time ago. This adds evidence to the theory of the object being a recent acquisition from perhaps elsewhere in the solar system. But just as it seemed as though Chiron was without a coma, one was spotted in 1989 by Karen Neech and Mike Belton, both from National Optical Astronomy Observatories. It was a mix of ice and dust with a diameter of 320,000 kilometers! A follow-up observation in 1990 by Bobby Bus and Ted Bowell of Lowell Observatory found that cyanogen gas was present in the coma. It was present on low amounts but was very visible because of its fluorescent nature (Ibid).

As the 1990’s continued on, the coma’s brightness fluctuated heavily, with changes as much as ±30-50%. Scientists suspect it was because of different levels on Chiron being exposed at different rates to the solar wind. Bobby decided to look at past plates to see if coma readings from the past could shed light. He was able to find a coma from 1969-1972 when Chiron was at aphelion (19.5 AU), and on top of that it was even brighter at that point then when at perihelion! What the heck?! It should be way too cold at that point for anything, even carbon dioxide, to sublimate (29-30).

KBO?
KBO? | Source

Clearly, scientists needed to try and find some more clues to see if it was once a Kuiper Belt object, and they decided to do this by comparison. And when they did that, they found some similarities – with Triton and Pluto. At the time, they were both suspect Kuiper Belt objects and had chemical similarities with Chiron. Also, all three had dark surfaces which were crusty, with Chiron being bright because of the coma reflecting light. Otherwise, it too was found to have a similar surface during quiet periods. In fact, only 0.1-1 % of the surface of Chiron was needed to sublimate in order to be as bright as it was recorded to be (30).

After all of this analysis, scientists felt confident that it was at one time a member of this family but wanted to know how it got to its present orbit and where the other objects like Chiron were. After all, if something could knock Chiron inward, why not other objects? Yes, the gravity of the gas giants made the orbit of anything around there questionable at best, with an average lifespan of 50 to 100 million years according to simulations by Bret Glodman and Martin Duncan of Queen’s University. And perhaps some objects are: comets. Some of these seem to come from past Neptune and come careening towards the Sun. Known as long period comets, they could be knocked out of the Kuiper Belt by gravitational effects and sent inward, according to work in the early 80s by Julio Fernandez of the University of Montevideo. This was further backed by simulations later in the decade by Martin Duncan, Thomas Quinn, and Scott Tremaine, which implied that no other mechanism could explain the source of long period comets. So…would Chiron happen to be one of these and simply fell into a semi-stable orbit? Does that make it a Kuiper Belt Object in reality? (30)

And then a study released in 2000 showed how Chiron processes water ice. Observations and spectrum analysis by Luu, Jewitt, and Trujillo showed the presence of water ice with carbon particulate, olivine, in a distribution consistent with a comet distribution and not a deeper, mantle level layer. Additional observations showed the coma-like feature gaining strength and fluctuation, just like in the past. Any gases like carbon monoxide or nitrogen which sublimate in conditions around Chiron kick up enough material to scatter it across its surface, impacts its ability to further sublimate, causes fluctuations in its brightness and water release, and creates the loose surface layer, all of which had been confirmed by previous observations and is in support of a Kuiper Belt Object which has been subjected to the inner solar system (Luu 5-7).

The main consensus amongst the science community is that Chiron is a comet and a minor planet. It is also a pioneer member of the centaurs, a group of objects between Jupiter and Uranus. But, as we have seen with Pluto, designations can change depending on new data. So stay tuned.

Works Cited

Luu, Jane X., and David C. Jewitt, Chad Trujillo. “Water Ice in 2060 Chiron and its Implications for Centaurs and Kuiper Belt Objects.” Astrophysical Journal Letters 04 Feb. 2000. Print.

Kowal, C.T. and W. Liller, B.G. Masden. “The Discovery and Orbit of 2060 Chiron.” International Astronomical Union 1979: 245, 248-9. Print.

Stern, Alan. “Chiron: Interloper from the Kuiper Belt.” Astronomy Aug. 1994: 28-32. Print.

© 2016 Leonard Kelley

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