The issue of NATURE published today contains a letter discussing the discovery and likely significance of a 500-km object recognized in the transneptunian region of the solar system in late 1996. News of the object, designated 1996 TL66, was in fact first published on Minor Planet Electronic Circular 1997-B18 as long ago as January 30. In addition to its unusually large size, the significance of the object lies in the high eccentricity of its orbit, which takes the object from a distance of 35 astronomical units from the sun at its closest point to some 130 astronomical units at its most distant. One astronomical unit is approximately the earth's distance from the sun and 30 astronomical units is that of Neptune.
1996 TL66 was first imaged last October by Jane Luu, Harvard- Smithsonian Center for Astrophysics, and Dave Jewitt, University of Hawaii, during an observing run with two of Jewitt's students on the 2.2-m telescope the University of Hawaii maintains on Mauna Kea. At first, and as the result also of follow-up observations with the Smithsonian Astrophysical Observatory's 1.2-m telescope in Arizona a month later, it appeared probable that the object was a "plutino", one of two main classes of object hitherto identified in the Kuiper Belt (an extensive swarm of icy bodies, presumably proto-comets, identified in recent years as orbiting the sun beyond the orbit of Neptune).
"Plutinos", meaning "little Plutos", is a generic name given to the class of Kuiper Belt members with orbits that come very close, and sometimes even cross, the orbit of Neptune. Despite their often extreme proximity to Neptune's orbit, the plutinos do not in fact have the possibility of encountering Neptune itself, because the periods of revolution about the sun of the plutinos and Neptune are precisely in a ratio of three to two. This means that, after three revolutions of Neptune and two of a plutino (about 500 years), the relative positions of the objects in their orbits repeat, and this cycle does not give the bodies an opportunity to pass within 10 or more astronomical units of each other. Although the cycle may break down eventually, it seems likely that it will continue to repeat for perhaps tens or hundreds of millions of years, thereby preventing devastating encounters between a plutino and Neptune. Pluto, a 2400-km object discovered in 1930, has been known since 1964 to exhibit precisely this type of motion, and it should therefore be considered as the first known member of the Kuiper Belt; the second member of the group would then be Pluto's satellite Charon, discovered in 1978 and having about half the diameter of Pluto.
The plutinos contrast with what may be called the "cubewanos", in recognition of their prototype 1992 QB1, also discovered by Jewitt and Luu. Cubewanos, which comprise perhaps 60-70 percent of the known objects in the Kuiper Belt, travel in orbits that are substantially more nearly circular and closer to the plane of Neptune's orbit than the plutinos. And whereas the plutinos orbit the sun at an average distance of 39 astronomical units, cubewanos have average distances over the range 42-46 astronomical units. They are therefore well beyond Neptune at all times.
After Carl Hergenrother measured 1996 TL66 with the Arizona telescope in December, continuing calculations by Brian Marsden showed that the supposition that 1996 TL66 is a plutino had to be incorrect. At this stage it became evident that the orbit had to be substantially larger and more elongated than that of a plutino. Nevertheless, the orbit's precise character was unclear, something that needed to be settled with further observations. Unfortunately, 1996 TL66 was by then starting to sink into the evening twilight, and no time had been allocated for follow-up in January on any of the suitable professional telescopes. It was at this stage that the help of an amateur astronomer, Warren Offutt, was solicited. At his observatory 1000 meters up in the mountains near Cloudcroft, New Mexico, Offutt has a well-equipped 0.6-meter telescope and electronic imaging device. He was already known by then as the only amateur in the world to have observed a member of the Kuiper Belt (other than Pluto) using amateur equipment, so it seemed likely that he would be able to obtain some measurements that would clinch the situation before 1996 TL66 was lost to view. Offutt was happy to oblige and obtained critical observations on January 10 and 11, thereby providing confirmation of the developing suspicions concerning the 1996 TL66 orbit.
Are there other objects like 1996 TL66? The ease with which it was found, at the start of new wide-field survey, suggests that there are. Furthermore, some of its colleagues may already have been inadvertently detected. For every two Kuiper Belt candidates that are discovered, typically one is lost. Out of the 40 or so Kuiper Belt candidates known, four of those lost possibly do have some of the characteristics of 1996 TL66, notably, indications that their orbits are quite highly inclined to that of Neptune. Perhaps 10 percent of what we think of as the Kuiper Belt may therefore belong to this "scattered" population.
So what is 1996 TL66? Physically, it is probably much the same as the other icy bodies out there, including Pluto. Dynamically, some of the Kuiper Belt objects, perhaps principally the plutinos, form the reservoir that eventually yields the typical short-period comets that at their most distant are near the orbit of Jupiter. If, eventually, the accumulated gravitational attractions of Neptune, Uranus, Saturn and Jupiter conspire to dislodge a plutino, a plutino can be dragged into the vicinity of those planets, to spend then perhaps another million years in a very unstable dynamical situation, bouncing back from one planet to another. This is known as the "centaur" stage, something currently being exhibited by seven known objects. The first of these, named Chiron following its discovery in 1977, does sometimes exhibit a coma or tail and is the largest object known to do so. Eventually, Jupiter wins, sending an object in toward the orbits of Mars and the earth. About a hundred of these short-period comets are known, all of them considerably smaller than Chiron but thereby indicative of the large number of smaller centaurs and plutinos that must exist. In very general terms, 1996 TL66 could be considered in the same category, perhaps also to move up the centaur route-- or once to have been a centaur. Perhaps, indeed, it is destined to move farther out than it is now. Perhaps it is on its way out toward the Oort Cloud. The Oort Cloud of comets surrounding the solar system at a distance of some 20 percent of the nearest star is believed mainly to represent icy bodies ejected from the vicinity of Uranus and Neptune. As many as a trillion proto-comets seem to exist in the Oort Cloud, drawn out from a planar to a near-spherical collection as the result of the gravitational effects of passing stars and giant molecular clouds. To gather a trillion comets takes a long time, and most of the formation of the Oort Cloud must have taken place long ago. The existence of 1996 TL66, and presumably of other bodies in its class, indicates that the process of maintaining the Oort Cloud may still be going on at some small level.
1997 June 5
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