[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

1996 TL66: A New Type of Transneptunian Object


http://cfa-www.harvard.edu/cfa/ps/pressreleases/1996TL66.html

PRESS INFORMATION SHEET:

1996 TL66: A NEW TYPE OF TRANSNEPTUNIAN OBJECT

Produced at the Harvard-Smithsonian Center for Astrophysics (CfA),
Cambridge, Massachusetts, U.S.A.

----------------------------------------------------------------------------

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

----------------------------------------------------------------------------

This page may not be copied onto other Web sites, but other sites may place
"clickable" pointers to this page. Photo-copies of this information sheet
may be distributed for educational purposes, provided that no charge is made
in doing so; such photocopies must include full credits. Journalists and
writers may quote from this "information sheet", provided that proper credit
is given. Further information is available by writing to Mail Stop 18;
Smithsonian Observatory; 60 Garden St.; Cambridge, MA 02138; U.S.A.

----------------------------------------------------------------------------