The Kuiper Belt

1948: Edgeworth predicts there should be more stuff beyond Neptune. Why? "Surface density" of solar system drops off abruptly beyond Neptune.  This doesn't look like a sensible way to distribute material when the solar system was made.

1951: Gerald Kuiper predicts, on the basis of comet orbits, that there must be a reservoir of "short period comets" extending from Neptune out to 100-200 AU, generally in the plane of the ecliptic.

1951-1978: work of Edgeworth and Kuiper profoundly ignored.  Why? A typical comet is about 5 km in diameter, compared to Pluto with d = 2274 km. So an absurdly enormous comet might be 100 times smaller than Pluto, in diameter.  Such an object would be 100 x 100 = 10,000 times smaller than Pluto in surface area and therefore 10,000 times fainter than Pluto, in reflected sunlight.  For all practical purposes, this makes giant comets at the distance of Pluto too faint to be seen with any telescope built before the 1990s.  Thus, all things being equal, the predictions of Edgeworth and Kuiper were untestable, and therefore ignored, by astronomers.

1977: Chiron found.  a = 13.6 AU. diam = 200 km.  Not a comet.  Not an asteroid. Behaves like a comet, sometimes.  On a very unstable orbit (out to 18.8 AU, in to 8.4 AU; perturbed by Jupiter and Saturn and Uranus) with lifetime of a few million years, at most.  Must have originated somewhere else.  Where?

1981: Fernandez revisits comet orbit question, confirms Kuiper's work.  Suggests that Chiron probably came from "Kuiper Belt."

1986:   David Jewitt, UH, decides there could be more Chiron-like objects out beyond Pluto, big enough to be found with big telescopes in Hawaii.  Most folks thought him crazy to try.  He was.  In 1989 he published the results of his first survey.  Nothing.  Nada.  1990 more of the same.

Jan 23, 1992: Pholus discovered (by "Spacewatch" group).  D = 200 km. Orbit crosses Saturn, Uranus, Neptune (a = 20.3 AU; e=0.57).  Unstable orbit, like Chiron.

30 Aug 1992: 1992 QB1 discovered by Dave Jewitt and Jane Luu.  [Look at discovery images].  This led to an evergrowing avalanche of additional discoveries:

30 Aug 1992: 1992 QB1.  a = 43.93 AU, e = 0.069, d = 283 km
28 Mar 1993: 1993 FW.  a = 43.80 AU, e = 0.048, d = 286 km
14 Sep 1993: 1993 RO.    a = 39.20 AU, e = 0.197, d = 139 km
15 Sep 1993: 1993 RP.     a = 39.3 AU   d = 96 km
16 Sep 1993: 1993 SB.     a = 39.44 AU, e = 0.323,  d = 188 km
17 Sep 1993: 1993 SC      a = 39.50 AU, e = 0.183,  d = 319 km

Object definitions:

Trans-Neptunian Objects (TNOs) or Classical Kuiper Belt Objects (CKBOs): able to survive for the age of the solar system without the special protection offered by resonances to the Plutinos, simply because they are already Neptune-avoiding; semi-major axes between about 42 and 48 AU (with an outer edge to the CKBO region at 50 AU) and small eccentricities.

Scattered Disk Objects (SDOs) or  Scattered Kuipper Belt Objects (SKBOs): large, eccentric, inclined orbits with perihelia near 35 AU; Neptune can exert some dynamical control over these objects.  They appear to be in the process of being removed, or scattered out of, the Kuiper Belt.

Centaurs: Objects that are now on unstable, giant planet crossing or near-crossing orbits. These include Chiron and Pholus.

Plutinos: see below

As of 18 February, 2003, there were

The biggest?

The pace of discovery can be seen at:  EKO news: object list.

Further information can be found from the EKO news archives.

A plot of the outer solar system shows the positions of the now known objects.  Why are do some areas seem to have "lines" of discovered objects?

What about the Plutinos?

There is one other special subgroup of KBOs, known as the Plutinos.  These all have orbits with semi-major axes between 39.37 and 40.05 AU, compared to Pluto at 36.529 AU.  Based on Kepler's 3rd law, we know that all of these objects must have orbital periods almost identical to that of Pluto.

As it turns out, the orbital periods of Pluto (247.7 yrs) and Neptune (164.82 yrs) are in the ratio of 3:2 (actually 3.006 : 2.0000).    All the Plutinos also share this relationship with Neptune and thus all of these objects complete two orbits around the Sun for each three orbits of Neptune.  Plantary scientists believe that this special orbital situation acts to stabilize these objects orbits, even though most, but not all, of them are Neptune crossers, like Pluto.

[Others are defining Plutinos as having orbital periods such that they are at either 3:2 or 4:3 resonances with Neptune.]

[Jewitt has also created the designation 'twotinos' for objects at the 2:1 resonance with Neptune.]

From Dave Jewitt's KBO page, one can follow the links to Plutinos to find a plot of the semi-major axes of KBOs to see the pile-up at the 3:2 resonance.

How Many Legs Does a Dog Have?

[quoted from Dan Green, quoted by Dave Jewitt]

Ian Ridpath [1978, Astronomy 6(12), 6] put it well when he recited Abraham Lincoln's riddle, in which

Lincoln asked somebody, "If you call a tail 'a leg,' how many legs does a dog have?
The person offered, 'Five?', to which
Lincoln replied, 'No, four --- [merely] calling a tail 'a leg' does not make it a leg!"
To which Ridpath added: "If you call Pluto 'a [major] planet,' how many [major] planets does the sun have?"
Is Pluto a Planet?

According to Dave Jewitt,

      So, bluntly put, one has two choices. One can either regard Pluto as the smallest, most peculiar planet moving on the most eccentric and most inclined orbit of any of the planets or one can accept that Pluto is the largest known, but otherwise completely typical, Kuiper Belt Object. The choice you make is up to you, but from the point of view of trying to understand the origin and significance of Pluto it clearly makes sense to take the second option. Pluto's eccentricity and inclination were pumped up along with the eccentricities and inclinations of the 25,000 other Plutinos (diameters > 100 km), probably driven by the radial migration of Neptune. The processes that shaped the orbits of the KBOs are the same ones that gave Pluto its prominent dynamical characteristics.

       Some people see this as a demotion of Pluto from Planet-hood. I think that it can reasonably be portrayed as a promotion. Our perception of Pluto has been transformed from a singularly freakish and unexplained anomaly of the outer solar system to the leader of a rich and interesting family of Trans-Neptunian bodies whose study will tell us a great deal about the origin of the solar system.

     So, we have discovered minus one planet and plus one Kuiper Belt. It seems like a fair trade to me.