All this talk about plutons (the newly defined solar-system objects of which Pluto is the prototype) has me thinking about the Kuiper Belt. And TNOs. And all the minor bodies of the solar system. (And Kuiper, BTW, rhymes with hyper.)
The Kuiper Belt, unlike the (theoretically) far more distant Oort Cloud, is not a theoretical home of space objects. The Kuiper Belt really exists. It’s the area of space extending from the orbit of Neptune out to a distance of 50 AU from the sun. (An AU or astronomical unit, BTW, is the approximate mean distance from the Earth to the sun.)
Objects within the Kuiper Belt are collectively referred to as Kuiper Belt Objects, or KBOs. Pluto and Charon are KBOs.
Some scientists prefer the term TNOs, for Trans-Neptunian Objects, for these bodies. My understanding of the reason for this preference is that it’s because astronomer Gerard Kuiper is not regarded as the first scientist to predict the existence of the belt of distant objects that bears his name. Kuiper didn’t “discover” the Kuiper Belt; nor did he predict its existence. Apparently he simply wrote a paper mentioning objects beyond Pluto.
Other scientists of the era also made reference to the existence of distant objects in that approximate area of space, and even did it in ways that some view as more predictive of the Kuiper Belt than Kuiper’s work was. So in order not to give credit when credit is probably not due, some scientists use the term TNO.
The two terms are not synonymous. A KBO is generally regarded to be an object that orbits only within the Kuiper Belt. A TNO can be any distant body that orbits the edge of the solar system and reaches perihelion past the 50-AU terminus of the Kuiper Belt. Sedna, for example, is a distant body that approaches the sun no more closely than 76 AU, making it a TNO but not a KBO.
Some TNOs/KBOs seem to be icy bodies, made of organic materials like Triton tholin, a nitrogen-rich substance made from organic compounds that have been irradiated by solar ultraviolet light. According to Wikipedia, Sedna’s spectra suggests that it is “24% Triton tholin, 7% amorphous carbon, 26% methanol ice with 33% methane.” Its mean surface temperature is 12 K, or a freezing-cold twelve degrees Kelvin above absolute zero. Damn.
Other denizens of the Kuiper Belt include the cubewanos, KBOs that orbit beyond Pluto and are not controlled by the gravitational influence of Neptune. The first cubewano discovered was (15760) 1992 QB1. Similar objects discovered after that one were called “QB1-o’s” — or cubewanos.
Also in the Kuiper Belt are the plutinos, the KBOs of the inner part of the Kuiper Belt that are gravitationally controlled by Neptune. Wikipedia defines plutinos as TNOs with “a 2:3 orbital resonance with Neptune.” Twotinos have a 2:1 orbital resonance with Neptune.
SDOs,or Scattered Disc Objects, are a subset of the TNOs. The innermost SDOs are found within the Kuiper Belt, but their “habitat” extends past the limits of the Belt, both farther from the sun and also farther below the ecliptic plane.
Are there objects closer in from Neptune? Past the inside terminus of the Kuiper Belt? Yep. Centaurs are the icy planetoids that orbit between Jupiter and Neptune. There are also the Trojan asteroids, asteroids of Neptune with the same orbital period as the planet.
And of course there’s the asteroid belt, the region between Mars and Jupiter where at least tens of thousands — and possibly millions — of asteroids orbit and collide. The largest asteroid here is Ceres, a body that made news recently by qualifying for planethood under the IAU’s controversial new proposed official definition of planet.
The Oort Cloud, the “postulated spherical cloud of comets” (Wikipedia) out beyond Pluto, is hypothesized to exist from 50,000 to 100,000 AU from the sun — far, far beyond the outer limits of Kuiper’s belt. This part of space is nearly one light-year from the sun, or almost 1/4 of the distance from the sun to Alpha Centauri, the nearest star system to Earth.
Here’s a site with a little more information about Gerard Kuiper and the question of assigning credit for predicting the existence of the Kuiper Belt:
Thanks to Johns Hopkins University for the graphic in this post.