The Kuiper Belt


The Kuiper Belt is a large group of objects, most of which orbit farther out from the sun than Neptune. One of these objects was first discovered in 1930. Astronomers could see that something was disturbing Neptune's orbit. Looking at the part of the sky which would have held something responsible for disturbing Neptune's orbit, a small body was discovered. It was then considered the ninth planet and named Pluto.



Pluto is 1413 miles across. It swings around the sun in a highly eccentric orbit. Unlike the eight planets, which all orbit around the sun in the same plane, called the ecliptic, Pluto's orbit is inclined seventeen degrees. An excellent page showing what can be seen of Pluto and its moons also shows moving pictures of its largest moon, Charon, circling it. The top diagram on that page also shows Pluto's highly inclined orbit. The reason it is so hard to get a picture of Pluto is that its average distance from the sun is 39.5 AU. It ranges from 29.7 to 49.4 AU.


One interesting feature of Pluto’s highly elliptical orbit is that it brings it closer to the Sun than Neptune. For about 20 years out of its 247.68 year orbit, Pluto comes within Neptune’s orbit. The last time this occurred was on January 21, 1979. It become more distant from the Sun than Neptune again on February 11, 1999.

Pluto’s dynamic orbit also causes its atmosphere to change dramatically. When the dwarf planet is closest to the Sun, it maintains a thin atmosphere of nitrogen, methane and carbon monoxide. But as Pluto moves further away from the Sun, its temperature drops down, and the atmosphere freezes out and falls to the ground as snow. (from "How Many Miles is Pluto From the Sun?")

Pluto's average density is about twice that of water, so it is concluded that it may be up to 50% rock mixed with various sorts of ices. Pluto's spectrum (the light that is reflected from Pluto) shows that its surface has frozen nitrogen, methane and carbon monoxide. Pluto's rotation period is 6.39 Earth days, but it spins the opposite way, which is called retrograde. In other words, the sun rises in the west on Pluto. This is caused by the degree of its tilt -- 118 degrees. It takes 248.6 Earth years to orbit the sun once. It has a very thin atmosphere of carbon monoxide and nitrogen.

Charon, its major moon, is 728 miles across, but its density is much less than Pluto's, at 1.25 that of water. This tells us that there is not much rock involved. The differences in density between these two objects show they formed independently and/or came from different parts of a larger body. This is also supported by the fact that Charon's surface is predominantly water ice. An interesting fact about Charon and Pluto is that they are so locked into each other's movement that the same face on Pluto is always facing the same face on Charon.

Charon was not discovered until 1978, and starting in 1992, astronomers began discovering that Pluto was not alone out there, but was one of a great number of objects circling the sun with various orbits and inclinations. Because they are beyond Neptune, they are referred to as "trans-Neptunian objects." At least 70,000 have been identified. Most of them are in a thick band orbiting in the ecliptic. They are classified in terms of their shapes and the sizes of their orbits.

(Note: two of the Kuiper Belt objects have very similar names: Charon and Chiron. If you remember that the one spelled with an 'a' is the one that goes AROUND Pluto and the one spelled with an 'i' is INDEPENDENT, you will remember which is which.)

May 18, 2015 -- a very good short video regarding our ongoing approach and what is interesting the scientists.

On the science fromt today, there is a whole lot of information about Pluto.

Video summary – short but very good.

Full NASA Report



In terms of distances of orbits from the sun, they can be classified into five groups:

1. Centaurs -- these bodies have moved in toward the sun from their original position outside of Neptune, probably due to interactions with the large planets, primarily Neptune. They now orbit from Jupiter to Neptune in highly eccentric orbits, and do not show any mathematical resonance with any planet. They cross the orbits of all four giant planets and thus have interacted with all of them. Their orbits are not yet stable and the interactions continue. These orbits range from a few on the solar system ecliptic to orbits that are angled up to 80 degrees.

Chariklo is the largest (163 miles across), followed by Chiron (145 miles across), and both have been found to have apparent rings. Whether these rings are material ejected from these dwarf planets or space debris collected by them, we don't know. Brightening events have been spotted on Chiron which appear to be the results of jets of material being ejected. Centaurs were so named because they seem to have a double nature, just like the mythical beast. They are Kuiper Belt Objects (KBO), but they share some of the characteristics of asteroids and comets. They appear to be rock and ice, and when they get close enough to the sun they display comet-like comas, or hazy areas surrounding them, which may be due to evaporated ices.We have seen this happen with four Centaurs.

2. Plutinos -- Their orbits are all similar to Pluto's, and all are trapped in a 3:2 resonance with Neptune. The average distance from the sun is the same as Pluto's, at 39.4 AU. The inclinations of their orbits can range from the ecliptic up to thirty-five degrees. Pluto is, itself, the largest of these objects.

Approximately 1/4 of the known trans-Neptunian objects are Plutinos. ... By extrapolating from the limited area of the sky so far examined, we have estimated that the number of Plutinos larger than 100 km diameter is 1400, to within a factor of a few, corresponding to a few % of the total. The number is uncertain for several reasons. First, the Plutinos are observationally over-assessed due to their being closer (brighter), on average, than the Classical KBOs giving rise to an observational bias in favor of the Plutinos. The intrinsic fraction is smaller than the actual fraction. Second, the initial orbits published by the IAU are little more than guesses, only weakly constrained by the limited orbital arcs. Pluto is distinguished from the Plutinos by its size: it is the largest object identified to date in the 3:2 resonance. ("The Plutinos")

3. Classical Kuiper Belt Objects -- Their orbits range from 42 to 48 AU and they are in a 2:1 orbital resonance with Neptune. The Classical KBO 'edge' is at about 50 AU, which marks the limit of Neptune's influence. Their orbits are the least eccentric of the KBOs, and they appear to have long-term stability. A few are eccentric enough to orbit out to about 55 AU before coming back in again. The "classical" name refers to what astronomers first suspected would be out there. They expected a thin band of objects. It was to their surprise to find so many, in such a thick band, and of such variety. The first recognized KBO was "1992 QB1" -- the pronunciation of which gave rise to the nickname of "Cubewanos" for the classical belt objects.

Classical KBOs with a low orbit inclination are referred as "cold," while those with higher inclinations get the moniker of "hot." About 475 Classical KBOs are known. The largest one is Makemake at 940 miles across, with an orbit inclination of 29 degrees (making it "hot"). It swings out to 52 AU in its orbit before coming back in. It was first nicknamed the "Easter Bunny" as it was discovered shortly after Easter in 2005.

4. Scattered Kuiper Belt Objects -- These objects have highly eccentric orbits, ranging from a perihelia of nearly 35 AU and orbit out to an aphelia of beyond 200 AU. They are also highly inclined to the ecliptic -- up to 35 degrees. Many astronomers consider objects in this region to be the source of many periodic comets. The first recognised example was 1996 TL66 (eccentricity = 0.6, perihelion distance = 35 AU, diameter = 600 km). The largest known object in the Scattered section is Eris (1445 miles across). It has a moon named Dysnomia.

5. Detached Kuiper Belt Objects -- Very highly eccentric orbits, ranging from a perihelia of 40 -80 AU and then swinging out to distances up to 1000 AU. Inclinations to the ecliptic are moderate. All are beyond Neptune's influence (Neptune orbits the sun at an average of 30 AU). Several of this type are known, having been discovered since 2000. It is quite possible comets are also originating from this group of objects. The largest known is Sedna, at somewhere around 650 to 950 miles across. It is so distant that the size can only be guessed at using both its brightness and its occultation (when it passes behind or in front of other objects). Its perihelion distance is 76 AU, which means that is as close to the sun as it gets.


The surface of KBOs have been found to be made up of frozen carbon monoxide, methane and water ices mixed with dirt and rock. Many have been found to have crystalline water ice on their surfaces. On Pluto's moon Charon there are ammonia hydrates as well as water ice crystals. It appears the ammonia is keeping the water in a liquid state despite the low temperatures. We have picked up what appear to be cryo-volcanoes, which means volcanoes spouting liquid water which freezes and crystallizes on the surface. This means the ammonia is keeping water liquid in the interior, with some obvious heating going on which is pushing the water to the surface.

Many KBOs have a red color, which can be shown to be due to the exposure of methane to radiation. We can see this on the Centaur named Pholus, which has a very red color. This color can also be seen on the Classical Belt Object Quaoar. Quaoar has solid methane and ethane on its surface along with crystal water ice. In other cases, carbon is precipitated from the methane, giving rise to a black surface. We have seen this black color consistently on comets.


Astronomers have had some fun with names out here. There is a KBO named Santa, for instance. So yes, Santa is real. Doubt he visits on a yearly basis, though. Makemake used to be the Easter Bunny. There is one named Rudolph. Most names of astronomical objects have to do with mythologies in different cultures. A great number of KBOs have not yet been named. Name one if you like!


Kuiper Belt Objects are not large. The illustration below shows that. At this point we are not sure if Eris or Pluto is larger, but all the others are much smaller. We suspect Triton, Neptune's moon, is a captured KBO. We also suspect that Saturn's moon, Phoebe, is also a captured KBO. Miranda, Uranus's moon, might also fall into this category as might a number of other small moons of the gas giants.


KBO sizes compared

This fun illustration shows the Earth at the top and, to the side, our moon. Under our moon is Mercury, the smallest planet. Directly under the Earth are the three largest asteroids from the Asteroid Belt: Vesta, Ceres, and Pallas. To their left is Neptune's moon, Triton. In the middle is the state of Texas, as a size comparison. Everything else in the illustration is a Kuiper Belt Object. Note what tiny moons several of them have.


Illustration courtesty of
Introductory Astronomy