4.2: Everything Else

Small Bodies, Belts, and the Oort Cloud

There are many, smaller objects in the Solar System, remnants of planetary formation and results of collisions.  Some of these objects are gathered in belts, but many are on their own.  All of the orbit the Sun. 

Moons

Asteroids

Asteroids are rocky bodies that orbit the Sun like miniature planets, mostly found in the space between Mars and Jupiter. This region of the solar system is called the asteroid belt. Asteroids are too small to be seen without a telescope; the first of them was not discovered until the beginning of the nineteenth century. Some of the smallest moons of the planets, such as the moons of Mars, are most likely captured asteroids.  Figure \(\PageIndex{7}\) is an image of the astroid, Bennu.  Bennu has an irregular shape and its surface is covered with boulders and dust.

Figure \(\PageIndex{7}\): Asteroid Bennu. This mosaic image of asteroid Bennu is composed of 12 PolyCam images collected on Dec. 2 by the OSIRIS-REx spacecraft from a range of 15 miles (24 km). (BennuAsteroid, Public Domain; NASA/Goddard/University of Arizona via Wikimedia Commons)

The Asteroid Belt

The asteroids all revolve about the Sun in the same direction as the planets, and most of their orbits lie near the plane in which Earth and other planets circle. The majority of asteroids are in the asteroid belt, the region between Mars and Jupiter that contains all asteroids with orbital periods between 3.3 to 6 years  contextualize Figure \(\PageIndex{11}\). Although more than 75% of the known asteroids are in the belt, they are not closely spaced (as they are sometimes depicted in science fiction movies). The volume of the belt is actually very large, and the typical spacing between objects (down to 1 kilometer in size) is several million kilometers. Several spacecraft, including Galileo, Cassini, Rosetta, and New Horizons all traveled through the asteroid belt without incident.

Figure \(\PageIndex{11}\): Asteroid Belt . Diagram of the asteroid belt. (Asteroid Belt, Public Domain; JPL via NASA Image and Video Library)

Comets

Another class of small bodies is composed mostly of ice, made of frozen gases such as water, carbon dioxide, and carbon monoxide; these objects are called comets. Comets also are remnants from the formation of the Solar System, but they were formed and continue to orbit the Sun in distant, cooler regions—stored in a sort of cosmic deep freeze.

Comets differ from asteroids primarily in their icy composition, a difference that causes them to brighten dramatically as they approach the Sun, forming a temporary atmosphere. In some early cultures, these so-called “hairy stars” were considered omens of disaster. Today, we no longer fear comets, but eagerly anticipate those that come close enough to us to put on a good sky show.

A comet is a relatively small chunk of icy material (typically a few kilometers across) that develops an atmosphere as it approaches the Sun. Later, there may be a very faint, nebulous tail, extending several million kilometers away from the main body of the comet. Comets have been observed from the earliest times: accounts of comets are found in the histories of virtually all ancient civilizations. The typical comet, however, is not spectacularly bright in our skies. Instead, comets have the appearance of a rather faint, diffuse spot of light somewhat smaller than the Moon and many times less brilliant. (Comets seemed more spectacular to people before the invention of artificial lighting, which compromises our view of the night sky.)  Comets are active when close to Sun and dormant when far away.

Figure \(\PageIndex{12}\): Comet Neowise. This image of the Neowise Comet, was taken by a photographer in Wisconsin on July 19, 2020. (Neowise Comet Wisconsin, Public Domain; WI-Photos via Wikimedia Commons)

Like the Moon and planets, comets appear to wander among the stars, slowly shifting their positions in the sky from night to night. Unlike the planets, however, most comets appear at unpredictable times, which perhaps explain why they frequently inspired fear and superstition in earlier times. Comets typically remain visible for periods that vary from a couple of weeks to several months. We’ll say more about what they are made of and how they become visible after we discuss their motions.

Note that still images of comets give the impression that they are moving rapidly across the sky, like a bright meteor or shooting star. Looking only at such images, it is easy to confuse comets and meteors. But seen in the real sky, they are very different: the meteor burns up in our atmosphere and is gone in a few seconds, whereas the comet may be visible for weeks in nearly the same part of the sky.

Dwarf Planets

 optional page - will delete if no time = large asteroids and comets

1. brief description of 5 dwarf planets, more later
2. images - just get the real ones, not the artist concept

The Kuiper Belt

Trans-Neptunian objects (TNO) are any solar system minor planet that orbits the sun at a greater average distance than Neptune. Pluto is now considered a TNO, as is Eris. As of July 2014, over 1,500 trans-Neptunian objects have been cataloged and of these, some 200 have been designated as dwarf planets. From Earth, astronomers study TNO heat emissions, colors, and spectra. The Kuiper Belt is a region beyond the orbit of Neptune at 30 Astronomical Units (AU) to about 50 AU from the sun. It is sometimes called the Edgeworth–Kuiper Belt. The Kuiper Belt is much larger than the Asteroid Belt. It’s about twenty times as wide and twenty to two hundred times as massive. Kuiper Belt objects, called KBOs, are composed of rock and metal, like the asteroids, but also frozen ices like ammonia, methane, and water. Trans-Neptunian objects are Kuiper Belt objects (KBOs), but KBOs are not TNOs because the distance range of KBOs from the sun is much farther out in the solar system.

At least two moons – Saturn’s Phoebe and Neptune’s Triton – are believed to be captured KBOs. To date, over 1,000 KBOs have been discovered and around 100,000 KBOs are suspected to exist.

KBOs are fragments from the original solar system’s protoplanetary disc that did not combine to form the more-massive and larger bodies. Astronomers believe that belts of fragments like the solar system’s Kuiper Belt are present around other stellar systems being studied.

The Kuiper Belt is a large area of space beyond Neptune that is the source of many comets. At such a great distance from the Sun, the objects in the Kuiper Belt do not receive enough heat from the Sun to create the large tail that we see in Figure \(\PageIndex{12}\).  Instead, they are a chunk of ice and dust floating in space.  Astronomers study the Kuiper belt in two ways. New, more powerful telescopes allow us to discover many of the larger members of the Kuiper belt directly. We can also measure the composition of short-period comets that are thought to originate in the Kuiper belt, where small gravitational perturbations from Neptune can gradually shift their orbits until they penetrate the inner solar system. More than a thousand Kuiper belt objects have been discovered, and astronomers estimate that there are more than 100,000 with diameters larger than 100 kilometers, in a disk extending out to about 50 AU from the Sun.  Contextualize figure 

 

Figure \(\PageIndex{13}\): The Kuiper Belt. This illustration includes an estimate of the number and position of possible Kuiper Belt Objects with the Solar System for scale. (Candidate Kuiper Belt object, Public Domain; A. Cochran (University of Texas) and NASA/ESA via Wikimedia Commons)

Following its successful flyby of Pluto, the New Horizons spacecraft explored the Kuiper Belt. The team eventually identified a member of the belt, Arrokoth, that could be reached by a slight change in the spacecraft trajectory. New Horizons flew past Arrokoth on January 1, 2019, at a distance of only 3,500 km, obtaining images and other data. The target turned out to be a contact binary - two objects that orbit so close to each other that they have merged into one, Figure \(\PageIndex{14}\).

Figure \(\PageIndex{14}\) Ultima Thule. This composite image of the primordial contact binary Kuiper Belt Object 2014 MU69 (official designation: (486958) Arrokoth, nickname: Ultima Thule - the bigger part is Ultima, the smaller one Thule) – featured on the cover of the May 17 issue of the journal Science – was compiled from data obtained by NASA's New Horizons spacecraft as it flew by the object on Jan. 1, 2019. The image combines enhanced color data (close to what the human eye would see) with detailed high-resolution panchromatic pictures. (UltimaThule CA06 color 20190516, Public Domain; NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko, via Wikimedia Commons)

The Oort Cloud

By following their orbits backward, we can calculate that newly discovered comets typically reach points farthest from the Sun at values near 50,000 AU (more than a thousand times farther than Pluto). This clustering of aphelion distances was first noted by Dutch astronomer Jan Oort, who, in 1950, proposed an idea for the origin of those comets that is still accepted today.

It is possible to calculate that a star’s gravitational sphere of influence—the distance within which it can exert sufficient gravitation to hold onto orbiting objects—is about one third of its distance to the nearest other stars. Stars in the vicinity of the Sun are spaced in such a way that the Sun’s sphere of influence extends a little beyond 50,000 AU, or about 1 light-year. At such great distances, however, objects in orbit about the Sun can be perturbed by the gravity of passing stars. Some of the perturbed objects can then take on orbits that bring them much closer to the Sun (while others might be lost to the solar system forever). Oort suggested that these newly observed comets are examples of objects orbiting the Sun near the edge of its sphere of influence. As a result, orbits of those comets had likely been disturbed by nearby stars, eventually bringing them close to the Sun where we can see them. The reservoir of ancient icy objects from which such comets are derived is now called the Oort Cloud.

Astronomers estimate that there are about a trillion comets in the Oort cloud. In addition, we estimate that about 10 times this number of icy objects could be orbiting the Sun in the volume of space between the Kuiper belt (which is gravitationally linked to Neptune) and the Oort cloud. These objects remain undiscovered because they are too faint to be seen directly and their orbits are too stable to permit any of them to be deflected inward close to the Sun. The total number of icy or cometary objects in the outer reaches of the Solar System could thus be on the order of 10 trillion. Contextualize figure 

Figure \(\PageIndex{15}\): The Oort Cloud. This illustration includes an estimate of the billions of comets in the Oort cloud with the Kuiper Belt and Solar System for scale. (Kuiper oort, Public Domain;  NASA via Wikimedia Commons)