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Small Worlds in the Solar System

AboutBook Cover of Small Worlds

Since the beginning of our history, humans have looked up at the sky and marveled at what they saw. The changing face of the Moon, the brilliance of the Sun, the movement of the planets, all of these and more awed our ancestors. In an attempt to understand, they created stories and prescribed meanings to the objects that filled the sky. Some also created elaborate models to reproduce the motion of objects in the heavens.

Today, scientists understand more about these celestial bodies. Some are unbelievably large, while others are smaller than a particle of dust. In this book, we will be exploring “small bodies” found in our solar system. Our solar system consists of one star, the Sun, and all the objects currently held in orbit by the gravity of our star. The solar system is just one part of a much, much larger group of stars that form the Milky Way Galaxy. 

So what is a small body?

A small body is an object in space such as an asteroid or comet. Interplanetary dust, Kuiper Belt Objects, material in the Oort Cloud, planetary satellites and, yes, even Pluto and other dwarf planets can be considered “small bodies.”

Oort Cloud

The Oort Cloud is a collection of small bodies at the far reaches of our solar system (Tactile 1). These objects are distributed in every direction and are not constrained to the primary orbital plane on which we find most planets, comets and asteroids. Countless icy bodies not confined to the ecliptic, or the plane on which Earth orbits, are found at vast distances from the Sun. The Oort Cloud is thought to be the birthplace of many long-period comets who rarely, if ever, get pulled in towards the inner solar system only to be flung back out towards the Oort Cloud due to the gravity of the Sun. These icy bodies are part of the family of “small bodies” in our solar system. As a whole, if the inner region of the Oort Cloud (about 2000 Astronomical Units, or AU from the Sun) were the size of a basketball (12” in diameter), the entire solar system from the Sun out to the orbit of Neptune would be about the size of a BB (3/16” in diameter)! The entire Oort Cloud extends from about 2000 AU to over 67,000 AU!

Note: An AU is a unit of distance and stands for an Astronomical Unit, which is the average distance from the Earth to the Sun and is about 150,000,000 km. Therefore, to say that Jupiter is found at 5 AUs means that it is about 5 times farther than the average distance from the Earth to the Sun. In miles, an AU is about 93,000,000 mi.

Kuiper Belt

The region extending outwards from Neptune’s orbit and generally in the same plane of the planets also contains many small bodies including those of the short-period comets, or those comets that take less than 200 years to make one orbit. This region is referred to as the Kuiper Belt and extends roughly 30 to 50 AU; although some suspected Kuiper Belt objects are found much further than 50 AU.

The New Horizons spacecraft will fly by Kuiper belt object, 2014 MU69, on December 31st 2018. Images from groundbased telescopes, Hubble telescope and the NASA Startospheric Observatory for Infrared Astronomy (SOFIA) indicate that MU 69 is an elongated object and may have two lobes or a binary shape. Stay tuned!

What is a comet?

The Oort Cloud and Kuiper Belt seem to be the birthplace of comets, therefore it is important to learn what these small bodies called comets are. A comet is a ball of ice and gas mixed with dust and rock that orbits the Sun. They date back to when the solar system was formed. Comets are often called “dirty snowballs” because they consist of a mixture of ice, rock and dust fragments.

A solid surface made of ice, rock and dust forms the nucleus of a comet. Tactile 2 details the nucleus of a comet discovered in 1969 called Comet 67P/Churyumov–Gerasimenko. The comet has two distinct lobes suggesting that this nucleus may be the result of the collision of 2 comets in the past – again highlighting the dynamic nature of small bodies! The “mountains” on the right edge of the nucleus are 100’s of meters high. Note also the outgassing occurring near the upper center of the core. As a comet is “pulled” into the inner solar system by the gravity of the Sun, the solar radiation pressure and increased temperature cause the material in the nucleus to vaporize, releasing gas and dust formerly frozen in the nucleus. When that happens, those on Earth are sometimes able to see the coma and the tail as detailed in Tactile 3. The coma is simply the gas and dust illuminated by the Sun directly surrounding the nucleus. Because of the outward radiating pressure from the Sun called solar wind, some of the material in the coma is “blown” away from the comet forming the tail. Here is where an interesting twist enters the story: The dust and gas are affected differently by the solar radiation. The more dense and massive dust is less affected by the solar radiation and retains more of a curved path due to the orbiting nucleus. The less dense gas is easily carried away from the nucleus and thus trails straight out away from the nucleus pointing directly away from the Sun. This is illustrated in Tactile 3 which shows the anatomy of a comet as it nears our Sun.

What are asteroids?

Although the distinction between comets and asteroids is not always easy to make, asteroids tend to be small rocky or metallic bodies that orbit the Sun mostly between Mars and Jupiter in what is called the Asteroid belt. Tactile 4 (not to scale by size or by distance) illustrates the location of the asteroid belt in relation to the other planets. Asteroids can be as big as 1000 km (600 miles) in diameter down to the size of small pebbles. Although they orbit the Sun, most are too small to be considered planets. The larger asteroids, on the other hand, may be classified as dwarf planets. They are not considered planets as they have not cleared the area around them of surrounding debris.

Tactile 5 highlights the asteroid Vesta, which has a diameter of 530 km. Vesta is unlike most asteroids in that it has different layers in its interior. These are exposed in the three “snowman” craters that can be found towards the top of the tactile and to the left of the ridges and valleys. The asteroid Vesta dispels the myth that all asteroids are uniform hunks of rock.

Asteroids are thought to be leftover material from the formation of our Solar System. They are classified into three major types: C, S, and X depending upon the abundance of rocky, metallic or clay like material.

Debris from comets and asteroids can often enter the Earth’s atmosphere and sometimes even collide with Earth. Smaller debris from comets and asteroids are often referred to as meteoroids. The terms “meteoroid,” “meteorite,” and “meteor” are often used interchangeably, but scientists define these using three different terms. Meteoroids are small parts of an asteroid or comet that are orbiting the Sun. When a meteoroid enters Earth’s atmosphere it is called a meteor and will usually vaporize and streak across the sky. Meteors are often called “shooting stars” or “falling stars” but have nothing to do with distant stars.

“Meteorites” are pieces of space debris that enter Earth’s atmosphere and stay intact to hit the Earth. They are primarily pieces of asteroids. They can be as small as a grain of rice or as large as a boulder. Most meteors and meteorites are categorized as either iron (mainly made of iron), stony (mainly made of minerals other than iron), or stony-iron (composed of both).

Asteroids, comets and meteoroids routinely hit the Earth. However, most of these are small and hardly noticeable. The larger the object, the more dangerous it is to the Earth.

If an asteroid more than 400 meters (a quarter-mile) wide hit the Earth, it could cause a global disaster such as the one that resulted in the extinction of the dinosaurs. However, these are extremely rare. Smaller asteroids or comets are still hazardous because, depending on their speed, angle of impact, and size, they can cause injuries, destroy cities, or cause deadly tSunamis. Scientists from NASA’s Near Earth Object program continually scan and track the motion of many of these objects, to proactively mitigate any potential collision issues with Earth.

Aside from a potential threat, asteroids are also a possible source for natural resources. Many of Earth’s metals and minerals are seriously depleted. Mining asteroids could help with this problem in the near future.

Unlike comets, most large asteroids are found in relatively stable orbits around the Sun and do not intersect with Earth’s orbit. One of these asteroids, Ceres (Tactile 6) is the largest known asteroid and can be referred to s a dwarf planet. It also sports curious features similar to those found on comets and also hints at a place of origin in the Kuiper belt. Notice the bumps in a crater at the center of Tactile 6. These bright spots are referred to as “Spot 5” and were discovered by the Dawn spacecraft in 2015. They are found in a crater about 80 km wide and are thought to be brighter than the surrounding region as they contain a type of salt that probably originated from salty water erupting onto the surface and subsequent evaporation, hinting at recent geologic activity. Material has also been observed, both directly and indirectly, erupting onto the surface of Earth, Jupiter’s moon, Io, and Saturn’s moon Enceladus.

Dwarf Planets

As the name implies, these planets are small bodies and smaller than the 8 known “major” planets in our solar system. From the Sun outward, the “major” planets include Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune (Tactile 3). Pluto, which was once classified as a major planet, resides in the outermost region of our solar system but it does not adequately fulfill all the requirements for maintaining major planet status.

The International Astronomical Union is the only recognized body that can officially name objects in space. They have determined that planets must:

  1. Orbit around the Sun
  2. Have sufficient mass for it to take on a nearly round shape, and
  3. Clear the neighborhood around its orbit

Many Dwarf Planets, on the other hand, are much smaller and fail to meet the 3rd criteria of clearing their neighborhood around its orbit. Pluto (Tactile 7), for example, orbits the Sun and has sufficient mass for it to take on a nearly round shape, however, it has not cleared its neighborhood around its orbit. In other words, it is not massive enough to either capture other objects in its orbit, or to fling them out of its orbit due to gravitational forces. Therefore, Pluto and other objects meeting a similar fate are considered Dwarf Planets. However, let’s be clear that the demotion of Pluto to dwarf planet status has not altered Pluto itself or its significance in our solar system or the fascinating science yet to be discovered about this object. Tactile 7 is based off recent data acquired from the New Horizons mission and highlights some of the dynamic terrain found on its surface.

Other named dwarf planets include Ceres, Haumea, Makemake and Eris; but many other yet to be discovered objects may meet the criteria of being considered dwarf planets.


Other small bodies in our solar system comprise what we call satellites. Satellites are most often thought of as “moons” orbiting around planets. However, the term can also refer to pieces of asteroids or comets, which orbit around other small objects. Note that as used here, the term ‘satellites’ refers to natural satellites and does not refer to the man-made variety, or spacecraft!

Our solar system is filled with intriguing satellites, some of which may even harbor oceans! Mercury and Venus have no satellites. The Earth has one satellite, our Moon. The Moon orbits around the Earth taking a little over 27 days to complete one revolution with respect to the background stars. Mars has two satellites named after the Greek characters Phobos and Deimos. Tactile 8 details the satellites of Mars to scale with each other and to scale by distance from Mars. Notice the large crater on the top left of Phobos. Notice also the odd shape of Deimos. What would it be like to have 2 moons orbting Earth?

The larger gas giant planets of Jupiter, Saturn, Uranus and Neptune harbor a wide variety of satellites waiting to be investigated by future scientists and explorers. Will you be one of them?

As a whole, small bodies in our solar system provide a window into what the early solar system may have looked like and the materials that formed it. Furthermore, the continued study of small bodies in our solar system will benefit humans by providing better methods for protecting our own planet and providing possible sources of resources in the future.

Created by: 

Dr. Cassandra Runyon, College of Charleston, Department of Geology & Environmental Geosciences, Charleston, South Carolina

Dr. David Hurd, Edinboro University of Pennsylvania, Planetarium and Department of Geosciences, Edinboro, Pennsylvania

Cynthia Hall, College of Charleston, Department of Geology & Environmental Geosciences, Charleston, South Carolina

John Matelock, Tactile Illustrator, Cambridge Springs, Pennsylvania

Ken Quinn, Consulting and Tactile Graphics Evaluator, Erie, Pennsylvania

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