This is a longish chapter, but one that I hope readers will find interesting and educational. We’ll review the structure of the Solar System, along with the meanings of the different terms used to categorize the various objects it contains. This will provide a good foundation for the discussions that follow.
At its simplest, our Solar System comprises the Sun and 8 planets. When I went to school there were 9 planets, because Pluto was included, but Pluto has since been reclassified as a dwarf planet. (I’ll explain the difference between “planet” and “dwarf planet” shortly.)
The 8 planets are organized into 2 groups of 4 planets each. This isn’t a coincidence, and I’ll explain why. The 4 inner planets — Mercury, Venus, Earth, and Mars — are called terrestrial planets. The 4 outer planets — Jupiter, Saturn, Uranus, and Neptune — are called giant planets. Perhaps unsurprisingly, giant planets are much larger than terrestrial planets.
Separating these 2 groups is the Asteroid Belt, which is a large collection of about a million small objects, called asteroids, orbiting between Mars and Jupiter. These objects range in size from about 1 meter up to 946 kilometers in diameter, which is the largest asteroid, Ceres. Ceres is also a dwarf planet.
Beyond Neptune is the transneptunian region, where many small objects are found called transneptunian objects (TNOs). These include Pluto and other dwarf planets, and comets. The transneptunian region is organised into large groups of objects with similar orbital characteristics: the Kuiper Belt, the Scattered Disc, and the hypothetical Hills Cloud and Oort Cloud.
The Solar System is thus organized into 3 major regions:
- The inner Solar System, where the Sun, terrestrial planets, and asteroids are found.
- The outer Solar System, where the giant planets are found.
- The transneptunian region, where all dwarf planets except for Ceres are found, plus the comets and many other small objects.
The original meaning of the word “planet” was “wanderer”, referring to objects that looked like stars, but moved across the sky, unlike the stars and constellations that appear fixed in place. In the ancient world, the Sun and Moon were classified as planets, and Earth was not. The 7 planets of antiquity were the 7 wandering celestial objects that could be seen with the naked eye: The Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn.
The current definition of “planet”, according to the IAU (International Astronomical Union), is an object that:
- Orbits the Sun.
- Is gravitationally rounded, which is to say, its mass is large enough for gravity to have pulled it into an rounded shape.
- Has cleared its orbital neighborhood of smaller objects; or, to put it another way, it is not part of a large group of objects in the same region of space.
The difference between terrestrial and giant planets is that giant planets have huge atmospheres of hydrogen and helium gas. These are the most abundant gases in the Universe, but also the lightest, and terrestrial planets are not large enough for their gravity to hold onto them.
The terrestrial planets can be thought of as 2 pairs: the hot terrestrials, Mercury and Venus, which are too close to the Sun for water. Both of these worlds are very dry. Then there are the cool terrestrials, Earth and Mars, which have an abundance of water.
The giant planets can also be thought of as 2 pairs. Jupiter and Saturn are the larger; having formed closer to the Sun, they accumulated a lot more material and especially a lot more hydrogen and helium gas. These 2 are known as gas giants. Uranus and Neptune have less gas, but large icy cores, and are therefore known as ice giants.
The frost line
The reason we have these 2 distinct groups of planets, with the smaller, rocky, terrestrial planets close to the Sun, and the larger, gassy, giant planets farther out, is because of water.
Hydrogen and oxygen are 2 of the most abundant elements in the Universe, and they combine rather easily to form water, which means we find water throughout the Universe. In the vacuum of space, water doesn’t exist as a liquid; only as ice or water vapor.
The frost line (also known as the snow line) is the distance from a star beyond which it’s cold enough for water to freeze. Closer to the star than the frost line, water exists only as vapor, except for in certain places on the surface of planets or moons where the conditions are suitable for liquid water or ice.
The frost line was at about 3 AU from the Sun during the formation of the Solar System, when the Sun was cooler, but is now at about 5 AU. (As a reminder, an “AU” means an “Astronomical Unit”, which is equal to the distance from Earth to the Sun, or about 150 million kilometers.) For context, Mars orbits at about 1.5 AU (below the frost line), and Jupiter orbits at about 5.2 AU (above the frost line).
Closer to a star than the frost line, water vapor is blown away by the solar wind. Thus, as the terrestrial planets were forming, they didn’t accumulate water; just rock and metals.
Farther from a star than the frost line, water freezes to ice, which can be accumulated by planets as they accrete. Because the giant planets were formed from the abundant ice in the outer Solar System, as well as rock and metals, they grew much more massive; so massive, in fact, that they were also able to hold onto the light gases hydrogen and helium. As these gases are super-abundant, the atmospheres of the giant planets grew very large.
The water on Earth and Mars wasn’t obtained during their formation but was delivered to them later by comets, which are icy objects that formed in the transneptunian region. Comets fall into the inner Solar System and sometimes crash into the surfaces of planets and moons, depositing ice. The cool terrestrial planets were able to hold onto this water by having low enough temperatures to prevent it from boiling away into space. On Luna, Mercury, and Venus, however, the temperatures were too high, and almost all of the water brought to these worlds by comets sublimed to vapor and was lost to space. We do find water ice in the floors of permanently shadowed craters at the poles of Luna and Mercury, because comets crashed there, but the temperatures in these places are always so low that the ice has never sublimed to vapor.
Dwarf planets and minor planets
Astronomers used to consider everything orbiting the Sun as either a major planet, minor planet, or a comet. This has varied slightly and everything orbiting the Sun is now classified as a planet, dwarf planet, or small Solar System body (SSSB). Both classification systems exist side by side. The term “planet” is simply shorthand for “major planet”. A dwarf planet is a minor planet that is gravitationally rounded. All other minor planets, plus comets, are SSSBs.
The IAU Minor Planet Center (MPC) maintains a database on all minor planets and comets. Most are numbered. At the time of writing, the MPC database lists 516 386 numbered minor planets, 241 240 unnumbered minor planets, and 4 014 comets. The full designation of Ceres, the first minor planet ever found, is actually “1 Ceres”, and Pluto’s is “134340 Pluto”. Pluto’s number is large because it was only recently classified as a minor planet, and was assigned the next available number.
The distinction between major and minor planets is not based on whether or not the object is gravitationally rounded. For a long time, Ceres was considered a planet, but it became clear that it shared its orbital neighborhood with many small objects (i.e. the Asteroid Belt). This marked it as a different kind of object than the major planets, which had already mopped up almost all of the small objects in their orbital neighborhood. The category of “minor planet” was then introduced, with Ceres as the first member.
Pluto was also believed to be orbiting by itself and not in a group, and was therefore considered a major planet. Being so far from the Sun, no other objects were detected orbiting near Pluto for a very long time. Eventually, however, many objects were found orbiting beyond Neptune, and it was realized that Pluto also shared its orbital neighborhood with a large number of minor planets (i.e. the Kuiper Belt). This raised the question of whether Pluto should also be reclassified as a minor planet, or if Ceres should again be classified as a major planet, or if some new classification was needed for these unique objects.
Ultimately it was decided that a new definition of “planet” was needed (the IAU definition given above) along with a new category of object, namely “dwarf planet”, to describe gravitationally rounded objects that orbit in the same region of space as large groups of other small objects. Dwarf planets are considered a subcategory of minor planet because they’re found among large populations of other minor planets.
There are currently 5 official dwarf planets: Ceres, Pluto, Haumea, Makemake, and Eris. However, there are hundreds of candidate objects that may be classified as dwarf planets once we learn more about them. These include Quaoar, Sedna, Orcus, Salacia, Ixion, and many others, which are as yet unnamed. We don’t yet know how many dwarf planets there are, but there could be hundreds or even thousands of them.
There are trillions of SSSBs. An SSSB is anything in the Solar System bigger than about a meter wide, except for planets, dwarf planets, and moons; therefore, it includes all comets and minor planets except for dwarf planets.
Asteroids are minor planets, typically of the inner Solar System, although trojans and centaurs (see below) are also sometimes considered types of asteroid. Only the largest asteroid, Ceres, is a dwarf planet; all others are SSSBs. The vast majority of asteroids are found in the Asteroid Belt, orbiting between Mars and Jupiter.
Not all asteroids are found in this region of space, however, and many can be found orbiting between and near the terrestrial planets. Those that come close to Earth are called near-Earth asteroids (NEAs). These are an important group primarily because they’re the first asteroids that will be explored and mined, and because some of them have the potential to hit Earth.
The 4 largest asteroids are Ceres, Vesta, Pallas, and Hygiea. These 4 alone comprise almost half the mass of all asteroids combined.
A trojan is a type of minor planet with the same orbit as a planet, leading or trailing the planet by 60º. These positions in a planetary orbit are points of gravitational stability known as the L4 or L5 Lagrange points.
Almost all currently known trojans are co-orbital with Jupiter. It’s estimated there could be about a million Jupiter trojans larger than 1 kilometer in diameter, although it is believed that Neptune may have 10–100 times more large trojans than Jupiter. No Saturnian trojans have yet been discovered, probably because of Jupiter’s powerful gravitational pull.
|Planet||# known trojans|
Centaurs are minor planets with orbital radii between that of Jupiter and Neptune. They have characteristics of both asteroids and comets, hence the name. There are an estimated 44 000 centaurs with diameters greater than 1 kilometer.
Comets are icy objects with long, elliptical orbits that bring them from the transneptunian region into the inner Solar System. As they come closer to the Sun than the frost line, the icy nucleus of the comet begins subliming to water vapor, causing dust and water vapor to be released into space. The solar wind blows this material away from the comet, giving it a long, visible tail called a coma.
Comets spend most of their time in the transneptunian region, only descending into the inner Solar System for a small fraction of their orbital period.
There are a great many objects in the Solar System that are too small to be categorized as SSSBs. These are roughly classified as:
- Meteoroids, which are between about 1 mm and 1 m in diameter.
- Micrometeoroids, which are between about 1 µm and 1 mm in diameter.
- Nanometeoroids (also known as cosmic dust), which are smaller than about 1 µm in diameter.
Objects that orbit other objects in space are called satellites. Those made by humans are called artificial satellites, whereas those formed by natural processes are called natural satellites. The natural satellites of stars are planets, minor planets, and comets; those of substellar objects are called moons.
|Planet / dwarf planet||# known moons|
Many SSSBs also have moons. In total, about 330 of these minor-planet moons have been discovered so far.
Some moons in our Solar System are massive enough to have become gravitationally rounded. There’s no official term for these objects, but the term “major moon” is by far the most common. Conversely, natural satellites that are not massive enough to have become rounded are known as “minor moons”.
Our Solar System has 19 major moons:
- Orbiting Earth: Luna (the Moon)
- Orbiting Jupiter: Io, Europa, Ganymede, and Callisto
- Orbiting Saturn: Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Iapetus
- Orbiting Uranus: Miranda, Ariel, Umbriel, Titania, and Oberon
- Orbiting Neptune: Triton
- Orbiting Pluto: Charon
The transneptunian region
Beyond Neptune, we find a vast region of the Solar System aptly named the transneptunian region. This is by far the largest region of the Solar System and the one we know least about. As far as we currently know, it’s entirely populated by minor planets and comets, although astronomers have not ruled out the possibility that one or more major planets may yet be found in this region of space. It extends as far as approximately 100 000–200 000 AU, which is about the largest possible orbital radius for an object orbiting the Sun.
The transneptunian region includes:
- The Kuiper Belt, which comprises small objects in stable orbits between about 30 and 55 AU. It includes the dwarf planets Pluto, Haumea, Makemake, and others, their moons, plus lots of SSSBs.
- The Scattered Disc, which comprises minor planets in unstable orbits between about 30 and 100 AU. These orbits are unstable because they come close enough to Neptune that its gravity perturbs them. It includes the dwarf planet Eris and others, and their moons, plus lots of SSSBs.
- The Hills Cloud (also known as the “inner Oort Cloud”), a hypothetical disc-shaped region that extends from about 250–1 500 AU to about 20 000–30 000 AU. It potentially contains billions of minor planets, plus, potentially, 5 times as many comets as the Oort Cloud.
- The Oort Cloud, a hypothetical spherical region that may extend from the Hills Cloud to as far as 50 000–200 000 AU. It is also believed to contain billions of minor planets and comets.
Objects that orbit beyond Neptune are collectively known as transneptunian objects (TNOs). All TNOs discovered so far are minor planets or comets, although it’s believed possible that another major planet may yet be found beyond Neptune. TNOs are further classified according to their group, and thus as called Kuiper Belt Objects, Scattered Disc Objects, Hills Cloud Objects, and Oort Cloud Objects.
The heliosphere and heliopause
The heliosphere is a bubble in space defined by the interaction between the solar wind and the interstellar medium. The boundary of the heliosphere is called the heliopause, which is where the force of the solar wind is balanced by the stellar winds of other stars. The heliopause is found in the transneptunian region, approximately 100 AU from the Sun upwind of the stellar winds, and about 200 AU downwind.
Roughly speaking, the Kuiper Belt and Scattered Disc are within the heliosphere, whereas the Hills and Oort Clouds are beyond it.
Beyond the heliopause is the interstellar medium, where the stellar winds from other stars are stronger than the solar wind. However, the Sun’s gravity dominates that of nearby stars out to a much greater distance, which is why it is believed that billions or trillions of objects orbit the Sun beyond the heliosphere in the Hills and Oort Clouds.
In a similar way to how the Earth’s magnetosphere blocks radiation from reaching the surface, the heliopause blocks about 75% of galactic cosmic rays. Thus, cosmic radiation beyond the heliopause is approximately 4 times greater than inside it, which may be a significant deterrent to space explorers and settlers planning on leaving the Solar System.