The solar system consists of the Sun and the planets, natural satellites, asteroids, meteoroids and comets that orbit it.
These objects range in size from microscopic dust to the largest planets. They all have certain characteristics that scientists use to classify them. These include size, temperature and composition.
Planets
In our solar system, a planet is a space object that orbits a star. It’s made up of gas and dust, and it attracts more and more gas and dust to its surface as it revolves around the Sun.
There are eight known planets, with many moons, asteroids and comets. They range in size from Mercury through Neptune, and they all have ring systems.
Astronomers believe that our planets are the result of the collisions and accumulation of debris left over from a nebula that formed the Sun. In the early stages of its life, a cloud of material ejected from a dying star was picked up by gravity, and it swirled about the forming sun. The leftover gas and dust that didn’t absorb into the Sun became the earliest stars in our solar system, eventually forming planets.
The planets, in order of increasing distance from the Sun, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Some of them, like Saturn and Jupiter, are large, low-density gas giants, while others, like Uranus and Neptune, have icy atmospheres that contain water and other chemicals that form ices.
As a result of the formation and evolution of our solar system, astronomers have a lot to learn about how and why planets are formed. They’ve analyzed data from the planets themselves, their moons and asteroids; they’ve used space probes to examine their atmospheric composition and interiors; and they’ve studied rocks that come down from the Moon by meteors and those returned by astronauts.
But defining what makes something a planet has never been easy. The issue has been a lively one, with three arenas for discussion: characteristics (physical properties of the body itself), circumstances (primarily questions regarding orbital properties), and cosmogony (formation mechanisms and histories). It’s all up for debate!
Moons
The moon is a solar system space object that revolves around a planet. Although they come in a variety of shapes and sizes, most planetary moons were probably formed from the discs of gas and dust circulating around planets during early solar system formation.
But some planetary moons may have formed elsewhere and been “caught” by larger worlds, orbiting around their center of gravity. These are called exomoons, and they can help astronomers understand the formation and evolution of the solar system.
Our own Earth has one natural satellite, but many larger gas giants have dozens or even hundreds of moons. And astronomers are finding new moons all the time!
They have a range of chemical compositions, and some moons have atmospheres and even oceans beneath their surfaces. These moons are essential to understanding the planets they orbit and can reveal information about their history and environment.
Mars has two small moons, Phobos and Deimos. These moons are likely captured asteroids, but they’re difficult to see in telescopes because of their close proximity to Mars.
Jupiter has a number of moons, including Callisto and Enceladus. Both are nitrogen-rich, and Enceladus is known to emit jets of gas and dust that may harbor liquid water beneath its surface.
Pluto has Charon, and astronomers have theorized that the moon is the result of a giant impact with a larger object in the distant past. However, it’s possible that Pluto and Charon are two separate bodies, orbiting around a common center of mass that lies outside of either object.
Some moons are very similar to their host planets, while others are quite different. Often these differences aren’t clear until they’re explored in depth.
Asteroids
Asteroids are rocky and metallic objects that orbit the Sun. They come in all sorts of shapes and sizes, from tiny specks to huge ones hundreds of miles wide. Scientists think that they’re the leftovers of the material that formed all the planets in our solar system billions of years ago.
In the early days of our Solar System, millions of small lumps of rock and metal were floating around the outer edges of the nascent planets’ orbits. Some of these lumps stuck together to form larger lumps that then became planets. Others fell out of these orbits and landed on Earth or other planets in our solar system, raining down on us or burning up as glowing meteors that look like stars.
These smaller chunks of rock and metal are grouped together in an area called the asteroid belt, which lies between the orbits of Mars and Jupiter. The smallest of these asteroids are called meteoroids, but the largest can be as big as the state of New York.
Asteroid compositions are varied, but they’re all mostly made up of the same kind of rocky stuff. Most of these chunks are composed of a type of rock called carbonaceous chondrite, which has a distinctive black colour. Other rocky asteroids, which are sometimes called “S-type” asteroids, contain silicates and clays.
Most asteroids have a lot of rubble on their surface, and these craters are often found in a spherical shape, suggesting that they were once rounded, but broke apart. Some asteroids are so big that they have companion moons.
Some of these asteroids are even close enough to Earth that they could crash into it. This has happened once, when an asteroid sliced a 6.2-mile-wide crater into Earth’s surface in Mexico 6.5 million years ago. The impact is thought to have wiped out 75% of all the species on Earth, including dinosaurs.
Meteorites
Meteorites, or pieces of meteors that have fallen to Earth’s surface, are an essential part of our understanding of our solar system. They provide clues to the formation and evolution of the planets, their moons and other planetary debris, and to the origin of the solar nebula.
While most meteors burn up as they pass through the atmosphere, some large meteoroids may strike the Earth’s surface and cause an impact. These impacts are rare; however, they do occur and can have devastating effects on the Earth.
The majority of the meteorites that fall to Earth are fragments of asteroids, which formed when our solar system was first formed. They have a variety of compositions, including metal (mainly iron), stony-irons (containing mixed minerals), and stones.
Stony-iron meteorites, for example, are likely mixtures of igneous and metamorphic rocks that once formed in the mantle of a massive asteroid. These are called chondrites; about 87 percent of stony meteorites in collections have chondrite compositions.
They are a major component of many meteor showers and are a source of dazzling displays of light, known as shooting stars. Other smaller pieces of extraterrestrial debris – like grains of dust from comets – also cause meteors.
There are a number of different meteorite types, including those that contain microscopic spherules or “chondrules” of metals and other elements that have condensed out of a gas. About 5 percent of chondrites are carbonaceous chondites, which are high in carbon and volatile elements.
They are largely rocky, although they can be composed of ices and other organic materials, too. They are particularly common in the asteroid belt, between Mars and Jupiter. They can form when a large asteroid hits another one, or when they break away from a comet.
Spacecraft
Spacecraft are essentially vehicles or robots that have been designed to leave Earth’s atmosphere and travel through space. They include satellites orbiting Earth, robots sent to other planets, and vehicles sent to the Moon carrying human astronauts.
Space craft are important as solar system space objects because they have given scientists valuable information about planets, moons and other space objects. Scientists have used spacecraft in three main ways to explore the solar system: from a distance by satellites; on-site from landers and rovers; and in the case of giant gas planets, by sending probes into their atmospheres.
Scientists use spacecraft to study the Earth, Venus, Mars, Jupiter, Saturn, a comet, and several asteroids. They also use satellites to investigate the Sun.
A spacecraft may be small and light or large and heavy, depending on its mission. A probe, for example, is smaller and lighter than a satellite because it doesn’t have to orbit a planet, like a satellite does.
When a spacecraft is launched into space, it must be able to survive the vacuum of space and the temperature extremes. For manned spacecraft, the thermal control subsystem is particularly important.
This subsystem controls the temperature of a spacecraft’s interior, which may be hundreds of degrees Fahrenheit or warmer in the presence of plasmas. Depending on the type of spacecraft and its mission, it can be passive or active, requiring the selection of materials with specific radiative properties.
A spacecraft’s command and data subsystem can be a separate electronic device, or it can be integrated with the other subsystems. Often, an uplink transmitter and receiver are combined into one electronic device. Transmitters send radio signals from a spacecraft to Earth, which are received by receivers. The resulting signal is then converted into binary code, or a form that can be read by computers on Earth.