Our Solar System contains the Sun, major planets with their satellites, numerous small bodies (asteroids, comets, meteoroids, Kuiper belt objects), and dust. The Solar System has three major zones: the inner solid planets and asteroids, the large gaseous planets, and the ice-dominated small bodies that make up the Kuiper belt (a circumstellar disc in the outer Solar System, extending from the orbit of Neptune to approximately fifty AU from the Sun) and Oort Cloud (a theoretical cloud of predominantly icy planetesimals proposed to surround the Sun at distances ranging from two thousand to two hundred thousand AU).
The Sun is the star at the centre of our Solar System that is the source of light and heat for life on Earth. Observations of the Sun across the whole electromagnetic spectrum have revealed that it is highly variable at some wavelengths but not at others. The age of the Solar System is approximately four point six billion years.
Solar systems around other stars are now thought to be quite common. It is now believed that there are more planets then there are stars in the Milky Way galaxy (the galaxy that contains the Solar System, with the name describing the galaxy’s appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye).
The Solar System
Our Solar System contains a wide variety of objects that reflect their evolutionary state. The Solar System can be subdivided into three zones: the inner zone with terrestrial planets Mercury, Venus, Earth, and Mars along with the asteroid belt out to nearly five Astronomical Unit (AU); the middle zone with giant gas planets Jupiter, Saturn, Uranus, and Neptune occupying the region between five and thirty AU, and the Kuiper belt extending from thirty to sixty AU which contains Pluto the largest object and the distance Oort Cloud of comets from fifty thousand to one hundred thousand AU.
The largest planet is Jupiter, exceeding the Earth more than ten times in size and more than three hundred times in mass. While the inner planets possess only three satellites (Earth’s Moon; Phobos and Deimos of Mars), the bulk of satellites belongs to the giant planets, the largest being four Galilean satellites of Jupiter (Io, Europa, Ganymede, and Callisto), a satellite of Saturn (Titan), and a satellite of Neptune (Triton), which are comparable in size to our Moon or even to the planet Mercury.
A new theory in the formation of the Solar System involves a process of gravitational resonances with Jupiter resulting in bringing both asteroids and Kuiper belt objects sunward thereby bombarding the inner Solar System after the first eight hundred and fifty million years. Perhaps as much as sixty to eighty percent of the water on the Earth may have come from this bombardment referred to as the “late heavy bombardment” as determined from samples from the Moon.
Inner planets experienced dramatic changes in their evolution that impacted on their structure, geology, landforms, and atmospheres. Unlike our gaseous giant planets that are essentially unmodified in structure and composition since the time of their origin approximately four point six billion years ago, the rocky inner planets have undergone dramatic changes throughout their evolution caused by geologic processes (volcanism, plate tectonics, and weathering).
These processes are closely related to the differentiation processes in the planetary interiors (heavier material sinks to the core) and result in different patterns of landforms and the formation of an atmosphere and hydrosphere (except Mercury). Dynamics including tidal interactions between the planets, satellites, and rings, as well as collisions and impacts; these dynamics are responsible for many intriguing phenomena, such as widespread volcanism on Io, a suspected deep-water global ocean on Europa, geyser-like eruptions on Enceladus, and methane lakes on Titan.
Small bodies of the Solar System
Small bodies of the Solar System are asteroids, comets, meteoroids, and meteor dust. Asteroids are mostly located in the Main asteroid belt between the orbits of Mars and Jupiter. The largest asteroid, Ceres, is about one thousand kilometres in diameter. Gravitational resonances with Jupiter have carved large gaps in the number of asteroids at certain locations in the asteroid belt. Asteroids that move into these gaps are either flung out of the Solar System or thrown inward to become hazards to the Earth.
Thee special groups of hazardous asteroids at eccentric orbits approaching or crossing the Earth’s orbit have been identified (Apollo, Amor, and Aten) and are referred to as Near Earth Objects (NEOs). The asteroids reaching a few kilometres in size that may impact on the Earth represent a potential life threat to our planet. Today we know of more than nineteen thousand NEOs for which approximately nine hundreds of them are greater than one kilometre in size.
Meteorites are collisional fragments of asteroids that fall through our atmosphere and are found on the Earth. Asteroids and meteorites are commonly classified by their composition as stony, iron, and iron-stony. The most primitive meteorites (carbonaceous chondrites) come from the outer part of the Main belt.
Comets are regarded as the most primitive bodies of the Solar System, with the main families being located in the Oort cloud and Kuiper belt. Comets are classified into short-period (orbiting the Sun for less than two hundred years), and long-period (having periods more than two hundred years). The main family of comets are located in the Oort cloud at distances greater than fifty thousand AU and in the Kuiper belt beyond the orbit of Neptune.
Comets are icy bodies consisting of nucleus, atmosphere (coma), and tail. Very porous nucleus is associated with a “dusty snow ball” because it is composed mainly of water ice with other frozen gases and dust; when a comet approaches the Sun, the coma and tail are developed due to sublimation of surface ices, and then are partially ionised. Comets are distinguished as the most pristine bodies or remnants of the outer planets formation, many of which were thrown out by gravity perturbations to the periphery of our Solar System forming the Oort Cloud.
Currently, there are more than four thousand known exoplanets. The most successful method of finding an exoplanet is the transit method in which the planet passes in front of its parent star blocking a small amount of its light that can be measured. Using the transit method on approximately one hundred and twenty-five thousand faint (distant) stars, NASA’s Kepler space telescope found approximately two thousand five hundred of these exoplanets and in addition, more than five thousand unconfirmed planets.
Because of observational biases, a large fraction of the known exoplanets are giant planets in tight orbits around their stars. In time, more systems like our Solar System, are expected to be found. From statistics, we now know that almost every star in the sky has planets with about twenty percent of the stars hosting Jupiter-mass planets and sixty percent super-Earths. Super-Earths are rocky planets with masses of about five to ten Earth masses. Some of the discovered planets are in the habitable zone of a star.
The habitable zone is the region around a star where a planet receives enough stellar flux to have a surface temperature that would allow liquid water, assuming the planet has an atmosphere that is thick enough to sustain this water. For stars colder than the Sun, the habitable zone will be closer in than that of the Sun, and for hotter stars, it will be further out. That is what can be said about the Solar System.