Mercury is one of the planets of the Solar System, an inner planet closest to the Sun. The smallest planet of the Solar System; 2 of the moons are larger than Mercury: Ganymedes (orbiting Jupiter) and Titan (orbiting Saturn). It is a rocky planet (terrestrial planet), the second most dense in the Solar System (5.43 g/cm³), due to its iron core. When observed from the Earth, it is always close to the Sun, never moving further away than 22 degrees from it, and it has phases, similarly to the Moon.
Venus is one of the planets of the Solar System, the second closest inner planet to the Sun. It is a rocky (terrestrial) planet, with great density (5.25 g/cm³). It is very similar to the Earth, in size and mass, and it is often called the sister of Earth.
The Earth is the third planet from the Sun in the Solar System. It is the fifth largest planet. The Earth is the terrestrial planet with the largest diameter, mass and density. The Earth is the only known planet in the Universe where life exists. According to current theories, the Earth formed 4.57 billion years ago, and life appeared on its surface within one billion years.
Mars is the fourth planet from the Sun in the Solar System, the outermost of the inner planets. It is 1.52 times as far from the Sun as the Earth. Its diameter is about half of Earth’s, its mass is only one tenth. It is a rocky planet (terrestrial planet).
Jupiter is the fifth planet from the Sun, second from the Earth, the closest of the outer planets. It is the largest planet in the solar System, its diameter is 142,984 km, with two and half times the mass of all the other planets combined.
Saturn is the second largest planet in the Solar System, a spectacular outer planet. It is a gas giant, or Jupiter-like (Jovian) planet.
Saturn is the most oblate planet, because of its high speed of rotation and low density. It is the least dense planet in the Solar System, the only one with a density less than that of water (0.69 g/cm³).
Uranus is one of the gas giants (Jupiter-like planets) of the Solar System, the fourth planet from the Earth, an outer planet. Its atmosphere consists of hydrogen (83%) and helium (15%), with some methane and ammonia. Methane in the upper atmosphere absorbs red light, giving the planet a pale blue-green hue. Few details of the atmosphere are visible. Clouds move along the latitudes, as is the case of Jupiter and Saturn, but they are much fainter.
Neptune is an outer planet, the outermost planet of the Solar System. It is 30 times farther from the Sun than the Earth, on a nearly circular orbit. It is the smallest of the gas giants, very similar to Uranus.
The Sun is one of the billions of stars in the Milky Way, located in the plane of our barred spiral galaxy, in the Orion spiral arm. The Sun, and the whole solar system orbit around 27,000 to 28,000 light years away from the centre of the disc, which measures 50,000 light years in radius. It takes roughly 240 million years for the Sun to complete one orbit. The environment of the solar system is sparse, the nearest stars – Proxima Centauri and the double system of Alpha Centauri – are 4.2–4.4 light years away from us, and there are only 11 stars within 10 light years.
By solar system we mean the Sun and all the variously sized celestial bodies orbiting it. The solar system is the area where the gravitation of the Sun is dominant. This is a sphere of about 2 light years in radius; on its border the gravity of the Sun is equal to the gravity of the nearest stars. The solar system is completely filled with solar wind, a continuous flow of electrically charged particles emitted by the Sun.
The solar system consists of the Sun, the planets, the moons of the planets, asteroids and comets, meteoroids and interplanetary matter i.e. dust and gas. Eight planets orbit the Sun; six of these have moons, the exceptions being Mercury and Venus.
In the order of their distance from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Planets can be divided into two separate groups: four terrestrial or Earth-type planets and four gas giants, also called Jovian planets. Terrestrial planets are closer to the Sun. They are smaller and denser, rotate more slowly, and have thinner atmospheres and weaker magnetic fields.
All of the planets orbit the Sun on nearly the same plane, in the same direction, their motion is direct, which means they move in an anticlockwise direction, as seen from the North Pole of the Earth. Except for Venus and Uranus, their rotation is direct too. The Sun also rotates in this direction.
Planets are kept in orbit by the gravity of the Sun. The mass of the Sun is 750 times larger than the total mass of the planets. There is also gravitational force between the planets; therefore, they influence each other’s motion. As a result, their orbits might undergo slow, minor changes.
Besides planets, there are billions of small objects in the solar system. Asteroids can be found just about everywhere. Many of them have orbits that cross that of the Earth. Most of the asteroids are located in two zones. The inner asteroid belt is between Mars and Jupiter (where there are at least 1 billion asteroids larger than 1 km in diameter); the outer belt, i.e. the Kuiper belt – is located beyond the orbit of Neptune (where several thousand Pluto-like, icy asteroids have been discovered).
Since 2006, Pluto has no longer been considered a planet. Pluto and a few other large asteroids are called dwarf planets. The orbits of the majority of the comets are entirely different from that of other objects: they have elongated elliptical orbits with different orbital planes. As the 5–20 km large icy core evaporates near the Sun, a rare, spectacular tail is formed. Due to solar wind, this tail points away from the Sun. Billions of comets orbit in the Oort cloud, the outer region of the Solar system, 0.5–2 light years from the Sun.
Since 1995, numerous exoplanets have been discovered around hundreds of stars. In many of these systems, giant planets orbit the stars, therefore we can safely assume they are not similar to our solar system.