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Solar System

NASA image

Solar System

The Solar System comprises the Sun and the planetary system that orbits it. In addition to planets, the Solar System also consists of smaller objects such as moons, comets, asteroids, dwarf planets, dust and gas.


The Sun is the Solar System's star and by far its most massive component. It has 99.8% of the mass in the solar system. The sun’s radius is about 432,000 miles (695,500 km)- approximately 109 times Earth’s radius.
It is a nearly perfect spherical ball of hot plasma (gas whose temperature has been raised to such a high level that it becomes sensitive to magnetism). About 3/4 of the Sun's mass consists of hydrogen; the rest is mostly helium, with much smaller quantities of heavier elements, including oxygen, carbon, neon and iron.
It is the most important source of energy for life on Earth. The energy of the sun comes from nuclear fusion reactions that occur deep inside the sun’s core.

Inner Solar System

The inner Solar System is the region comprising 4 terrestrial - inner planets (Mercury, Venus, Earth, Mars), and the asteroid belt.
Inner planets have dense, rocky compositions, few or no moons, and no ring systems. They are composed largely of refractory minerals, such as the silicates, which form their crusts and mantles, and metals, such as iron and nickel, which form their cores. Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather; all have impact craters and tectonic surface features, such as rift valleys and volcanoes.

Asteroid Belt

The asteroid belt occupies the orbit between Mars and Jupiter. It contains tens of thousands, possibly millions, of objects over 1 kilometre in diameter. Asteroids are composed mainly of refractory rocky and metallic minerals and some ice. They range from a few metres to hundreds of kilometres in size. Asteroids smaller than one meter are usually called meteoroids.
The largest asteroid is Ceres. It has a diameter of 945 km (587 miles). It is composed of rock and ice.

Outer Solar System

The outer region of the Solar System is home to the giant outer planets (Jupiter, Saturn, Uranus, Neptune) and their large moons.
Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles, such as water, ammonia, and methane than those of the inner Solar System because the lower temperatures allow these compounds to remain solid.

The Eight Planets of the Solar System


Mercury is the planet nearest the sun. It is the smallest planet - it is only slightly larger than Earth's moon. Its surface also appears to be much like that of the moon. It has broad, flat plains, steep cliffs, and many deep craters similar to those on the moon. The craters formed when meteors or small comets crashed into the planet.
Mercury moves around the sun faster than any other planet. It goes around the sun once every 88 Earth days. (The Earth goes around the sun once every 365 days, or 1 year).
Mercury is dry, extremely hot, and almost airless. Because the planet is so close to the sun, Mercury's surface temperature can reach a scorching 840 °Fahrenheit (450 °C). However, since it doesn't have a real atmosphere to entrap any heat, at night temperatures can plummet to minus 275° F (minus 170 °C).


Venus is the third brightest object in Earth's sky after the Sun and Moon. Venus is sometimes referred to as the “morning star” and “evening star”. This dates back to ancient civilizations who believed that Venus was in fact two distinct stars appearing in the sky.
It is sometimes referred to as the sister planet to Earth, because their size and mass are so similar. Venus is also the closest planet to Earth.
Venus' atmosphere is mostly carbon dioxide and nitrogen gas with clouds of sulphuric acid. Because the atmosphere is so thick and dense the pressure is very high. The pressure is 92 times the pressure on Earth, enough to crush many things.
Venus is much hotter than Earth. All the carbon dioxide in the atmosphere acts like a blanket, trapping heat from the Sun. This effect is called the greenhouse effect and it is very strong on Venus. This makes the surface of Venus the hottest of any planet's surface in the Solar System with an estimated average temperature of 480 °C (896.0 °F). Venus rotates in the opposite direction to other planets. Most other planets rotate counter-clockwise on their axis, however Venus, like Uranus, rotates clockwise.


Earth is the largest of the 4 terrestrial planets. It is the most dense planet in the solar system. The density of Earth differs in each part of the planet – the core, for example, is denser than the Earth’s crust.
Earth’s atmosphere is composed of 78% nitrogen, 21% oxygen, and trace amounts of other gases including argon and carbon dioxide. The large amount of oxygen on Earth comes from our plant life’s consumption of carbon dioxide during photosynthesis.
Earth has a very powerful magnetic field. This field protects the planet from the effects of solar winds.
The Earth has an Ozone Layer which protects it from harmful solar radiation.
70% of the Earth’s surface is covered by water.


Mars is the 2nd smallest planet in the Solar System, after Mercury. Named after the Roman god of war, it is often referred to as the "Red Planet" because the iron oxide prevalent on its surface gives it a reddish appearance.
The planet's orbit path is more elongated than many of the other planets and this oval shaped orbit results in fierce dust storms that cover the entire planet and can last for many months.
In September 2015, NASA scientists announced that water exists in liquid form on the surface of Mars making it possible for life to be sustained on the Red Planet.
Mars was believed many times in the past to be home to intelligent life. This came from the discovery of lines or grooves in the surface called canali by Italian astronomer Giovanni Schiaparelli. He believed that these were not naturally occurring and were proof of intelligent life. More controversy came in 1976 when NASA took a picture of a remarkably human face and mysterious, pyramid-like structures in the Cydonia region of Mars.


Jupiter is the largest planet in the Solar System. Its volume is that of about 1,321 Earths. It has gaseous outer layers and may have a rocky core.
It has the shortest day of the eight planets. The planet rotates very quickly, turning on its axis once every 9 hours and 55 minutes.
Jupiter has a faint ring system around it. Its rings are mostly comprised of dust particles from some of Jupiter’s moons during impacts from comets and asteroids.
Jupiter has at least 67 moons; the largest ones are Io, Europa, Ganymede, and Callisto.
Jupiter’s Great Red Spot is an enormous storm that has been raging for over 300 years.


Saturn is the second largest in the Solar System, after Jupiter.
It has a large, beautiful and extensive ring system that encircles the planet. These rings are mostly made from chunks of ice and carbonaceous dust. They stretch out more than 12,700 km from the planet but are only a mere 20 meters thick.
Saturn is the least dense planet in the solar system. It is a gas giant because it is predominantly composed of hydrogen and helium.
Saturn has the fastest winds of any other planet in our solar system. These winds have been measured at approximately 1,800 km per hour (1,100 miles per hour).
Saturn has 150 moons and smaller moonlets. All of these moons are frozen – the largest of which are Titan and Rhea.
At Saturn's south pole, there is a feature that appears to be a hurricane with a giant eye, and at the south pole, there is a weird hexagon-shaped vortex.


Uranus is often referred to as the “ice giant”. While it has a hydrogen and helium upper layer like the other gas giants, Uranus also has an icy mantle which surrounds its rock and iron core. The upper atmosphere of water, ammonia and methane ice crystals gives Uranus its distinctive pale blue color.
It is the coldest planet in the solar system.
Uranus has 13 presently known rings. The rings are composed of extremely dark particles.
Like Venus, Uranus turns in a retrograde direction which is opposite to the direction Earth and the other six planets turn.


Neptune is not visible to the naked eye and is the only planet found by mathematical prediction. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet. Neptune was subsequently observed with a telescope on 23 September 1846.
Due to its blue coloration, Neptune was named after the Roman god of the Sea.
Like the other outer planets, Neptune possesses a ring system, though its rings are very faint. They are most likely made up of ice particles and grains of dust.
Neptune’s moon Triton is one of the largest in the solar system and has volcanoes that emit plumes of frozen nitrogen.

Note: Pluto, once considered the ninth and most distant planet from the sun was reclassified from a planet to a dwarf planet in 2006.

Dwarf Planets:
There are 5 officially recognised dwarf planets in our solar system. They are Ceres, Pluto, Haumea, Makemake and Eris.

The Planets and their Moons
Earth 1Moon
Mars2Phobos, Deimos
Jupiter62Io, Europa, Ganymede, Callisto, Amalthea, Himalia, Elara, Pasiphae, Sinope, Lysithea, Carme, Ananke, Leda, Metis, Adrastea, Thebe, Callirrhoe, Themisto, Kalyke, Iocaste, Erinome, Harpalyke, Isonoe, Praxidike, Megaclite, Taygete, Chaldene, Autonoe, Thyone, Hermippe, Eurydome, Sponde, Pasithee, Euanthe, Kale, Orthosie, Euporie, Aitne, plus others yet to receive names
Saturn33Titan, Rhea, Iapetus, Dione, Tethys, Enceladus, Mimas, Hyperion, Prometheus, Pandora, Phoebe, Janus, Epimetheus, Helene, Telesto, Calypso, Atlas, Pan, Ymir, Paaliaq, Siarnaq, Tarvos, Kiviuq, Ijiraq, Thrym, Skadi, Mundilfari, Erriapo, Albiorix, Suttung, plus others yet to receive names
Uranus27Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, Puck, Miranda, Ariel, Umbriel, Titania, Oberon, Caliban, Sycorax, Prospero, Setebos, Stephano, Trinculo, plus others yet to receive names
Neptune13Triton, Nereid, Naiad, Thalassa, Despina, Galatea, Larissa, Proteus, plus others yet to receive names

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