Our solar System

  Facts about our solar system

The solar system is a collection of planets, moons, asteroids, comets, dust and gas which orbit our local star, the sun. It includes the rocky inner planets Mercury, Venus, Earth and Mars; the gas giants Jupiter and Saturn; and the ice giants Uranus and Neptune. Between Mars and Jupiter is a collection of asteroids known as the asteroid belt, while beyond Neptune is where small icy bodies, like Pluto and comets, live.

About 4.6 billion years ago, a giant cloud of dust and gas known as the solar nebula collapsed in on itself and began to form what would eventually become the solar system's sun and planets. Meteorites, or pieces of space rock which have fallen to Earth, have helped scientists figure out the age of the solar system. Some of these small pieces have broken off from moons or planets and can yield interesting scientific information about the chemistry and history of their home body. Others have been traveling around the solar system since its beginning, before the planets even existed. The Allende meteorite, which fell to Earth in 1969 and scattered over Mexico, is the oldest known meteorite, at 4.55 billion years old. Scientists think the solar system formed when a nearby exploding star, called a supernova, triggered the collapse of the solar nebula. 

THE SUN

The sun is at the centre of the solar system and is its largest object, accounting for approximately 99.8% of the solar system's mass. The sun is a giant, raging ball of fire powered by nuclear reactions, and it provides the energy which sustains life on Earth. The yellow dwarf star is made up of gas — about 91% hydrogen and 8.9% helium, according to NASA. Compared with other stars, the sun is relatively small and just one of hundreds of billions of stars in our home galaxy, the Milky Way. The sun is about 26,000 light-years from the centre of our galaxy, according to NASA. The Milky Way is a spiral galaxy, with curved arms of stars emanating from its centre. The solar system is located in one of the smaller arms, called the Orion-Cygnus Arm or simply the Orion Arm. The Milky Way is huge compared with the solar system. If the solar system were the size of your hand, the Milky Way would be as big as North America, according to NASA. 

OUR SOLAR SYSTEM'S PLANETS

Eight confirmed planets and many dwarf planets orbit the sun. According to NASA, "the order and arrangement of the planets and other bodies in our solar system is due to the way the solar system formed." Rocky materials could withstand the young sun's immense heat, so the first four planets — Mercury, Venus, Earth and Mars — are small, with rocky surfaces. Beyond them, "materials we are used to seeing as ice, liquid or gas settled in the outer regions of the young solar system," NASA says, namely the gas giants Jupiter and Saturn and the ice giants Uranus and Neptune.

Mercury

Mercury is the closest planet to the sun. It is also the smallest planet in the solar system; at about 3,032 miles (4,879 kms) in diameter, it is only slightly larger than Earth's moon and has no moons of its own. Mercury has no atmosphere to protect it from the sun's relentless radiation, and surface temperatures can reach highs of 800 degrees Fahrenheit (427 degrees Celsius) during the day and plummet as low as minus 290 F (minus 179 C) at night. 

Venus

Venus is the second planet from the sun and the hottest planet in the solar system. Its atmosphere is a thick layer of mostly carbon dioxide gas that traps heat, allowing the planet's surface temperatures to reach a scorching 880 F (471 C). At 7,520 miles (12,100 km) in diameter, according to NASA, Venus is slightly smaller than Earth and, like our planet, has a core of molten iron. In 2020, astronomers announced that they had discovered phosphine in the atmosphere of Venus. On our planet, this chemical is almost always made by living creatures, leading some researchers to wonder if Venus' clouds might host life. However, the findings are controversial and have yet to be fully confirmed; indeed, other work has suggested that it would be too difficult for any creatures to survive in such a hellish place because of the lack of much liquid water. 

Earth

The third rock from the sun, Earth is the only planet in the universe known to have life. Its habitability is linked to the presence of liquid water. Earth is located in the so-called Goldilocks zone, meaning it orbits at the ideal distance from the sun to have liquid water; if it were any closer, the water would evaporate into a gas, and if Earth were farther away, the water would freeze. About 71% of our planet's surface is covered in water, according to the U.S. Geological Survey, and Earth's atmosphere protects the planet from solar radiation. Earth likely earned its name from the English and German words for "ground." The blue planet is the largest of the four rocky planets in the solar system, and it has one moon. Scientists think Earth's moon was formed from a piece of Earth that broke off when a giant object smashed into the young planet.

Mars

Mars is known as the Red Planet because of the iron-rich dust that covers its surface and gives it a rusty colour. Mars, the fourth planet from the sun, is home to the largest volcano in the solar system, Olympus Mons. The planet has a thin atmosphere, and without a thick protective shield, temperatures on Mars average around minus 80 F (minus 60 C). It is likely that liquid water — which would provide a home for life as we know it — existed on the Martian surface billions of years ago, and some scientists think Mars may still have liquid water on its surface today. Mars has two moons: Phobos, which may have formed from ancient rings around the planet, and Deimos, which may have been knocked into its current orbit by an ancient, no-longer-existing moon.

Jupiter

Jupiter is the biggest planet in the solar system. Unlike the inner planets, Jupiter is a gas giant, made up mainly of helium and hydrogen. Jupiter, the fifth planet from the sun, is twice as big as all of the other planets in the solar system combined, yet it also has the shortest day of any planet, taking 10 hours to turn about its axis, according to NASA. Jupiter is surrounded by dozens of moons, and its rings are faint and composed of dust. Deep in the planet's atmosphere, high pressure and high temperatures have compressed the hydrogen gas into a liquid, creating the largest ocean in the solar system, according to NASA.

Saturn

Saturn, the sixth planet from the sun, is the second-largest planet in the solar system. Saturn is best known for its prominent rings. Like Jupiter, Saturn is a gas giant composed of helium and hydrogen, and it is the least dense of the planets. Saturn's rings are made of billions of ice particles and rocks. The ringed planet also has dozens of moons, ranging in size from that of a sports field to the size of Mercury, according to NASA. One of Saturn's moons, Enceladus, is covered in an icy ocean that astronomers say makes this moon a promising candidate for extra terrestrial life.

Uranus

Uranus, the seventh planet from the sun, was the first planet to be discovered using a telescope, by British astronomer William Herschel in 1781. The ice giant is composed of heavier elements than its gas giant neighbours — a mixture of water, methane and ammonia ice. Unlike other planets in the solar system, Uranus effectively orbits on its side (with its axis almost pointing toward the sun), and it "rolls" like a ball as it travels around the sun. Methane gas in Uranus' atmosphere makes the planet appear green-blue. The planet has 13 rings and 27 moons.  

Neptune

Neptune, the eighth planet from the sun, was discovered in 1846. However, even before observing the planet for the first time, scientists predicted its existence, because of its effect on Uranus' orbit. Neptune is so far from the sun that it takes 4.15 hours for sunlight to reach the planet, according to Cornell University. (For comparison, it takes about eight minutes for sunlight to reach Earth.) When the light arrives at Neptune, it is 900 times dimmer than what we see on Earth, according to NASA. Neptune is made up of water, methane and ammonia surrounding a small, rocky core. Strong winds on the planet propel clouds of frozen methane at speeds of up to 1,200 mph (2,000 km/h). Neptune has 14 known moons, one of which was rediscovered after going missing for 20 years.

WHY IS PLUTO NOT A PLANET?

Pluto, once considered the ninth planet in the solar system, was reclassified as a dwarf planet in 2006 because it failed to meet one of the criteria in the definition of a planet. The International Astronomical Union defines a planet as a celestial body that orbits the sun, has enough gravity to pull itself into a round or almost-round shape, and has cleared the neighbourhood around its orbit. Pluto has not cleared the neighbourhood around its orbit, so it's not considered a planet. Some astronomers believe this reclassification is unfair and that Pluto should be reinstated as the ninth planet in the solar system. Looking at the historical literature, Philip Metzger, a planetary scientist at the University of Central Florida, found that before the 2006 decision the "clearing the neighbourhood" rule appeared only in a single 1801 paper. He and his colleagues concluded that this Pluto-excluding rule is "arbitrary and not based on historical precedent," Live Science reported, and that therefore Pluto should still be considered a planet. Pluto sits in the vast Kuiper Belt, a region beyond Neptune which contains trillions of icy objects.