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 our solar system's sun and planets. Our solar system hosts the sun at its center — a star so large that its gravitational pull keeps numerous planets, dwarf planets (such as Pluto), comets and meteoroids orbiting around it.
How old is our solar system?
Meteorites, or pieces of space rock that have fallen to Earth, have helped scientists figure out the age of the solar system. Some of these small pieces of space rock, or meteoroids, have broken off moons or planets, and can yield interesting scientific information about the chemistry and history of their home body; others have been traveling around our solar system since that primordial dust-cloud collapse, before those planets even existed. The Allende meteorite, which fell to Earth in 1969 and scattered over Mexico, is the oldest known meteorite, dated to 4.55 billion years old.
How did our solar system form?
Scientists believe a nearby exploding star, called a supernova, may have triggered the collapse of our solar nebula. According to this theory, the supernova's explosion sent shock waves through space and those shock waves pushed parts of the nebula closer together, leading to collapse. The supernova may have even seeded material into the nebula, and this jettisoned material would have drawn even more matter toward the nebula's growing mass.
The sun is at the center of our solar system and is its largest object, accounting for 99.8% of the solar system's mass. Our sun is a giant, raging ball of fire powered by nuclear reactions, and it provides the energy that sustains life on Earth. The life-giving star is a yellow dwarf star made up of gas: about 91% hydrogen and 8.9% helium, according to NASA. Compared with other stars, the size of the sun is relatively small and it's just one of hundreds of billions of stars in our home galaxy, the Milky Way.
One star in a galaxy
The sun is between 25,000 and 30,000 light-years from the supermassive black hole that forms the center of our galaxy. The Milky Way is a spiral galaxy, with curved arms of stars emanating from its center. Our solar system forms one of the smaller arms, called the Orion-Cygnus Arm, or simply the Orion Arm.
If our solar system were the size of your hand, the Milky Way would cover North America, according to NASA Jet Propulsion Laboratory's Night Sky Network. The boundary of the sun's gravitational influence extends about 122 astronomical units (AU), where one AU is the distance between the Earth and the sun, or is about 93 million miles (150 million kilometers).
Our solar system's planets
Eight confirmed planets and at least five dwarf planets orbit our 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, and 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 is the closest planet to the sun, and it is also the smallest planet in the solar system, only slightly larger than Earth's moon. Mercury has no atmosphere to protect it from the sun's relentless radiation and daytime surface temperatures can reach highs of 800 degrees Fahrenheit (430 degrees Celsius) before plummeting as low as minus 290 F (minus 180 C) at night. Mercury was named after the Roman messenger of the gods because of its speedy rotation around the sun. This small planet has no moons.
Named after the Roman goddess of love, Venus is the hottest planet in our 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). Venus is slightly smaller than Earth, and, like Earth’s outer core, it also has a core of molten iron. "Almost all the surface features of Venus are named for noteworthy Earth women — both mythological and real," says NASA. "A volcanic crater is named for Sacajawea, the Native American woman who guided Lewis and Clark's exploration. A deep canyon is named for Diana, Roman goddess of the hunt." Similar to Mercury, there are no moons orbiting Venus.
The third rock from the sun, Earth is the only planet known to harbor life in the universe. Its habitability is linked to the presence of liquid water. Earth is located in what is known as the "Goldilocks Zone," orbiting 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; and Earth's atmosphere protects the planet from solar radiation. Earth is the only planet not named after a god. Earth likely earned its name from the English and German words for "ground." Our blue planet is the largest of the four rocky planets in our 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 young Earth.
Mars is known as the Red Planet, due to the iron-rich dust that covers its surface and gives the planet a rusty color. This red color led the ancient Romans to name Mars after their god of war. The Red Planet is also 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). This makes it unlikely that liquid water — and thus life as we know it — could exist on the Martian surface, although scientists think that it once might have. Mars is currently the only known planet to be solely inhabited by robots, Quartz reported. Mars has two moons, Phobos and Deimos.
Jupiter is the biggest planet in the solar system. Unlike its four neighbors closer to the sun, Jupiter is a gas giant, mainly made up of helium and hydrogen. It is named after the king of the Roman gods, who is also known as Zeus in the Greek pantheon. Jupiter is twice as big as all of the other planets in the solar system combined, and yet it also has the shortest day of any planet, taking 10 hours to turn about its axis. 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 temperature have compressed the hydrogen gas into a liquid, creating the largest ocean in the solar system, according to NASA.
Saturn, the sixth planet from the sun, is the second-largest in the solar system. The planet is best known for its prominent rings. Like Jupiter, Saturn is also a gas giant composed of helium and hydrogen, and it is the least dense of the planets. The planet's rings are made up of billions of ice particles and rocks, and Saturn's biggest ring, Phoebe, spans an area almost 7,000 times the planet itself. The ringed planet also has 82 moons, ranging in size from that of a sports field to the size of planet Mercury. One of Saturn's moons, Enceladus, is covered in an icy ocean which, according to astronomers, makes the moon a promising candidate for extraterrestrial life.
Uranus was the first planet to be discovered using a telescope. It is also the only planet to be named after a Greek deity, Ouranos the sky god, rather than a Roman one. The seventh planet from the sun is an ice giant. It is composed of heavier elements than its gas giant neighbors, being a mixture of water, methane and ammonia ice. Also 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. Uranus also has 13 rings and 27 moons.
Scientists predicted Neptune existed before they observed it for the first time because of its effect on Uranus' orbit. NASA's Voyager 2 spacecraft is the only mission to have visited the ice giant. Neptune, named after the Roman sea god, is so far away from the sun that it takes 4 hours for sunlight to reach the planet. (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. About 80% of the Neptune's mass 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.
What happened to Pluto?
Our solar system has at least five dwarf planets: Ceres, Pluto, Eris, Haumea and Makemake. 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 neighborhood around its orbit.
Pluto, once the ninth planet in the solar system and named after the Roman god of the underworld, was demoted to dwarf planet status in 2006 because it failed on the third point of the definition for a planet: It had not cleared its neighborhood of space objects. Pluto sits in the vast Kuiper Belt, a region beyond Neptune that contains trillions of objects.
Some astronomers believe this reclassification of Pluto as a dwarf planet to be unfair, and that it should be reinstated as the ninth planet in our solar system.
There is a possible candidate to fill Pluto's vacancy as the ninth planet: Planet X, or Planet Nine. California Institute of Technology researchers Mike Brown and Konstantin Batygin published a paper in 2016 in the The Astronomical Journal, hypothesizing that a planet 10 times larger than Earth was orbiting the sun about 20 times farther away than Neptune does.
"All those people who are mad that Pluto is no longer a planet can be thrilled to know that there is a real planet out there still to be found," Brown said at the time. "Now, we can go and find this planet and make the solar system have nine planets once again."
However, according to NASA, Planet Nine's existence is theoretical at this point because the planet has not been directly observed.
Beyond the planets
Beyond the ice giant Neptune, the solar system extends to the Kuiper Belt and Oort Cloud. The Kuiper Belt, whose existence was only confirmed in 1992, is 30 to 55 AU wide, according to NASA.. The belt's most famous inhabitant is the dwarf planet Pluto, but it also contains trillions of frozen objects, many of which are remnants of our early solar system. The Oort Cloud is in the solar system's icy outer reaches, and although it has been hypothesized since the 1950s, it has never been observed.