The Solar System: A Guide to Things Orbiting Our Sun
The solar system is made up of Sun and everything around it, including planets, moons, asteroids, comets and meteorites. It extends from the sun, called Sol by the ancient Romans, and passes through the four inner planets, through the asteroid belt to the four gas giants and to the disc in the form Kuiper Belt and far beyond to the teardrop heliopause. Scientists estimate that the edge of the solar system is about 9 billion miles (15 billion kilometers) from the sun. Beyond the heliopause lies the giant, spherical Oort cloud, which is thought to surround the solar system.
How was the solar system discovered?
For millennia, astronomers have tracked bright spots that seemed to move among the stars. The ancient Greeks called them planets, which means “wanderers”. Mercury, Venus, March, Jupiter and Saturn were known in antiquity, and the invention of the telescope added the asteroid belt, Uranus, Neptune, Pluto and many moons of these worlds. The dawn of the space age saw dozens of probes launched to explore our system, an adventure that continues today.
There have been five man-made objects so far, Traveler 1, Traveler 2, New Horizons, Pioneer 10 and Pioneer 11, which crossed the threshold into interstellar space.
Related: How much of the solar system is made up of interstellar matter?
The discovery of Eris launched a series of new discoveries of dwarf planets, and eventually led the International Astronomical Union to revise the definition of a “planet”. Revision changed Plutofrom planet to dwarf planet status in 2006, a decision that remains controversial – especially after the New Horizons mission discovered immense geological variety over the world in 2015.
Astronomers are now on the hunt for another planet in our solar system, a true ninth planet, after proof of its existence was unveiled on January 20, 2016. The so-called “planet nine“, as scientists call it, is about 10 times the mass of Earth and orbits the sun as close as 200 astronomical units (AU). An AU is the distance from earth to sun — approximately 93 million miles (150 million km).
How did the solar system form?
Many scientists believe that our solar system formed from a giant, rotating cloud of gas and dust known as a solar nebula. When the nebula collapsed due to its gravity, it spun faster and flattened into a disk. Most of the materials were pulled towards the center to form the sun. Other particles inside the disk collided and stuck together to form asteroid-sized objects called planetesimals, some of which combined to become asteroids, comets, moons and planets .
the solar wind of the sun was so powerful that it swept away most of the lighter elements, such as hydrogen and helium, from the innermost planets, leaving behind mostly small, rocky worlds. The solar wind was much weaker in the outer regions, however, driving gas giants composed mostly of hydrogen and helium.
The sun is by far the largest object in our solar system, containing 99.8% of the mass of the solar system. It radiates most of the heat and light that makes life possible on Earth and perhaps elsewhere. The planets orbit the sun in oval-shaped paths called ellipses, with the sun being slightly off-center from each ellipse. NASA has a fleet of sun-watching spacecraft, like the Parker Solar Probe, to learn more about its composition and to make better predictions about solar activity and its effects on Earth.
indoor solar system
The four inner planets – Mercury, Venus, Earth and Mars – are made mostly of iron and rock. They are known as terrestrial or Earth-like planets because of their similar size and composition. Earth has one natural satellite – the moon – and Mars has two moons – Deimos and Phobos.
Between Mars and Jupiter is the asteroid belt. Asteroids are minor planets, and scientists estimate there are more than 750,000 with diameters greater than 1 km and millions of smaller asteroids. The dwarf planet Ceres, about 950 km in diameter, resides here. A number of asteroids have orbits that bring them closer to the solar system, which sometimes causes them to collide with Earth or the other inner planets.
Earth is surrounded by a flotilla of spacecraft, and Mars has also been visited by many spacecraft. Some of the most important Mars missions include the Rover of Perseverance, Overview lander, The Tianwen-1 rover, Rover Curiosity, the Opportunity and Spirit the rovers, the Mars reconnaissance orbiter (which takes high-resolution photos from orbit) and Viking landers and rovers.
Related: Why do the planets of the solar system orbit in the same plane?
Venus has been explored by American, European and Soviet spacecraft, such as Venera 13 over the decades. NASA announced in June 2021 that it would launch two missions to Venus by 2030 – an orbiter called VERITAS and an atmospheric probe called DAVINCI+.
Mercury has hosted several flybys and two long-duration missions: MESSENGER (now complete) and BepiColombo.
outer solar system
The outer planets – Jupiter, Saturn, Uranus and Neptune – are giant worlds with thick outer layers of gas. Between these planets they have dozens of moons with a variety of compositions, ranging from rocky to icy and even volcanic (as in the case of Jupiter Io.) Almost all of the mass of the planets is made up of hydrogen and helium, giving them compositions close to that of the sun. Beneath these outer layers they have no solid surfaces – the pressure of their thick atmospheres liquefies their interiors, although they may have rocky cores. Rings of dust, rock and ice surround all these giants, Saturn being the most famous.
Comets are often known as dirty snowballs and are mostly made up of ice and rock. When a comet’s orbit brings it closer to the sun, some of the ice in its central core turns into gas that erupts from the sunlit side of the comet, which the solar wind carries outward to form a long tail. Short-period comets that complete their orbit in less than 200 years are thought to come from the disc-shaped Kuiper belt, while long-period comets that take more than 200 years to return are believed to come from the spherical Oort cloud. .
Jupiter and Saturn have each been visited by multiple spacecraft and have also hosted long-range missions, including Juno and Galileo to Jupiter, and Cassini to Saturn. Uranus and Neptune, however, have only been seen in one spacecraft flyby – that of Voyager 2 in the 1980s. Some scientists are working on creating a Uranus or Neptune orbiter to fly there in the around 2030s. Scientists also make observations from the ground, to track long-term changes in weather patterns and cloud formations in the gas giants.
Astronomers have long suspected that a band of icy material known as the Kuiper Belt existed beyond Neptune’s orbit extending about 30 to 55 times the distance from Earth to the sun, and since the last decade of the 20th century until now, they have found more than a thousand of these objects. Scientists estimate that the Kuiper Belt is likely home to hundreds of thousands of icy bodies more than 100 km wide, as well as a trillion or more comets.
Pluto, now considered a dwarf planet, lives in the Kuiper Belt. It’s not the only one – recent additions include Makemake, Haumea and Eris. Another Kuiper Belt object called Quaoar is probably massive enough to be considered a dwarf planet, but it hasn’t been classified as such yet. Sedna, about three-quarters the size of Pluto, is the first dwarf planet discovered in the Oort Cloud. NASA’s New Horizons mission performed the first ever flyby of the Pluto system on July 14, 2015.
If Planet Nine exists, it orbits the sun at a distance 20 times farther than Neptune’s orbit. (Neptune’s orbit is 2.7 billion miles from the sun at its closest point.) The strange world’s orbit is about 600 times farther from the sun than Earth’s orbit is from the star. Scientists haven’t actually seen Planet Nine directly, and some astronomers debate its existence, which has been inferred by its gravitational effects on other Kuiper Belt objects.
Related: What if Planet Nine was a baby black hole?
Beyond the Kuiper Belt lies the very edge of the solar system, the heliosphere, a vast teardrop-shaped region of space containing electrically charged particles emitted by the sun. Many astronomers believe that the edge of the heliosphere, known as the heliopause, is about 9 billion miles (15 billion km) from the sun.
The Oort cloud lies well beyond the Kuiper belt and theoretically extends between 5 and 100,000 astronomical units (AU), the distance between the sun and the Earth (about 93,000,000 miles or 150 million kilometers), and is home to up to 2 trillion icy bodies.