While the infant Sun was still collecting material to start fusing hydrogen, tiny dust particles in the disk around it randomly collided and stuck to each other, growing in just a few years to objects hundreds of meters across. This process continued for several thousands of years, forming kilometer-sized objects big enough to gravitationally attract each other. This led to more collisions and accretions, forming Moon-sized protoplanets in less than a million years. In the inner, hotter part of the solar disk, the planets grew primarily from rocks and metals because it was too warm for water and other volatiles—substances that evaporate at room temperature—to condense. Up to hundreds of these worlds collided and combined in the inner solar system for about 100 million years until only four large bodies remained: Mercury, Venus, Earth, and Mars. The inner planets didn’t get as big as the outer planets because the percentage of rocks and metals available in the Universe—and thus our solar system’s starting materials—is lower compared to hydrogen, helium and volatiles like water ice. Just after this point we think a Mars-sized planet collided with Earth. The resulting debris coalesced to form the Moon. Mercury may have experienced a high-speed collision with another planet that stripped off Mercury’s outer layer, which would explain why the planet’s core makes up so much of its volume. The resulting debris may have spread out into space instead of forming a moon. In the outer, cooler part of the disk, gases and water ice were dominant. The Sun’s weaker gravitational influence in this region, combined with the presence of significantly more material, meant protoplanets there grew faster and became large enough to attract light elements like hydrogen and helium. Jupiter formed less than 3 million years after the birth of the solar system, making it the eldest planet. Saturn formed shortly after, amassing less material since Jupiter gobbled such a large portion of the outer disk. With little hydrogen and helium left, the next planets to form––Uranus and Neptune––accumulated more ices like water and ammonia. This is why we call them ice giants. Some simulations show that additional ice giants may have formed that were later kicked out of our solar system. Jupiter didn’t allow planets to form in the asteroid belt as its gravity pulled on dozens of Moon-and Mars-sized baby planets there, causing them to either collide and shatter with other bodies or leave the region. This process took a few ten million years after Jupiter’s formation, leaving the asteroid belt with only small bodies of rock, ice and metal that collectively weigh less than 1% of Earth’s mass. Ceres, the largest object in the asteroid belt, is considered an outlier because it has plenty of organics and water ice, which means it likely formed farther away and then migrated into the belt. Small Worlds Stick TogetherWhile the inner terrestrial planets were forming, baby planets beyond Neptune were colliding and sticking together to form planet-like worlds like Pluto and lumpy, icy bodies like Arrokoth. These objects formed what we now know as the Kuiper belt, though the belt was much denser than it is today. Just as Earth’s Moon formed after a collision between Earth and another world, similar smashups in the Kuiper belt created moons, some of which are relatively large. This may have been the case with Pluto and Charon. Jupiter’s huge mass attracted a dense disk of material that eventually coalesced into 4 planet-like moons: Io, Europa, Ganymede, and Callisto. Saturn’s moon Titan formed the same way. Some outer planet moons like Triton at Neptune may have been independent worlds captured by the giant planets’ gravity fields. That, as far as we know, was the end of the beginning. Planets and other small worlds didn’t grow any further as the young Sun’s strong solar wind blew most of the leftover dust and gas into interstellar space.
The solar system consists of the Sun and everything that orbits, or travels around, the Sun. This includes the eight planets and their moons, dwarf planets, and countless asteroids, comets, and other small, icy objects. However, even with all these things, most of the solar system is empty space. The solar system itself is only a small part of a huge system of stars and other objects called the Milky Way galaxy. The solar system orbits around the center of the galaxy about once every 225 million years. The Milky Way galaxy is just one of billions of galaxies that in turn make up the universe. At the center of the solar system is a star called the Sun. It is the largest object in the solar system. Its diameter, or distance through its center, is 865,000 miles (1,392,000 kilometers). In addition, the Sun contains more than 99 percent of all the material in the solar system. The Sun is a very hot ball of hydrogen and helium gases. It has a temperature, at its core, of more than 28,080,000° F (15,600,000° C). It constantly changes the hydrogen in its core into helium. This process gives out huge amounts of radiation, or energy. Living things on Earth depend on this energy, in the form of light and heat. The gases that surround the Sun shoot out a stream of tiny particles called the solar wind. It flows outward through the whole solar system. The solar wind is what causes auroras, or displays of colored light in the night sky in parts of Earth. In the Northern Hemisphere these auroras are called the northern lights. After the Sun, the largest objects in the solar system are the planets. In order from closest to the Sun, these planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Most of them orbit the Sun in paths shaped like circles. Most of the planets have at least one moon. However, they vary widely in size, temperature, and makeup. Scientists used to call Pluto the ninth planet. But in 2006 scientists decided that several objects in the solar system, including Pluto, should be called dwarf planets. Millions of small chunks of metal and rock called asteroids also orbit the Sun. Most asteroids are found in a ring between Mars and Jupiter. They are believed to be debris, or bits of material, left over from collisions between other bodies in the solar system. The largest asteroids are hundreds of miles in diameter, but most are much smaller. Small asteroids regularly fall to Earth or burn up in the sky as glowing meteors. Comets are small chunks of dirt and ice. Billions of them orbit the Sun in very long paths shaped like ovals. When they are closest to the Sun, the Sun’s radiation causes them to glow. Most comets are too small or too distant ever to be seen from Earth. Comets come from two parts of the outer solar system: the Kuiper Belt and the Oort Cloud. Beyond Neptune lies the Kuiper Belt, a flat ring of millions of small, icy objects. These objects orbit the Sun at a very great distance. They are mostly 30 to 50 times farther from the Sun than Earth is. At the outer reaches of the solar system is the Oort Cloud. It is a huge cloud of countless small, icy objects. The Oort Cloud surrounds the rest of the solar system. The solar system was formed about 4.7 billion years ago. It probably started as a loose cloud of gas and dust. Scientists think that a force called gravity pulled parts of the cloud together into clumps. The largest clump was squeezed together so tightly that it got very hot. This clump eventually became the Sun. Over millions of years the other clumps became the planets. The Sun’s strong gravity eventually pulled the planets into their orbits. Over time some of the leftover clumps became asteroids, comets, and other small, icy objects. In 1957, the Soviet satellite Sputnik 1 became the first human-made object to orbit Earth. Since then, scientists have sent many spacecraft to explore various parts of the solar system. Spacecraft have carried astronauts into orbit around Earth, to the moon, and to human-made space stations. Other spacecraft, called probes, have carried cameras and scientific equipment but no astronauts. Space probes have landed on the planets Mars and Venus, on asteroids, and on Titan, which is one of Saturn’s moons. In addition, space probes have flown past all the planets in the solar system. They have taken many photographs and collected much valuable information. The solar system is also known as a planetary system. Since the 1990s scientists have found many planetary systems beyond our solar system. In these systems, one or more planets orbit a star—just as the eight planets in our solar system orbit the Sun. These planets are called extrasolar planets. Finding other planetary systems is not easy, however, because extrasolar planets appear much dimmer than the stars they orbit. As space probes travel farther away from Earth, they are likely to discover more extrasolar planets. |
When was the solar system formed
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