Planet Formation

On a clear winter night above Long Island, look south toward the constellation Orion. Just below the three stars of Orion's Belt, a soft fuzzy patch glows in the sword of the hunter. That patch is the Orion Nebula, also called M42. Even though it looks tiny from your backyard in Long Beach, it is one of the largest, closest, and most active stellar nurseries in our part of the Milky Way Galaxy. It sits about 1,344 light-years from Earth and is roughly 24 light-years across.

The Orion Nebula is a giant cloud of hydrogen gas, helium, and tiny grains of dust. Astronomers call this kind of cloud a nebula. Inside it, gravity is squeezing parts of the cloud together right now. When a clump of gas gets dense enough, it heats up and a new star ignites. The brightest young stars in the heart of the nebula — known as the Trapezium Cluster — are less than one million years old. By comparison, our Sun is about 4.6 billion years old.

The most exciting discovery in Orion did not come from looking at the stars themselves, but at what surrounds them. Using the Hubble Space Telescope, scientists imaged more than 150 small dark or glowing disks orbiting these young stars. These disks are called proplyds, short for protoplanetary disks. They are flat, spinning pancakes of gas and dust, and they are the exact place where new planets are forming. Proplyds give us a direct view of something we cannot see in our own solar system anymore: planet-making in action.

Why does this matter for Earth? Because our own Sun and planets were born from a cloud just like Orion's. About 4.6 billion years ago, a cold cloud of dust and gas — the solar nebula — began to collapse under its own gravity. As it collapsed it spun faster (the same reason a skater spins faster when she pulls her arms in), and it flattened into a disk. The center grew hot and dense enough to become our Sun. The leftover material around the Sun became Earth and the other planets.

Inside the disk, tiny dust grains stuck together. Larger clumps swept up smaller ones, growing into planetesimals a few kilometers wide. Some planetesimals collided and grew into protoplanets. Close to the hot young Sun, only metals and rocky silicates could stay solid. This is why Mercury, Venus, Earth, and Mars are small and rocky. Beyond a boundary called the frost line — between Mars and Jupiter — the disk was cold enough for water, methane, and ammonia to freeze. The protoplanets out there grew huge and pulled in light gases like hydrogen and helium, becoming the gas giants Jupiter, Saturn, Uranus, and Neptune. Finally, a strong solar wind from the young Sun blew away the leftover gas, ending planet construction.

So when you look at Orion this winter, you are looking at a younger version of your own home. Every atom in your body, the Atlantic Ocean, and the bedrock under Long Island was once spinning in a disk just like the proplyds being photographed there right now.

Our solar system has eight planets, and they fall neatly into two very different families. The split is not random — it traces back to where each planet was born inside the solar nebula and which side of the frost line it formed on.

Terrestrial planets (Mercury, Venus, Earth, and Mars) are the four inner planets. The word terrestrial means "Earth-like." They are small, with diameters between about 4,879 km and 12,756 km. They are made of rock and metal — silicates, iron, and nickel — giving them high densities of roughly 4 to 5.5 g/cm³. They have thin or no atmospheres, few or no moons, and solid rocky surfaces you could (in theory) land a spacecraft on. They formed close to the young Sun, where temperatures were so high that only metals and rocky silicates could stay solid. Light gases and ices simply could not condense there — they evaporated or were blown outward by the Sun's strong solar wind.

Jovian planets (Jupiter, Saturn, Uranus, and Neptune), also called gas giants or "Jupiter-like" planets, are the four outer planets. They are enormous — Jupiter is about 11 times Earth's diameter. They are made mostly of hydrogen, helium, and ices (water, methane, ammonia), with low densities of about 0.7 to 1.6 g/cm³. Saturn's density is so low that it would actually float if you had a bathtub big enough. Jovian planets have thick, deep atmospheres with no solid surface, many moons (Jupiter has 95 known, Saturn 146), and ring systems — Saturn's are the most famous, but all four have at least faint rings.

The frost line (also called the snow line) is the invisible boundary that separates these two families. It sits roughly between the orbits of Mars and Jupiter, in the same region as today's asteroid belt. Inside the frost line, temperatures during planet formation were too warm for water, methane, and ammonia to freeze — only rocky and metallic materials could become solid grains. Outside the frost line, temperatures dropped below the freezing point of these "ices," so much more solid material was available. Protoplanets out there grew huge fast. Once big enough, their gravity began pulling in the most abundant gases in the nebula — hydrogen and helium — turning them into gas giants.

Beyond Neptune lies a cold region called the Kuiper Belt, home to Pluto and other dwarf planets. Even farther out sits Eris, with a stretched, highly eccentric orbit (e = 0.436 according to the ESRT) that takes it more than 96 AU from the Sun. These tiny icy bodies are leftover material from the original solar nebula — frozen time capsules from 4.6 billion years ago.