Solar systemhow the planets and the Moon formed

The solar system formed inside a molecular cloud. 4.6 billion years ago, the shock wave of a nearby supernova triggered the collapse of the solar nebula, the Sun then ignited, planets grew layer by layer around it, and the Moon came from a giant impact. From the collapse of the solar nebula to the cooling of Earth's surface 4 billion years ago, when rain fell for millions of years to make the primordial oceans, there are eight key events.

This era runs from 9 billion to 9.7 billion years, and covers our own small corner of the universe.

Eight events

• 4.6 billion years ago · Solar nebula collapses: A nearby supernova's shock wave triggers the collapse.
• 4.57 billion years ago · Sun ignites: The core temperature reaches 15 million degrees.
• 4.56 billion years ago · Frost line: Beyond the frost line, icy material condenses.
• 4.55 billion years ago · Jupiter forms: The giant planet clears out its orbit.
• 4.54 billion years ago · Proto-Earth: A molten ball of rock.
• 4.5 billion years ago · Theia impact: Debris from the collision forms the Moon.
• 4.1 billion years ago · Late Heavy Bombardment: Comets and asteroids deliver water.
• 4 billion years ago · Primordial oceans: The surface cools and rain falls for millions of years.

Traces of the solar system's birth

The decay products of aluminum-26 in meteorites show that a star had died nearby just as the solar system was born. The paper's title asks "Fossil or fuel?" (Lee, Papanastassiou and Wasserburg, The Astrophysical Journal Letters, 1977). How old is Earth? Lead isotopes in meteorites give about 4.55 billion years. To remove lead contamination, the researcher later also helped drive the ban on leaded gasoline (Patterson, Geochimica et Cosmochimica Acta, 1956).

The Moon and the planets

Computer simulations show that a Mars-sized body hit Earth at an angle and the debris came together as the Moon (Canup and Asphaug, Nature, 2001). Why is Mars so small? Jupiter moved inward, was then pulled back by Saturn, and on the way ate up Mars's raw material (Walsh et al., Nature, 2011).

Where the water came from

A violent rearrangement of the giant planets' orbits flung many small bodies into the inner solar system and caused the Late Heavy Bombardment. This is the Nice model (Gomes, Levison, Tsiganis and Morbidelli, Nature, 2005). Liquid water may have appeared even earlier, though. A zircon grain the size of a grain of sand suggests Earth may have had liquid water only about 150 million years after it formed. The paper's title speaks of evidence for continental crust and oceans 4.4 billion years ago (Wilde, Valley, Peck and Graham, Nature, 2001).

Comparing this evidence with the timeline reveals an interesting difference. The timeline puts the primordial oceans at 4 billion years ago, while the zircon study pushes the appearance of liquid water back to 4.4 billion years ago. Likewise, the meteorite-based age of Earth, about 4.55 billion years, is not far from the timeline's proto-Earth at 4.54 billion years ago. The timeline gives a rough order that is easy to follow, and the exact years change as research is updated.

The solar system is not the only one

A giant planet was found around 51 Pegasi, circling its star in 4 days. The solar system was no longer the only one, and the discoverers won the 2019 Nobel Prize for it (Mayor and Queloz, Nature, 1995). The Kepler telescope stared at one patch of sky and looked for new worlds by the faint dimming when planets pass in front of their star, and went on to find thousands (Borucki et al., Science, 2010). Around one small red dwarf, seven Earth-sized planets are packed together, and several of them may have liquid water (Gillon et al., Nature, 2017).

Want to read the originals? Search these titles

• Lee et al. (1977): Aluminum-26 in the early solar system: Fossil or fuel?
• Patterson (1956): Age of meteorites and the earth
• Canup and Asphaug (2001): Origin of the Moon in a giant impact near the end of the Earth's formation
• Walsh et al. (2011): A low mass for Mars from Jupiter's early gas-driven migration
• Gomes et al. (2005): Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets
• Wilde et al. (2001): Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago
• Mayor and Queloz (1995): A Jupiter-mass companion to a solar-type star
• Borucki et al. (2010): Kepler planet-detection mission: introduction and first results
• Gillon et al. (2017): Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1

How the game shows it

In the game, the time range of the Solar system era is "9 billion – 9.7 billion years" and its resource is planetesimals. The generators are Molecular clouds, Protoplanetary disk, Planetesimal collisions and Protoplanets, roughly the order of "the cloud becomes a disk, small bodies in the disk collide and grow, and finally become planets".