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454 BCE
Planetary Accreation
Early on, our Solar System was a disk of dust and gas in orbit around the proto-Sun. The solid materials collided with each other and accreted to form gradually larger bodies, until the Solar System's four terrestrial planets (Mercury, Venus, Earth, and Mars) were formed. -
451 BCE
Formation of the Moon
Earth was born about 4.5 billion years ago, and scientists think the moon arose a short time later. The leading explanation for the moon's origin, known as the Giant Impact Hypothesis, was first proposed in the 1970s. It suggests the moon resulted from the collision of two protoplanets, or embryonic worlds. One of those was the just-forming Earth, and the other was a Mars-size object called Theia. The moon then coalesced from the debris. -
440 BCE
Planetary Cooling
Leftover heat remained from when Earth is formed and radioactive particles that inhabited Earth both warmed up Earth significantly. Therefore, it has not cooled as rapidly as some other planets. As planetary bodies cool slowly, heat diminishes. -
390 BCE
End of Heavy Bombardment
New dating of lunar rocks add to a growing body of evidence that the Moon and Earth were pelted by a flurry of large meteorites during a relatively brief geologic time span about 3.9 billion years ago. Known as the "Late Heavy Bombardment," or LHB, this period of heightened meteorite activity would have had important implications for life on Earth, since it coincides roughly with the time that scientists think the first primitive bacteria appeared on our planet. -
150 BCE
Core Formation
Earth's rocky core formed first, with heavy elements colliding and binding together. Dense material sank to the center, while the lighter material created the crust. The planet's magnetic field probably formed around this time. Gravity captured some of the gases that made up the planet's early atmosphere.
