In the Beginning There was Proto-Earth and Theia

4.6 billion years ago, gravitational forces pulled enough star dust and rock together to form the planet Proto-Earth, and it began to orbit the sun, but another planet–Theia–shared the same orbit for 20-30 million years. According to the Giant Impact Hypothesis, the shared orbit eventually led to a collision between these 2 planets. Ejecta from the impact circled the Earth’s orbit for millions of years in a ring not unlike the ring that currently circles Saturn. Gravity consolidated the ring into the moon which had a much closer orbit then than it does today.

20-30 million years after Proto-Earth formed it was struck by another planet that occupied the same orbit.
The impact of the collision between Proto-Earth and Theia caused ejecta to orbit earth in a ring. This ring eventually consolidated into the moon.

Convincing evidence supports the Giant Impact Hypothesis. Earth’s spin and the moon’s orbit have similar orientations. Earth’s high momentum of rotation suggests the occurrence of an ancient impact. Analysis of moon dust indicates it was once molten–the result of a fiery impact that melted the ejecta that eventually accreted to form the moon. (Scientists refer to the consolidation of material into a planet as accretion.) The moon has a small core, suggesting most of Theia’s core fused with Proto-Earth’s core. The moon has small quantities of volatile elements, indicating most vaporized upon impact. Isotope ratios of zinc and oxygen on the moon are identical with those on Earth.

This is what the early Earth looked like after the moon consolidated. Widespread tectonic activity combined hydrogen and oxygen in rocks into water vapor released by volcanoes.

Earth is the most unusual planet in the solar system, and from what scientists can determine is unlike any known planet from other solar systems based on the limited data they can glean from such distant bodies. Water is abundant on Earth, while most planets are absolute deserts. Moreover, Earth is surrounded by an oxygen-rich atmosphere. Scientists wonder how earth accumulated water because they believe heat from the early sun was so great that most of the water on Proto-Earth boiled off. The prevailing theory posits meteorites and/or comets from the outer solar system delivered water to earth. However, a new study suggests water always existed in earth’s rocks. Scientists examined 13 inner orbit meteorites, known as enstatite chondrite meteorites, and determined they have the same isotopic ratios of various elements, including hydrogen, found in rocks on earth. Hydrogen and oxygen in earth rocks combined to form water that was released as vapor during volcanic activity. Proto-Earth likely formed from the accretion of enstatite chondrite meteors and asteroids within just a 5-million-year timespan. It’s mind-boggling to imagine how the consolidation of lifeless rocks led to our weird planet rife with so many different lifeforms.


Piani, L.; et. al.

“Earth’s Water may have been Inherited from Material Similar to Enstatite Chondrite Meterorites”

Science 369 (6907) August 2020

Schiller, M.; M. Bizzarro, and J. Siebert

“Iron Isotope Evidence for very Rapid Accretion and Differentiation of the Proto-Earth”

Science Advances 6 (7) 2020

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