Earth's water is actually a natural byproduct of the planet's fast formation

The research team was led by Prof. Martin Bizzarro from the Globe Institute at the University of Copenhagen. The team comprised Prof. Martin Schiller and Ph.D. student Isaac Onyett from the Globe Institute at the University of Copenhagen. 

This study changes our fundamental understanding of the Earth's formation and improves the possibility of finding other habitable planets in the universe.

Debunking the chance event theory

Silicon is commonly present in different types of meteorites. The team used silicon isotopes to understand the timescales and building blocks of terrestrial planets like Earth and Mars. 

They analyzed over 60 meteorites and planetary bodies to establish a genetic relationship between the planets and celestial bodies like asteroids and comets. 

The team found that certain meteorites, called chondrites, the oldest and most primitive rocks in the solar system, had an excessive amount of a silicon isotope called μ³⁰Si. In comparison, planets in the inner solar system, like Mars and Earth, had a deficit of this silicon isotope. 

This finding suggests that chance collisions between the Earth and water-rich asteroids and comets did not affect the presence of water on Earth.

A new understanding of planet formation

Based on their observations on the silicon isotope, the team proposed a new theory to explain planet formation. 

According to their theory, Earth was formed rapidly by amassing small particles of dust in the accretion disk around the newly formed Sun. The accretion disk is a large rotating disk of gas, dust, and icy particles. 

Once the Earth was large enough, it acted like a vacuum cleaner, with its gravitational force rapidly pulling up all the dust, gas, and icy particles. "And that makes it grow to the size of Earth in just a few million years," explained Onyett in a press release.

Since the Earth vacuumed up icy particles along with other dust and gas, the water present on Earth today is a result of the planet's formation and not a chance event.

By studying the nucleosynthetic composition of silicon and the correlation with the ages of celestial bodies, the team concluded that chondritic bodies (previously thought to be planetary building blocks) were not responsible for the formation of terrestrial planets in the solar system. Instead, the early-formed differentiated asteroids played a significant role in planet formation.

These findings are crucial for unraveling the mechanisms and timescales involved in planet formation and also help our search for habitable planets. 

"This theory would predict that whenever you form a planet like Earth, you will have water on it. If you go to another planetary system where there is a planet orbiting a star the size of the Sun, then the planet should have water if it is in the right distance," said Bizzarro in the same press release.

The findings of the study are published in the journal Nature.

Study abstract:

Understanding the nature and origin of the precursor material to terrestrial planets is key to deciphering the mechanisms and timescales of planet formation. Nucleosynthetic variability among rocky Solar System bodies can trace the composition of planetary building blocks. Here we report the nucleosynthetic composition of silicon (μ³⁰Si), the most abundant refractory planet-building element, in primitive and differentiated meteorites to identify terrestrial planet precursors. Inner Solar System differentiated bodies, including Mars, record μ³⁰Si deficits of −11.0 ± 3.2 parts per million to −5.8 ± 3.0 parts per million whereas non-carbonaceous and carbonaceous chondrites show μ³⁰Si excesses from 7.4 ± 4.3 parts per million to 32.8 ± 2.0 parts per million relative to Earth. This establishes that chondritic bodies are not planetary building blocks. Rather, material akin to early-formed differentiated asteroids must represent a major planetary constituent. The μ³⁰Si values of asteroidal bodies correlate with their accretion ages, reflecting progressive admixing of a μ³⁰Si-rich outer Solar System material to an initially μ³⁰Si-poor inner disk. Mars’ formation before chondrite parent bodies is necessary to avoid incorporation of μ³⁰Si-rich material. In contrast, Earth’s μ³⁰Si composition necessitates admixing of 26 ± 9 per cent of μ³⁰Si-rich outer Solar System material to its precursors. The μ³⁰Si compositions of Mars and proto-Earth are consistent with their rapid formation by collisional growth and pebble accretion less than three million years after Solar System formation. Finally, Earth’s nucleosynthetic composition for s-process sensitive (molybdenum and zirconium) and siderophile (nickel) tracers are consistent with pebble accretion when volatility-driven processes during accretion and the Moon-forming impact are carefully evaluated.