Earth is the only known planet that supports life. It has liquid water, a breathable atmosphere, and the right conditions for plants, animals, and humans to exist.
But when our planet first formed, it was anything but friendly to life. In fact, new research suggests that without a lucky collision with another celestial body, Earth may never have become habitable at all.
When the solar system formed about 4.6 billion years ago, it started as a vast cloud of gas and dust surrounding the young sun.
This cloud contained volatile elements such as hydrogen, carbon, and sulfur—substances essential for life.
But near the sun, where Earth and the other rocky planets—Mercury, Venus, and Mars—formed, temperatures were too high for these elements to condense into solid matter.
As a result, the early building blocks of Earth, known as proto-Earth, contained very little of these life-friendly materials.
By contrast, celestial bodies that formed farther from the sun, in cooler regions, were able to trap water and other volatiles in their structures.
Scientists have long wondered how Earth, born as a dry rocky world, later acquired the ingredients for oceans, an atmosphere, and eventually life.
A new study led by researchers at the University of Bern sheds light on this mystery.
Using highly precise measurements of isotopes—different forms of chemical elements preserved in meteorites and ancient rocks—the team reconstructed Earth’s chemical history.
They found that Earth’s fundamental chemical makeup was established surprisingly early, no more than three million years after the solar system’s birth. Unfortunately, that early mix of elements would have made Earth a barren, water-poor planet.
The research team used the decay of a short-lived radioactive isotope called manganese-53, which gradually transformed into chromium-53 over time, to measure the age of Earth’s building materials with exceptional accuracy.
This “cosmic clock” revealed that the proto-Earth’s chemistry was fixed long before life could have arisen.
“The proto-Earth was initially dry and rocky,” explains Dr. Pascal Kruttasch, the study’s first author. “That means something had to happen later to deliver the volatile elements, such as water, that are essential for life.”
The leading candidate for this event is a dramatic collision with another planet-sized body known as Theia. Scientists believe Theia formed farther from the sun, where it could collect water and other volatiles.
At some point, it crashed into the proto-Earth. This catastrophic impact not only produced the material that eventually formed the moon, but also delivered the essential elements that made Earth habitable.
“The Earth does not owe its life-friendly conditions to a slow, steady process,” says co-author Professor Klaus Mezger. “Instead, it likely took a single, chance event—a collision with a water-rich planet—to set the stage for life.”
This discovery highlights how rare and fragile the conditions for life may be in the universe. If Earth’s history had unfolded even slightly differently, it might have remained a dry, barren world. The researchers hope future studies of the Theia collision will reveal more about how exactly this cosmic accident shaped both Earth and the moon, and why life was able to take hold here and not elsewhere.
In short, our planet’s ability to host life may not be a natural inevitability—it may be the result of a fortunate cosmic crash.
Source: University of Bern.
