Imagine Earth’s entire history compressed into a single day. In this cosmic calendar, as envisioned by Carl Sagan, humans wouldn’t arrive until the very last few seconds before midnight. Our species’ existence, spanning a few hundred thousand years, represents a mere blink in the vast expanse of our planet’s history. This begs the question: just how old is our planet, and how do we know its age?
Earth was born approximately 4.54 billion years ago, a mere 10 million years after the solar system itself emerged. This dramatic genesis began with a colossal cloud of gas collapsing under its own gravity, giving birth to the sun. Leftover fragments from this primordial cloud coalesced into planets. Mark Popinchalk, an astronomer at the American Museum of Natural History and New York University, offers a vivid analogy: “Think of early solar systems like a pizza. If the gas cloud that forms a star is a ball of dough, it might start as a blob but with some initial spin. The star forms from 99% of that ‘dough,’ but the rest retains that spin. Over time, it flattens out around the star, like a pizza. It’s from that remaining 1% of the ‘dough’ that all the planets are formed.”
However, the nascent Earth was a far cry from the lush, green world we know today. Immediately after its formation, it remained a molten sphere due to the relentless collisions that shaped it. Heavier elements, like iron, sank to the center, forming our planet’s core, while lighter elements bubbled to the surface. This process ultimately resulted in a layered Earth, comprised of a core, mantle, and crust.
Once the solar system settled down, and the constant bombardment of asteroids subsided, the oceans began to form, and almost instantaneously, life emerged. “While humans couldn’t exist for much of Earth’s history, cellular life has been continuously present for about 3.5 billion years,” Popinchalk explains. Recent research suggests this timeline may even stretch back as far as 4.2 billion years!
The key to deciphering this ancient history lies beneath our feet. Rocks, those seemingly inert fragments of our planet, hold the secrets to Earth’s age and its past conditions. Scientists employ a technique known as radiometric dating, which utilizes the varying amounts of radioactive elements within rocks to determine their age. However, Earth rocks can be deceptively complex. “Earth is an active, bustling place,” Popinchalk notes. “Volcanoes, weathering, and geological processes make it challenging to find rocks dating back to Earth’s formation.”
Fortunately, the moon, born from a cataclysmic collision with our planet in its infancy, lacks the pesky plate tectonics that constantly reshape Earth’s surface. Samples of moon rocks brought back by the Apollo missions have been instrumental in refining our estimate of Earth’s age. New samples from missions like Chang’e 5 are continuing to enhance our understanding of the moon’s history, shedding further light on our own planet’s past.
For nearby planets like Mars, we can dispatch rovers to collect rocks and analyze them to determine their age. But how do we unravel the ages of planets orbiting distant stars, beyond our reach? “The best way to learn about planets around other stars is actually to study the star itself,” Popinchalk reveals. “My expertise lies in estimating the age of a star by observing its spin rate. Young stars rotate rapidly, while older stars spin slowly. By measuring the spin rate of a star hosting a planet, we can estimate the star’s age and use a similar number for the planet.”
As we continue to discover and characterize new worlds beyond our solar system, we gain a deeper understanding of planet formation processes. This knowledge, in turn, illuminates the history of our own planet, allowing us to piece together the intricate tapestry of Earth’s remarkable journey.