How old is the Earth? It may seem like a simple question to answer. The typical rough estimate is that our planet is about 4.5 billion years old. But the closer planetary scientists get, the softer the story gets. Nuances about how our planet formed could change the age of Earth by half a billion years or so.
“It’s easy to talk about an age, but it gets more and more complex as you get closer,” says geology professor Thomas Lapen, who chairs the University of Houston’s department of Earth and atmospheric sciences. As scientists have tried to determine more precise measurements of Earth’s age, they have had to grapple with the details of how our planet came to be.
“When you are born, it is an instant in time,” explains Lapen. But planetary formation is a process that takes millions of years. To assign an age to Earth, astrophysicists, planetary scientists, and geologists have to determine what point in the process could be considered the birth of Earth.
When was the Earth “born”?
About 4.6 billion years ago, gas and dust swirled in orbit around the newly formed sun. During the first few million years of the solar system, particles collided and coalesced into asteroids and seed planets. Those space rocks kept crashing into each other, some getting bigger and bigger, shaping the solar system as we see it today.
But planets are not just big piles of rocks. As they accumulate material, these celestial bodies also differentiate into the layers of a core, mantle, and crust (at least in the case of Earth and the other terrestrial planets). Accumulation and differentiation take time, probably on the order of tens of millions of years. Some might consider a point in that stage of Earth’s formation the birth of our planet. But Lapen says that he thinks of it as the conception of the Earth, and the birth occurred later, when a catastrophic event also formed the moon.
[Related: June 29 was Earth’s shortest day since the invention of atomic clocks]
According to the widely accepted giant impact theory, during the chaos of our solar system’s early days, proto-Earth collided with another small Mars-sized body. When the two collided, the debris merged on the Earth-orbiting moon.
This impact is also thought to have essentially “reset” the materials that made up the planet, Lapen says. At that time, a thick magma ocean may have covered proto-Earth. Following the powerful collision, material from both bodies mixed and fused into the system of planets and moons we know today. Evidence for such a “reset” comes from terrestrial and lunar rocks that contain identical forms of oxygen, Lapen explains.
“Proto-Earth was, in all probability, either destroyed or changed in composition,” says Lapen. “In my opinion, the Earth was not the Earth as we know it until the moon formation event.”
If this event marked the birth of our planet, the Earth would be between 4.4 and 4.52 billion years old. But determining a more specific age for our planet requires examining ancient evidence.
Assigning a number to the age of our planet
Like detectives looking for clues to an ancient crime, planetary scientists have to look at the evidence that remains today when reconstructing our planet’s early history. But with all the turmoil during that chapter, the turbulent magma ocean, and intense geological rotation, proof can be hard to find.
One way to constrain Earth’s age is to look for the planet’s oldest rocks, Lapen explains, which formed just after the magma ocean hardened into a solid surface. For that date, scientists are looking for zircons discovered in the Jack Hills in Western Australia, the oldest known minerals.
To determine the age of these crystals, a team of scientists used a technique called radiometric dating, which measures the uranium they contain. Because this radioactive element decays into lead at a known rate, scientists can calculate the age of a mineral based on the ratio of uranium to lead in the sample. This method revealed that the zircons are approximately 4.4 billion years old.
These rocks suggest that the Earth-Moon system must have formed at some point before 4.4 billion years ago, because the rock record “would be erased by the moon formation event,” says Lapen. So the planet is no less than 4.4 billion years old. But how much older could it be? To answer that, Lapen says, scientists turn to other places, including the moon.
[Related: Here’s how life on Earth might have formed out of thin air and water]
The rocks on the body of Earth’s satellite are better preserved than those here, because the moon does not undergo processes such as plate tectonics that would melt and reorganize its surface. There are two main sources for these clues: in lunar meteorites that fall to Earth and in samples collected directly from the moon during NASA’s Apollo program.
Like proto-Earth, the young moon was also covered in an ocean of magma. The oldest rocks mined from the lunar surface may indicate when the lunar crust formed. The scientists performed radiometric dating on zircon fragments collected during the Apollo 14 mission, corrected the calculations for cosmic ray exposure, and determined that the lunar crust hardened approximately 4.51 billion years ago.
There would have been a period of time between the collision and the merging, the cooling and the differentiation of the bodies, says Lapen, so this date also has a window of uncertainty of about 50 million years.
“Coming out with the exact event is very challenging,” he says. Lapen estimates that the Earth-Moon system probably formed between 4.51 billion and 4.52 billion years ago, but some scientists say the calculations could be up to 50 million years apart.
Another way to narrow down that time window is to look at the rocks that existed when proto-Earth was forming. When the planets solidified from the debris around the young sun, not all of the material coalesced into the worlds and their moons that we see today. Some were preserved on asteroids or comets.
Sometimes those solar system time capsules reach us as meteorites falling to the surface of our planet. The oldest known space rock, Lapen says, is the Erg Chech 002 meteorite. It is believed to be a fragment of an igneous crust from a primitive protoplanet in the early Solar System. As such, the dating of the Erg Chech 002 meteorite provides a snapshot of a time when proto-Earth was likely at a similar stage in its conception.
“If the ‘birth of Earth’ is defined as the time of formation of the first proto-Earth or protoplanet core that eventually grew through accretion to form present-day Earth,” says Lapen, “then perhaps that was so long ago.” time as the age of [Erg Chech 002].” The scientists calculated that this piece of igneous crust crystallized approximately 4.565 million years ago.
Can the age of the Earth be refined?
On human time scales, an uncertainty of 50 million years when the Earth-Moon system formed sounds vast and imprecise. But on planetary time scales, particularly billions of years ago, “it’s a good estimate,” says Lapen.
“The further back we look, often the less accurate things are due to gaps in the record. It is a relatively short period of time, in which a lot of things happened: there was an impact, everything had to merge, cool and differentiate into resistant rocky bodies that have a core, a mantle and a crust”, he says.
However, scientists are not done. There is always an opportunity to get more precise and accurate measurements of Earth’s age, Lapen says, particularly when researchers get additional samples from the moon, meteorites, and asteroids.
The rocks acquired by China’s Chang’e 5 mission to the moon are still being studied, for example. NASA’s Artemis program also plans to collect lunar rocks. And NASA’s conservation team is also making more materials available for study from the Apollo program, which offers researchers the opportunity to study ancient lunar rocks with new technologies, Lapen adds.
If these new efforts reveal lunar samples of parts of the moon’s surface that crystallized earlier than previously studied, he says, that could further limit time estimates for when Earth and the moon were born.