Just how old is Mercury? Is it the oldest planet, or the youngest? Actually, you might be surprised to know that there is no oldest or youngest planet. Mercury is exactly the same age as all the rest of the planets in the Solar System: approximately 4.6 billion years old.
Astronomers think that our entire Solar System started out as a giant cloud of cold molecular hydrogen gas, left over from the Big Bang 13.7 billion years ago. Some event, like a nearby supernova explosion disturbed the cloud and caused it to begin collapsing. As it collapsed, various pieces broke off, each of which would eventually form a star like our own Sun.
Because of conservation of momentum from all the particles of hydrogen, the cloud began to spin as it collapsed, becoming a flattened disk. The newly forming Sun was at the center of this disk, with dust surrounding it. These dust particles collided together, forming larger and larger chunks of dust, rocks, boulders, asteroids, planetesimals, and eventually the planets we see today. All of the planets formed together at the same time.
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How do we know how old Mercury is? Astronomers have found meteorites on Earth, that fell from space at some point in the past. They use a technique called radiocarbon dating to determine how old the meteorites are. No matter where the meteorites come from; the Moon, Mars, or the asteroid belt, they’re always the same age: 4.6 billion years.
And so, how old is Mercury? The same as everything in the Solar System: 4.6 billion years old.
We have written many stories about Mercury here on Universe Today. Here’s an article about a the discovery that Mercury’s core is liquid. And how Mercury is actually less like the Moon than previously believed. How old is the Universe?
We have also recorded a whole episode of Astronomy Cast that’s just about planet Mercury. Listen to it here, Episode 49: Mercury.