More than 1.5 billion people use the internet every day, and they search for all kinds of stuff.   And good for them! We’re all about curiosity here at SciShow. That’s why we’ve worked with Google and YouTube to answer ten of the most popular questions searched on the internet.   This is The World’s Most Asked Questions.    And today’s question is: How old is Earth?   Our best estimate is that the planet is 4.54 billion years old, give or take 50 million years.   It’s taken scientists hundreds of years to narrow it down to that range.   So the better question really is, “HOW do we know how old Earth is?”   Well, for a long time, scientists just looked for the oldest parts of the planet’s surface and then determined the age of the rocks there.   The thing is, Earth is always churning around the plates that make up its surface, forming new crust and recycling older crust into the planet’s interior.    So, most rocks on the surface today are an average of 100 million years old. But geologists have found older rocks, mostly those that were once in the deepest layers of the crust, but then got thrust upward before they could be recycled.    The oldest of these are the Acasta Gneisses in northwestern Canada. By measuring the amounts of radioactive isotopes of elements in the rock, scientists were estimate their age at 4.04 billion years old.   But to find the oldest material of ANY kind on Earth, you have to go to Western Australia, where geologists have discovered crystals of zircon in sedimentary rocks. Zircon contains uranium, which decays at a fixed, predictable rate, so geologists were able to use that to determine that the crystals are 4.3 billion years old.   So, evidence like this tells us that Earth must be at least four billion years old. But we’ve found even more clues to Earth’s age … in space.   The moon isn’t geologically active like Earth is, so material from its surface can be dated pretty reliably. And the moon rocks brought back from the Apollo missions have been found to be 4.4 to 4.5 billion years old.   Since the moon formed about the same time as Earth, that helps narrow down Earth’s age even further.   Then there are meteorites which come from lots of different sources in space, but many are leftover debris from the formation of the solar system.   And sure enough, more than 70 meteorites have been dated to the same exact range as the moon rocks, 4.4 to 4.5 billion years ago.   So you put all this evidence together, and, our little Earth is pretty old.    So thanks for googling that, and let us know in the comments what the oldest thing you’ve ever interacted with is... besides the Earth. According to our unscientific research via our SciShow survey, viewers who follow us on Facebook are more likely than others to own something that’s over 300 years old. And those of you who own older things are less likely to get frequent hiccups.   Let us know what other questions you’d like us to answer by using the hashtag #WMAQ, for World's Most Asked Questions, and we’ll compile the most asked questions into a video at the end of the month.   Thank you for watching this episode of SciShow. If you have any more planetary questions, don’t forget to check us out at youtube.com/scishowspace.