Has Earth grown larger from the buildup of decaying vegetation through the ages?
— Robert in Spartanburg, S.C.
Earth isn’t getting bigger. It’s actually getting smaller!
Decaying vegetation does pile up across the planet, but not everywhere equally. Wind and rain erode the ground over time, and even where leaves and other vegetation do gradually accumulate, like peat bogs and river deltas, that material doesn’t add to Earth’s bulk.
Trees are built from air — well, air and water. The water comes from the rain and the ground, but most of the rest of a tree is carbon and oxygen that was extracted, by photosynthesis, from the carbon dioxide in the air. And that carbon dioxide came from somewhere else on Earth. None of these processes actually makes the Earth bigger or smaller — no mass is being created or destroyed. Atoms are just getting moved from one place to another.
But Earth’s size isn’t quite constant. Space around Earth is dusty; it’s full of asteroid debris, comet trails and ionized particles streaming away from the sun. And as our planet flies through that dust, our gravity vacuums it up.
The dust enters the atmosphere, drifts around and eventually settles on the surface. This steady flow of dust — along with occasionally larger chunks in the form of meteorites — adds about 43 tons of mass to Earth every year. It’s possible that a few molecules of the dust on your dresser recently arrived from another planet.
But that 43 tons a year is small potatoes compared to Earth’s mass, which is about 5,972,200,000,000,000,000,000 tons.
Moreover, and in spite of the added space dust, the planet is actually losing mass over all, because our atmosphere leaks. Gravity does a decent job of keeping Earth’s air wrapped around us, but a faint stream of lightweight gasses — mostly hydrogen, but also helium and oxygen — is continually escaping from the fringes of our atmosphere. These streams are particularly dense near the poles, where gas ionized by the sun flows out along the magnetic field lines in the form of the polar wind.
Thanks to our leaky atmosphere, Earth loses several hundred tons of mass to space every day, significantly more than what we’re gaining from dust. So, overall, Earth is getting smaller.
Don’t worry: At the current rate, it would take quadrillions of years for Earth to evaporate completely, millions of times longer than the expected lifetime of the sun. But if the air leak bothers you, you could always try to convince NASA to build a giant lid.
Randall Munroe is the author of the web comic XKCD and, most recently, “How To: Absurd Scientific Advice for Common Real World Problems.”