Is the Earth getting heavier?

12 October 2008

Question

Is the Earth getting heavier? As the human population grows, are we putting on weight as a planet?

Answer

Chris - Yes but perhaps not for the reason that he was suggesting.

People often think that, as we increase our population, the weight of the people comes from nowhere. People just weigh more and the Earth therefore weighs more. That's not true. The Earth is an isolated system where all the of the mass that was on it to start with doesn't go anywhere. You don't make mass from nowhere, so people have got to gain weight by taking mass from elsewhere on the Earth: in other words, eating food that's come out of the Earth and into their bodies.

But, considering the planet as a whole, does that get heavier over time? The answer is yes, it can.

Every year, Earth gains about the weight of two aircraft carriers landing on it: two "HMS Ark Royals", or about 40,000 tonnes-worth of debris, which lands on Earth from space.

You can demonstrate this for yourself. If you put a big plastic sheet or a white sheet on your grass in the garden on a nice day, leave it for a few hours and then run a magnet over it. You can often find specks have just fallen down from outer space and landed on your magnet. Debris, dust and other stuff raining in from space contributes a huge amount of weight to the Earth every single year.

Comments

If the earth gets heavier, does that affect the gravitational pull of the sun? Making earth closer to the sun as the earth gets heavier?

Yes, if the planet gains mass then it will exert an increased gravitational attraction upon other bodies. However, as mentioned in the answer above, the amount of mass added to the Earth every year may sound like a lot - of the order of a some tens of thousands of tonnes. Relative to the mass of the Earth though - which is about 6 million million million million kilograms (6x1024) the added mass is absolutely trivial makes a near-negligible different to Earth's gravitational field. Thus its effects may be disregarded in the grand scheme of things.

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