# Science Questions

## How many planets could have heard our radio signals?

Sun, 27th May 2007

### Question

We've been sending out radio waves for about 80 years, and given that they travel at the speed of light, the nearest star that could hear them would be about 80 light years away. How many planets fall within that range?

[We put this question to Maggie Turnbull as part of the Planets and Cosmology show]

Maggie Turnbull: You can imagine this sphere around the Earth, going out into space, that is filled with radio noise that we have emitted because of our technological activities. If you want to count the very first broadcasts, then the radius of that sphere around us is something like 80 or as much as 100 light years. I don't think in terms of light years, I think in terms of parsecs and 1 parsec is about 3 light years. If we say that this sphere is about 30 parsecs in radius, I know that there are about 3 to 5 thousand stars inside that sphere around us, and of those, about 2 thousand are sun-like stars. These are similar enough to our star that they could live long enough and have planets that could have life on them that are somewhat similar to our own.

This is a lot of stars, but you also have to consider that not every one of those stars is going to have planets and out of those that do, they are not all going to have Earth sized planets that are in the habitable zone.

So I would say that a reasonable number would be about 10% of those, and anywhere between 1 to as many as 5 hundred habitable planets are within that sphere of noise that we've created.

You have to keep in mind that most of the transmissions that are leaking out into space are doing just that, they are leaking, they are not beamed intentionally at any star system, so they are very weak by the time they get there. Also, all the signals that we leak out into space are not going equally in all directions, sometimes you have to be at a certain spot to be able to pick them up.

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