Could Gliesians be Watching Baywatch?

[Note: Click here for a more thorough treament of the viability of SETI and the (high) likelihood of extraterrestrial intelligence]

Gliese 581g is an earthlike planet orbiting the star Gliese 581, 20.3 light years away from us.  Discovered just last week by astronomer Steven Vogt, he announced that the odds of life on this exosolar planet are 100%.  That’s a pretty bold statement, even for a planet thought to be in a habitable zone of a star a little smaller and cooler than our sun.  Most astronomers are attributing his statement to being a little overexcited with his discovery.  But it got me wondering – if there were technologically advanced lifeforms on this planet, is it possible that they would be able to receive our radio or TV transmissions?  And, remember that when Gliese 581g might be receiving from us today is what we broadcast 20.3 years ago, such as episodes of Baywatch.  Alternatively, can we hear them, as SETI has been attempting to do for the past 30 or so years?

As it turns out, there isn’t much to worry about, unless we decide to send a very high-powered narrow directional message to a planet that just happens to be at the perfect level of technology which also just happens to have outpaced their social evolution dramatically.  Not likely there either, for reasons that I will discuss in an upcoming post (sorry, Stephen Hawking).

Here’s the deal.  Let’s take a typical TV broadcasting station operating 50000 watts on Channel 2.  Because radio waves attenuate proportional to the square of the distance from the transmitter, this signal will be pretty miniscule by the time it gets to the edge of our solar system.  In fact, by the time this 6 MHz wide signal at 60 MHz gets about 1.4E+11 km away, its power density will be at the same level as the corresponding power density of the cosmic microwave background (CMB) radiation in that frequency band (feel free to check my math – it’s a little rusty).  In addition, the signal will be an indecipherable mess because it will be intermixed with all of the other TV stations broadcasting on Channel 2.  So how far out exactly is 1.4E+11 km?  Turns out this is past Pluto, but barely the beginning of the Oort cloud in our own Solar system, or .015 light years.  That is .00075 of the distance to Gliese 581, across which distance the signal will be attenuated by a further factor of 1.8 million and CMB background noise will completely swamp out the signal.

So, zero chance of Gliesians kicking back and enjoying an interstellar episode of Baywatch.  And not much chance of us hearing accidental radio waves generated from their planet, even assuming they followed a similar technological evolution at the exact same time as us.  Sorry, SETI.