for National Geographic News
The discovery in the mid-1990s of the first planets outside our solar system gave new life to the simmering debate over whether humans are the only intelligent beings in the universe.
By now dozens of these so-called exoplanets have been washed by our radio and television broadcasts.
But is there anyone on the exoplanets to tune in to those shows?
Probably not on the exoplanets we can detect with current instruments. These gas giants have crushingly high gravities and deadly atmospheres.
Still, theories abound that life-supporting "other Earths"as yet undetectable by astronomersmight exist near such gas giants.
So if there are aliens on other Earths, what are they listening to, and why haven't they replied in kind?
Billions and Billions
One thing that the experts agree on is that the aliens can't be listening to anything much older than Orson Welles's famous 1938 radio broadcast "The War of the Worlds."
(Related news: "'War of the Worlds': Behind the 1938 Radio Show Panic" [June 2005].)
"The first radio experiments go back a hundred years," said Seth Shostak, senior astronomer at the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, California.
"But they were low power and low frequencies that don't make it into space. You don't get signals that make a serious job of getting out there until the 1930s."
That means that only 40 or 50 of the nearly 200 known exoplanets are close enough to have heard us.
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