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The Question

(Submitted April 23, 1998)

Imagine you were an alien roaming around in Space and you were interested in finding a planet that has life. What is the evidence that Earth has human life? What are the unambiguous indications of life on Earth?

The Answer

There are a combination of features of the Earth that you can detect from space that would be very hard to explain without invoking the presence of life. These are: the abundance of molecular oxygen; the presence of chlorophyll, the pigment used by plants in combining carbon dioxide and water to form sugar; and a trace amount of methane (which is hard to maintain with all that oxygen around).

It so happens that the Galileo spacecraft (see, for example http://www.jpl.nasa.gov/galileo/ ) reached its ultimate destination, Jupiter, via a circuitous route (for technical reasons), including two flybys of Earth. NASA attempted, and succeeded, in detecting the abovementioned signs of life on Earth during one of them.

(Also if you are talking about intelligent life forms, radio and TV broadcast signals would be a conclusive evidence.)

Of course, there is no guarantee that every planet with living beings will look like the Earth (it's hard to be sure when you have only one example!). In fact, Europa, a moon of Jupiter, seems to have a liquid ocean underneath its icy crust, and some scientists speculate that there could be life on Europa. In this case, the ice on the surface makes it difficult to prove or disprove this idea from a distance. (The JPL Galileo pages above have a lot of information about Europa.)

Best wishes,

Koji Mukai
for Ask an Astrophysicist

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