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Our best chance of discovering an extraterrestrial civilization is by listening out for any radio signals that they may send out. The Allen Telescope Array at Hat Creek Observatory, U.S.A., is doing just that. Since 2007, 42 radio dishes measuring 20 ft (6.1 m) in diameter have been directed toward nearby Sunlike stars. Eventually, 350 dishes will work together as one giant “ear.”

Signal search

Blasts from the past

Allen vis!tors

Some people think intelligent extraterrestrial life has visited Earth, although there is no evidence.

In 1947, Kenneth Arnold was flying his piane over the Cascade Mountains, U.S.A., when he suddenly saw nlne craft accompanying him. He said that they were “flying like a saucer would if you skipped it across water.” The term “flying saucer” has been use for alien spacecraft ever sińce.

UFO stands for “unidentified flying object"—something seen in the sky that cannot be identified. Some people think that UFOs are

visiting spacecraft, but they often turn out to be atmospheric phenomena, planets, aircraft, or weathr balie

I don t I believe it ■

The first an extraterrestrial might learn about life on Earth is from our television shows. We have been transmitting TV shows sińce 1932 and the signals from these shows have already traveled morę than 75 light-years from Earth, advertising our presence.

Exoplanets

^ M Our search for alien life started V lin the solar system, but it is now concentrated on planets orbiting other stars. These are called exoplanets, with the first one discovered in 1992.

We now know of morę than Uu430 exoplanets. The Kepler spacecraft is searching our region of the Milky Way for morę.

A elear indicator of life on a planet w O is oxygen in its atmosphere—this is what scientists look for on exoplanets.

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Stars and their exoplanets

51 Pegasi

The first Sunlike star found to have a planet.

2M1207

This star’s exoplanet was the first to be imaged directly.

55 Cancri

Five giant planets orbit this star in the Cancer constellation.

Gliese 581

One of its four planets is the closest in mass to Earth so far.

HD 209458

Water vapor was detected on the exoplanet of this star.

Quest!ons of lii

In 1961, astronomer Frank Drakę drew up a list of ąuestions to help calculate how many intelligent communicating cmlizations live in our Milky Way Galaxy:

01: How many stars does our galaxy contain?

02: What fraction of stars have planets?

03: How many wet, warm planets occur in each planetary system?

04: If the planet is wet and warm, what is the chance of life breaking out?

05: What is the chance of this life becoming intelligent?

06: What is the chance of this intelligent life being able to communicate with life elsewhere?

07: During what fraction of a planet’s existence will intelligent, communicating life exist?

In 1997, scientist Carl Sagan figured out that one million intelligent, communicating life forms exist in our galaxy, and the nearest is only 300 light-years away.

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