The EconomistFebruary 22nd 2020 751I
n 1990, ayear into the journey to Jupiter
of an American spacecraft called Galileo,
Carl Sagan, a well-known astronomer,
turned the probe’s instruments back to-
wards Earth. He wanted to find out whether
it was possible to detect evidence of life on
the planet from a distance.
Galileo took spectrographic measure-
ments of sunlight streaming through
Earth’s atmosphere and found methane
and oxygen, both indicators of living pro-
cesses. The probe also took photographs of
Earth at different wavelengths and uncov-
ered something called the “red edge”—a
sharp change in the reflectance of the plan-
et at red wavelengths, which Sagan as-
cribed to the presence of photosynthetic
plant life on the surface.
There was, however, a third clue—and
an indicator that life not only exists on
Earth, but has also developed intelligence.
This came from the narrowband electro-
magnetic radiation that was streaming
from Earth’s surface—in other words tele-vision and radio channels leaking into
space. “That, as far as we know, is an un-
mistakable indicator of technology and an
unmistakable indicator of life,” says An-
drew Siemion, an astrophysicist at the
Berkeley setiResearch Centre in Califor-
nia. “And indeed, it is the most detectable
signature of life on this planet as viewed
from a distant vantage.” Sagan’s experi-
ment thus confirmed what those engaged
in the search for extraterrestrial intelli-
gence (which is what setistands for) had
suspected—that the best way to find aliens
is to look for unnatural radio signals.
That search is accelerating. As Dr Sie-
mion told this year’s meeting of the Ameri-
can Association for the Advancement ofScience (aaas), in Seattle, better tele-
scopes, faster computers, the discovery of
thousands of planets circling stars other
than the sun, and an influx of money and
scientific talent are transforming the field.
What was once a fringe activity has now be-
come mainstream. The next decade, he
reckons, will allow astronomers to im-
prove their search for signals from outer
space a thousandfold.
setibegan 60 years ago with Project
Ozma. This was led by Frank Drake, an as-
tronomer at Cornell University. It was an
attempt to listen for radio waves that might
be coming from intelligent life in the vicin-
ity of two nearby sun-like stars, Tau Ceti
and Epsilon Eridani. Further projects along
this line followed, but no little green men
were detected. By the turn of the 21st cen-
tury people were starting to lose interest
and financial support for the endeavour
was dwindling.
The tide turned, though, in 2009 with
the launch of Kepler, an American probe de-
signed to look for planets orbiting other
stars. It found them. In spades. Astrono-
mers have now confirmed the existence of
more than 4,000 such exoplanets. More-
over, their models suggest that virtually ev-
ery star has a planetary system. With so
many potential habitats, and more being
discovered every month, the prospects of
finding alien life suddenly brightened.
They will brighten further with theThe American Association for the Advancement of ScienceIs there anybody out there?
SEATTLE
This year’s meeting of the AAAS featured the rhizosphere, human emotions,
mapping body cells and the search for extraterrestrial intelligenceScience & technology
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77 Reading faces
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