Sky & Telescope - August 2022 - 35

POTENTIALLY HABITABLE PLANETS: PLANETARY HABITABILITY LABORATORY / UNIVERSITY OF
PUERTO RICO, ARECIBO; WATER TEXTURE: POLYGRAPHUS / SHUTTERSTOCK.COM
(University of California, Berkeley), director of the Berkeley
SETI Research Center. Simple life appeared within Earth's
first billion years, but uncovering definitive evidence of life
from that time period has been difficult, even though we
live here. Finding biosignatures on distant exoplanets will be
even harder, he says.
We already know of one potentially habitable planet
within 10 light-years and several more within 20. Even with
the coming generation of 30-meter telescopes on the ground
and 6-meter ones in space, we only expect to find a couple
dozen potentially habitable planets close enough to detect
biosignatures. In contrast, we could see a laser beam shot
directly at us from a distance of up to thousands of lightyears,
and we might be able to register a powerful radar beam
aimed at Earth from across the galaxy.
Thanks to recent exoplanet discoveries as well as fresh
sources of funding, astronomers are doubling down on SETI.
Thirteen white papers submitted to the astronomy decadal
survey featured SETI work, says Siemion, a sharp rise from
just two a decade ago. There are a growing number who wonder
if gathering vast amounts of data from the galaxy could
yield an unexpected signal so unnatural that only intelligent
beings could have produced it.
A Long-term Quest
The term technosignature is new, but the basic idea remains
what it has been for decades: a signal that, to our knowledge,
nature cannot generate. Generally, such signals are
either narrow in frequency or short in time. A laser beam,
for example, would create a single-color signal spanning an
unnaturally narrow range of wavelengths. A bright signal
that lasts for only a nanosecond would also arouse suspicion.
The shortest natural pulses we know come from pulsars
and last 100,000 times longer: An outburst lasts at least as
long as it takes light to travel across the source, which is 0.1
millisecond for a 30-kilometer object. Light can travel only
30 centimeters in a nanosecond, and no natural object that
small could produce enough energy to be seen light-years
away. Such a short signal might come from intelligent life.
Frank Drake began the first SETI experiment in the spring
of 1960, when he turned the new 85-foot radio telescope at
the National Radio Astronomy Observatory's site in West
Virginia toward two nearby, Sun-like stars. Since then, SETI
researchers have picked up some promising signals but no
convincing evidence of extraterrestrials. Perhaps the most
famous is the " Wow! " signal, a bright, narrowband radio
burst that lasted 72 seconds, recorded in 1977 by the Big Ear
radio telescope at Ohio State University. Its origin remains a
mystery, and it has never repeated (S&T: May 2021, p. 84).
Over the years, the SETI quest has expanded to explore
two regions of the electromagnetic spectrum in which cosmic
noise is low and atmospheric transmission is high: radio
frequencies from 1 to 10 gigahertz and visible/near-infrared
wavelengths from 400 to 2300 nanometers. The biggest
changes over the past decade have been in funding and
sk yand tele scope .o r g * AUGUST 2022 35
POTENTIALLY HABITABLE An artist imagines the appearance of
known Earth-size planets in their star's habitable zone, where water
could conceivably remain liquid on a rocky surface. For comparison,
on this scale Proxima Cen b is about the size of Earth. The planets
are ordered by their distance from Earth, given in light-years (ly).
Proxima Cen b
4.2 ly
Ross 128 b
11 ly
GJ 1061 c
12 ly
GJ 1061 d
12 ly
Teegarden's Star b
12 ly
Teegarden's Star c
12 ly
GJ 273 b
19 ly
GJ 667 C e
24 ly
GJ 667 C f
24 ly
TRAPPIST-1 d
41 ly
TRAPPIST-1 e
41 ly
TRAPPIST-1 f
41 ly
TRAPPIST-1 g
41 ly
TOI-700 d
102 ly
K2-72 e
217 ly
Kepler-1649 c
301 ly
Kepler-296 e
545 ly
Kepler-186 f
579 ly
Kepler-1229 b
866 ly
Kepler-62 f
981 ly
Kepler-442 b
1,194 ly
http://www.SHUTTERSTOCK.COM http://www.skyandtelescope.com

Sky & Telescope - August 2022

Table of Contents for the Digital Edition of Sky & Telescope - August 2022

Contents
Sky & Telescope - August 2022 - Cover1
Sky & Telescope - August 2022 - Cover2
Sky & Telescope - August 2022 - 1
Sky & Telescope - August 2022 - Contents
Sky & Telescope - August 2022 - 3
Sky & Telescope - August 2022 - 4
Sky & Telescope - August 2022 - 5
Sky & Telescope - August 2022 - 6
Sky & Telescope - August 2022 - 7
Sky & Telescope - August 2022 - 8
Sky & Telescope - August 2022 - 9
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Sky & Telescope - August 2022 - 11
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Sky & Telescope - August 2022 - Cover3
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