WWW.ASTRONOMY.COM 33between 1995 and 2004, took observations
of approximately 1,000 nearby and mostly
Sun-like stars.
Because red dwarfs outnumber G-type
(Sun-like) stars by nearly 10 to one, if you
made a target list composed only of dwarfs,
the stars will, on average, be closer. Put
another way, if you look at 1,000 red dwarfs
instead of the same number of type G stars,
the dwarfs will be only half as far away, on
average. The signals from a hypothetical
E.T. civilization will be four times stronger
and easier to find.
In addition, red dwarfs’ low luminosity
has an upside for biology. While solar-type
stars have a main sequence lifetime of
roughly 10 billion years, the dwarfs last at
least 10 times longer. Given their low power
output, they are the eternal f lames of the
cosmos. In other words, every red dwarf
that has been born since the Big Bang is still
alive and kicking.
As Charbonneau puts it: “Red dwarfs
don’t show their age. ... They don’t age. A
planet around a Sun-like star is only habit-
able for about 6 billion years. Around a
bright type O or A star, you only have half a
billion years. But red dwarfs ... well, we
could find a 10-billion-year-old Earth.”
This longevity has important conse-
quences for brainy beings. For the first 3
billion years of life’s existence on Earth, all
organisms were microscopic. Intelligent,
technically sophisticated beings didn’t make
it onto the scene until 4.6 billion years into
the life of our home star. Given that changes
in the Sun’s brightness will make our planet
uninhabitable within only a billion years or
so, you could say that sentient life in the
solar system arrived just in time.
No such chancy circumstances apply to
red dwarfs, obviously. Their inhabitants
— if there are any — have plenty of time to
become smart and communicative. And at
least as important for SETI, to stay that way.
On average, a random red dwarf will be
several billion years older than a random
G-type star, and since intelligence seems to
require a while to evolve, older is better.
Scientists could expect a higher percentage
of dwarfs to house sentient beings.
Given these advantages, SETI research-
ers are considering more scrutiny of these
oft-neglected star systems. And how many
dwarfs do we need to examine before trip-
ping across an alien signal? Obviously, we
don’t know. But here’s a simple reckoning:16 percent of dwarfs have an Earth-sized
planet in their habitable zone. Suppose one
in 10 of these develops intelligent beings
after 5 billion years, and that sentience
remains present thereafter for one in 10
those. Then in a sample of 700 dwarfs,
there will be at least one that’s currently
home to intelligent beings.
That rough-and-ready estimate may be
unduly optimistic, but the numbers are still
compelling. Examining dwarfs for brainy
beings is, well, a no-brainer. Indeed, accord-
ing to Charbonneau, “This is where E.T.
lives!”
Doyle agrees: “If I were an inhabitant of
a planet around a dwarf, I would find it
hard to believe that any beings could exist
on a planet around a solar-type star. I would
probably argue for pointing radio telescopes
only at red dwarfs.”
But beings do exist around at least one
solar-type star — our star. Doyle’s comment
is an argument for thinking outside our
own box and realizing that in such a diverse
universe, life could arise in diverse environ-
ments. Sounds like good advice.DOWNLOAD SHOSTAK’S MAY 2012 ARTICLE “WHAT HAPPENS WHEN WE DETECT ALIEN LIFE?” AT http://www.Astronomy.com/toc.Extremophiles live in conditions on Earth that
do not seem conducive to life — what scientists
thought were uninhabitable zones on our planet.
Deinococcus radiodurans, listed in the Guinness
Book of World Records as the toughest bacterium,
can withstand 1,000 times as much radiation as
humans and repair all resulting DNA damage. It
also carries on through dehydration, acid baths,
and high and low temperatures. Organisms like
this remind us that life could be hiding in unex-
pected places on other planets. MICHAEL DALY/OAK RIDGE
NATIONAL LABORATORY
NASA’s Kepler space telescope revolutionized exoplanet science, discovering hundreds of confirmed
planets and thousands of candidates by watching for dimming when these planets transit their stars.
This plot shows simulations of the star systems by size and color (and thus by temperature). As scientists
analyze more of Kepler’s data, they find smaller planets around dimmer stars. NASA