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32 ASTRONOMY t FEBRUARY 2014
the emptier realms farther out, a hint that
perhaps the dwarfs do house some attrac-
tive worlds. Recent data from the Kepler
space telescope have shown this to be the
case. Harvard astronomers Courtney Dress-
ing and David Charbonneau have examined
3,897 red dwarfs that are under Kepler’s
loupe, including 64 that have planet candi-
dates. Within this sample, 16 percent have
an Earth-sized Goldilocks world. That’s a
stunningly large fraction if you extrapolate
to the whole galaxy — if 75 percent of the
galaxy’s 200 billion stars are red dwarfs,
then they house 24 billion habitable Earth-
sized planets.
Of course, red dwarfs, while as common
as phone polls, are not the only stars in the
sky. Roughly 20 percent of all stars are cous-
ins of the Sun — types G and K. Berkeley
astronomers Erik Petigura and Geoff
Marcy, together with their University of
Hawaii colleague Andrew Howard, have
analyzed Kepler observations of such sys-
tems and found that 22 percent are likely to
have a planet in the Goldilocks zone. That
adds another 9 billion worlds to the Milky
Way’s inventory of life-friendly locales.
However, it’s clear that the majority of
the Goldilocks worlds are hanging out
around the galaxy’s smallest stars. Accord-
ing to Charbonneau, “In terms of stars with
habitable worlds in the galaxy, the red
dwarfs dominate.”
This is all encouraging news for SETI
practitioners. But what about those other
red-dwarf bugaboos: low-grade sunlight
and sterilizing f lares? Just because dwarfs
sport habitable real estate is no guarantee
that they actually are inhabited.
Heath and colleagues reckon that the
photosynthetically relevant sunlight — the
light that plants actually can use — on red
dwarf worlds is 5 to 25 percent as strong as
that f looding Earth. But even this feeble
illumination would be enough to support
many interesting terrestrial plants, includ-
ing rice, wheat, and a number of vegetables.
In addition, there’s photosynthesis and then
t here’s photosynthesis. While most plants
use red light with a wavelength of about 670
nanometers, various kinds of exotic bacteria
perform their photosynthetic magic with
light waves of about 900 nanometers. That
lower-energy light is in the “near-infrared”
part of the spectrum — a perfect match for
the glow of red dwarfs. We can reasonably
expect that Darwinian evolution would
have little trouble producing rich f lora
appropriate for animal life around these
dim bulbs.
As for the f lares, it’s not hard to imagine
numerous adaptive strategies that life could
use. Obviously, simpler life could stay
underwater, cower beneath rocks, or armor
itself with shells. Another easy defense
would be opportunistic: Simply retreat into
nearby shelter as the stellar f lare slowly
builds in intensity.
A more intriguing possibility is that
organisms will develop biological means of
coping. Numerous terrestrial bacteria can
repair damage to their DNA caused by
ultraviolet light. Obviously, if earthly life-
forms can figure out how to heal radiation
damage, the f lora and fauna that evolve on a
planet orbiting a red dwarf — where f lares
might be a perennial environmental danger
— could do the same.
In addition, and as Henry notes about
his own observations: “We’ve spent more
than a decade studying the variability of red
dwarfs, and we rarely see a f lare. At visible
wavelengths, they hardly vary at all, usually
less than 2 percent. And those that emit
truly harmful X-rays are mostly dwarfs
younger than 100 million years. After that
brief period of youthful exuberance, they
seem to settle down.”
And indeed, as Doyle points out, f lares
might be only an inconvenience. “While red
dwarf stars have f lare activity, this only lasts
for less than an hour or so,” he says.
Advantages for SETI
Given the evidence that the majority of hab-
itable galactic real estate is situated in dwarf
star systems, SETI observers have adequate
incentive to focus their searches for extra-
terrestrial intelligence on these abundant
neighborhoods.
But wait! As they say on the television
commercials, there’s more.
SETI programs — including both radio
and optical experiments — often examine a
finite “target list” of a specific type of object.
As an example, the SETI Institute’s Project
Phoenix program, which was carried out
Just 20 light-years from Earth, the Gliese 581 red dwarf system is home to a nearly Earth-sized planet.
GJ 581g — 1.3 times Earth’s radius — hugs the middle of the habitable zone, orbiting its star every 37 days.
Scientists have found the largest number of
planets around Sun-like stars. However, the search
for exoplanets has focused largely on stars like our
own because scientists thought them most likely
to harbor life. New research, though, suggests
that the more-abundant red dwarfs (M-type stars)
might be suitable homes for extraterrestrials.
ASTRONOMY: ROEN KELLY, AFTER EXOPLANET ENCYCLOPEDIA
In 1960, astronomer Frank Drake undertook the
first search for extraterrestrial intelligence. Using
the 85-foot Howard Tatel Telescope in Green Bank,
West Virginia, he observed two Sun-like stars —
Tau (τ) Ceti and Epsilon (ε) Eridani — for purpose-
ful signals from smart aliens. He came up empty,
but efforts long continued to focus on star systems
similar to our own. NRAO
NASA/LYNETTE COOK