In a galaxy filled with billions of
stars, scientists searching for alien life need some way to pick out
those which are most likely to harbor habitable planets and moons. For
more than 150 years, an important tool in this screening process has
been the concept of a "circumstellar habitable zone."
Traditionally, this zone has been
defined as a narrow disk around a star where temperatures are moderate
enough that water on the surface of a planet can exist in a liquid
form. The idea is that where liquid water exists, life might arise.
Beginning in the latter half of the 20th
century, new information began to emerge that challenged the
traditional view. Scientists on Earth began finding rugged organisms
thriving in harsh conditions that were off-limits to most other
creatures. Meanwhile, images beamed back by robotic probes in space
revealed that other moons within our solar system were much more
interesting geologically—and perhaps biologically—than our own.
However, beginning a decade ago, planets
discovered around other stars began to reveal a diversity of planetary
systems that was beyond expectations.
More recently, scientists have gone back
and reexamined their ideas about the possibility of habitable planet
forming around red dwarf stars. Despite being the most abundant stars
in the galaxy, red dwarfs have traditionally been shunned by scientists
as being too small and too dim to support life. Those prejudices are
beginning to fade and the recent discovery
of a small, rocky world in orbit around a red dwarf 28,000 light-years
from our corner of the solar system has refueled speculations that
these stars might harbor planets with life.
Extremophiles
Extremophiles are a diverse group of organisms that thrive in harsh environments
intolerable to virtually all other creatures. Since the late 1960s,
scientists have discovered hundreds of different extremophile species,
most of them bacteria.
This hardy
group includes members that can survive scalding waters, subzero
temperatures, bone-crushing pressures, corrosive acid, extreme salt and
arid conditions. Extremophiles have been found that can withstand
massive doses of radiation, breath rust, eat sulfur, belch methane and
live without oxygen or sunlight.
"Finding
extremophiles on Earth has just been mind-blowing," said Carol Tang, a
researcher from the California Academy of Sciences who studies
extremophiles. "If you think about how there's very few places on Earth
where there isn't life, you can't think about the solar system and the
universe in a very limited way anymore."
Habitable moons
In 1979,
NASA's two Voyager spacecrafts shocked scientists with images they
beamed back of Jupiter's moon Europa. The images showed a shiny world
covered in water ice, but what was really remarkable was how smooth its
surface was.
Unlike our
own moon, Europa has relatively few impact craters. Because it doesn't
have an atmosphere to burn up incoming objects like asteroids,
scientists concluded that Europa had an internal heat source that kept
its waters fluid, allowing the moon to periodically repave its icy
shell and erase away the craters that must routinely be carved.
"Before the
Voyager missions, scientists used to think that the moons of the other
planets were old, rocky, battered bodies like our moons," said Cynthia
Phillips, a SETI planetary scientist.
Scientists
think Europa stays warm by a process called tidal heating. All moons,
including our own, are stretched and pulled by the planet they orbit.
Jupiter is so massive and its gravity so strong that it actually causes
Europa’s surface to bulge and shrink as it circles around in its orbit.
This constant motion generates friction and heat.
Saturn's cloud-covered moon, Titan,
is thought to be warmed by the same process. Other moons generate heat
through different means. Scientists recently discovered that Saturn's
moon Enceladus, for example, contains a mysterious hot spot
in its southern hemisphere that might be caused by radioactive material
left over from the moon's formation billions of years ago.
Widening zone
This
revelation, that not all the moons in our solar system are as dead and
barren as our own, meant that places outside the traditional habitable
zone might sustain liquid water and support life.
"If you have
a fairly sizable planet with plenty of internal energy to keep warm it
might not need to be close to the Sun," said biologist Ken Nealson from
NASA's Jet Propulsion Laboratory in California."It might have plenty of
energy to support a perfectly good biomass without having a lighted
surface."
Scientists
believe that beneath Europa’s icy shell lies an ocean vaster than
Earth’s. For this reason, many scientists figure the Jovian moon may be
a better bet for finding alien life than Mars.
"There might
have been liquid water on Mars in the past and there could have been
life then, but it's pretty unlikely that we'll find life living there
today," Phillips said. "But on places like Europa, there could be and
probably is water there today. Instead of looking at an extinct
biosphere, we could be looking at a currently active one."
Red dwarfs
Last fall, a group of about 30 scientists from different fields got together in Mountain View, California for a workshop
sponsored by the SETI Institute. The workshop was convened to answer a
single question: are the planets orbiting red dwarfs habitable?
SETI scientists will soon begin looking for radio signals from intelligent extraterrestrial life using the Allen Telescope Array and they wanted to know whether red dwarfs should be included in the list of stars to search.
Red dwarfs
are believed to make up about 85 percent of the stars in the universe,
but they are so small and so dim that scientists have traditionally
ignored them as possible havens for habitable planets.
One of the
main objections was that the habitable zones of red dwarfs would be
very narrow and very close to the stars. For a planet orbiting a red
dwarf to be warm enough to have liquid water, it would need to be
located closer to the star than Mercury is to our own Sun. At such a
close distance, the planet would become tidally locked to the red dwarf
the way our Moon is to Earth. Any water existing on such a planet would
be boiled away on the side facing the star and frozen solid on the
other.
In recent
years, however, new computer models have suggested that the situation
isn’t as impossible as it might seem. The models predict that if an
orbiting planet had a thick enough atmosphere, heat could be
redistributed from the lit side of the planet to the side that was dark.
As for the
criticism that a red dwarf’s habitable zone is very narrow, Todd Henry,
an astronomer at Georgia State University, has an interesting view.
Because there are so many more red dwarfs than stars like our Sun,
Henry has performed calculations suggesting that if the narrow
habitable zones of all the red dwarfs in our galaxy were combined, they
would equal the habitable zone of the all the Milky Way’s Sun-like
stars.
"You open up a lot more territories if you put [red dwarfs] back on the table," Henry said.