Two Exoplanets May Support Life
Basil
star Gliese 581. Scientists believe the pair could be inhabitable and
using new, super telescopes, astronomers are now searching for signs of
life.
The alien sun, glowing blood-red in the sky, provides little light and
heat. But it does shine day and night throughout the year, which lasts
only about two Earth months on this distant planet.
The other side of the planet, however, is in constant darkness, never
illuminated by the dwarf star. Extreme temperature differences trigger
mega-storms that make terrestrial hurricanes look like gentle breezes,
as powerful winds drive massive waves against the coastlines.
There is no doubt about it: this is not a cozy place. And yet it is
quite possible that it supports life.
This is the exciting conclusion that was reached by Harvard astronomer
Lisa Kaltenegger and her research team. The scientists performed an
extensive simulation of the planet Gliese 581 d, the results of which
will be described in an upcoming issue of the Astrophysical Journal.
"It's a fascinating new world," says Kaltenegger, who has been
conducting her research at the Max Planck Institute for Astronomy (MPIA)
in Heidelberg, Germany since the end of September. "There is actually a
possibility that it's habitable."
Water Could Flow
Gliese is considered the most exciting planetary system astronomers have
discovered to date outside our own solar system. A total of six planets
orbit the red sun in the Libra constellation. Only last week, US
scientists announced that they had discovered another planet in the
system that's even more similar to Earth. The smaller, rocky planet
Gliese 581 g also appears to orbit its sun in the "habitable zone."
Kaltenegger has already begun calculating the climate for this satellite.
The most important requirement to allow for the development of life as
we know it is that a planet is heated by its sun to a sufficiently
moderate degree that liquid water can form there. "If the ingredients
are right," says MPIA Director Thomas Henning, "it could happen almost
automatically."
Now the calculations of Kaltenegger's team are fueling the suspicion
that there might even be a few oases of life among the roughly 500
planets already discovered outside our solar system, that have remained
unnoticed until now.
At first Gliese 581 d, which was discovered in 2007, was also believed
to be an icy planet incapable of supporting life. Initial estimates
indicated that its orbital path was in fact too far from its star,
meaning it was but a flying ball of ice with temperatures constantly
hovering around minus 40 degrees Celsius (minus 40 degrees Fahrenheit).
But there was one factor the astronomers hadn't taken sufficiently into
account: the greenhouse effect.
The Aliens Would Need More Muscles
Gliese 581 d is seven times as heavy as the Earth, which puts it in the
class of the so-called super-Earth planets. There is every indication
that because of the massive rocky planet, powerful volcanoes once spewed
massive amounts of CO2 into the atmosphere, far more CO2, in fact, than
exists in our atmosphere. This could have produced a dense envelope of
air under high atmospheric pressure. The resulting greenhouse gas effect
would have caused temperatures to rise significantly above freezing,
leading to a thaw on the icy planet.
Based on her model calculations, Kaltenegger speculates that atmospheric
pressure on the planet could even be as high as seven or eight bars, a
level found on Earth at the bottom of lakes. "It would certainly be
extremely difficult to move around there," the astronomer explains,
"sort of like constantly wading in deep water."
Kaltenegger's calculations also indicate that gravity is higher on the
planet. Even a slender earthling would weigh about as much as an adult
male gorilla on Gliese 581 d.
"For land dwellers, in particular, it would certainly be advantageous to
have a few extra muscles," Kaltenegger speculates. "Or else the aliens
would have to crawl along the ground like snakes."
Shrubs Could Have Leaves Black As Coal
A noticeably more earth-like gravitational environment is likely to
prevail on the newly discovered neighboring planet, Gliese 581 g, with a
mass only three to four times that of the Earth. However, it is not yet
clear whether the planet's climate can support life. According to
initial rough estimates, temperatures on Gliese 581 g, which presumably
lacks a significantly warming greenhouse effect, are more freezer-like.
The possibility of vegetation on the Gliese planets is also a source of
speculation. If plant-like beings did exist there, they would probably
look extremely exotic to our eyes. But because the star provides so
little light, the alien plants would have to utilize all available light
for their photosynthesis. The consequence, as US scientist Nancy Kiang
has discovered, would be bizarre. According to Kiang, for grasses or
shrubs to thrive under a red sun, their blades and leaves would have to
be black as coal.
All of this, so far, is little more than conjecture, albeit with
scientific underpinnings. But with the right telescopes and measuring
instruments, it would, in fact, be possible, one day, to determine
whether there are trees and other vegetation growing on the two
earth-like Gliese planets.
To reach this determination, scientists would have to capture the light
from the planet and use it to decode the chemical composition of their
atmospheres. A high oxygen concentration alone would indicate that life
exists there, because oxygen is a highly reactive gas that can only
exist in small amounts in the atmosphere of an uninhabited planet. A
high oxygen concentration would mean that there must be organisms like
bacteria and plants on the planet to constantly produce more oxygen.
The Firefly And The Headlight
But analyzing the light coming from an extrasolar planet, or exoplanet,
remains an enormous technical challenge. Normal stars shine millions of
times more brightly than their dimly lit satellites. The task of
detecting a planet the size of Earth near a distant sun is about
equivalent to detecting a firefly flying next to a car headlight in
Cairo -- from Berlin.
Scientists have a long way to go before they can unlock the secrets of
the atmospheres of rocky planets like Gliese 581 d or Gliese 581 g,
which makes the breakthroughs astronomers have been able to announce in
recent years all the more astonishing. They have already been able to
study the atmospheres of larger exoplanets, at least indirectly. To do
this, they take advantage of mini solar eclipses that occur when a
distant planet passes in front of its sun. When this happens, the planet
is uniformly illuminated, leaving its chemical fingerprint on the light
emitted by the star.
For now, this trick only works with gas planets, which have enormous
atmospheres. The atmospheres of exoplanets analyzed to date contain
mostly hydrogen and helium, a composition very similar to that of
Jupiter and Saturn. This confirms that our solar system is not an
exception in the Milky Way.
Early this year, MPIA scientists even managed, for the first time, to
directly capture and analyze the light coming from a distant planet. To
do so, they used the world's most advanced observatory, the European-run
"Very Large Telescope" in Chile's Atacama Desert. Its eight-meter
mirrors are so photosensitive that they could detect a flashlight on the
moon. But even this is only good enough for only a very few of the
exoplanets. The impressive achievement was only possible because the
planet the scientists were studying, the gas giant HR 8799 c, is
unusually bright. HR 8799 c is still very young and as hot as a
flamethrower.
New Telescopes on The Horizon
Astrophysicists are confident in their ability to make rapid headway in
pushing the boundaries even farther, and they are eager to directly
photograph smaller and colder planets. "We are moving forward much more
quickly than expected," says Max Planck scientist Henning. "In as little
as five years, we could be far enough along to measure the atmospheres
of super-Earth planets orbiting small, relatively dim suns for the first
time."
Scientists like Henning are pinning their hopes on the next generation
of eyes in the sky. The James Webb Space Telescope, the successor of the
legendary Hubble Space Telescope, could be sent into space by as early
as 2014. The Europeans, for their part, plan to build another
observatory, the "Extremely Large Telescope," in the Atacama Desert.
With its 42-meter mirror, it would be the biggest telescope ever built.
There are even plans in the works to build a 100-meter telescope.
These super observatories should allow scientists to solve the mysteries
of the Gliese planets. And what if their remote diagnosis reveals that
there are, in fact, unknown life forms there? Could mankind launch an
expedition to explore the alien worlds?
The Gliese system is only 20.5 light-years away from Earth, making the
red dwarf star one of the 100 closest fixed stars -- a cosmic neighbor,
so to speak.
But even this relatively small interstellar chasm could not be crossed
with conventional rockets. To reach the twin earthlike planets,
terrestrial astronauts would have to travel for 400,000 years. Man has
only existed for half as long.