The Twin Earths of Gliese 581: Two Exoplanets May Support Life
Two planets similar to Earth have been discovered circling the dwarf star Gliese 581. Scientists believe the pair could be habitable 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.
Translated from the German by Christopher Sultan
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