Return to "Earth's Twin" -European Teams Says Gliese 581 May be Habitable

More than 10 years after the discovery of the first extra-solar planet, a European team of astronomers have confirmed that one of the planets might indeed be located within the habitable zone around the star Gliese 581.

Until a few years ago, most of the newly discovered exo-planets were Jupiter-mass, probably gaseous, planets. Recently, astronomers have announced the discovery of several planets that are potentially much smaller super-Earths with a minimum mass lower than 10 Earth masses.

In April, a European team announced in Astronomy & Astrophysics the discovery of two new planets orbiting the M star Gliese 581 (a red dwarf), with masses of at least 5 and 8 Earth masses. Given their distance to their parent star, these new planets -Gliese 581c and Gliese 581d- were the first ever possible candidates for habitable planets.

The expression “super-Earths”, which is often used to refer to exoplanets in the 2-10 Earth-mass range, might be confusing, as it indeed suggests that these planets are rocky planets that differ from the Earth only by their mass. But Gliese 581 c and d could very well be big icy planets, with a very different composition from the Earth.

Unlike Jupiter-mass giant planets that are mainly gaseous, terrestrial planets are expected to be extremely diverse: some will be dry and airless, while others will have much more water and gases than the Earth. Only the next generation of telescopes will allow us to tell what these new worlds and their atmospheres are made of and to search for possible indications of life on these planets. Early in the next decade, scientists will launch a new kind of telescope, the interferometry space telescope, which uses the interference of light beams to enhance the resolving power of telescopes. However, theoretical investigations are possible today and can be a great help in identifying targets for these future observations.

Two international teams, one led by Franck Selsis and the other by Werner von Bloh investigated the possible habitability of these two super-Earths from two different points of view.compute the properties of a planet’s atmosphere at various distances from the star. If the planet is too close to the star, the water reservoir is vaporized, so Earth-like life forms cannot exist. The outer boundary corresponds to the distance where gaseous CO2 is then unable to produce the strong greenhouse effect required to warm a planetary surface above the freezing point of water. The major uncertainty for the precise location of the habitable zone boundaries comes from clouds that cannot currently be modeled in detail.

W. von Bloh and his colleagues studied a narrower region of the habitable zone where Earth-like photosynthesis is possible. This photosynthetic biomass production depends on the atmospheric CO2 concentration, as much as on the presence of liquid water on the planet. Using a thermal evolution model for the super-Earths, they have computed the sources of atmospheric CO2 (released through ridges and volcanoes) and its sinks (the consumption of gaseous CO2 by weathering processes).

The main aspect of their model is the persistent balance hat exists on Earth between the sink of CO2 in the atmosphere-ocean system and its release through plate-tectonics. In this model, the ability to sustain a photosynthetic strongly depends on the age of the planet, because a planet that is too old might not be active anymore, that is, would not release enough gaseous CO2. In this case, the planet would no longer be habitable.

Both teams found that, while Gliese 581 c is too close to the star to be habitable, the planet Gliese 581 d might be habitable. However, the environmental conditions on planet d might be too harsh to allow complex life to appear. Planet d is tidally locked, like the Moon in our Earth-Moon system, meaning that one side of the planet is permanently dark. Thus, strong winds may be caused by the temperature difference between the day and night sides of the planet. Since the planet is located at the outer edge of the habitable zone, life forms would have to grow with reduced stellar irradiation and a singularly odd climate.

However, even under these strange conditions, it might still be habitable if its atmosphere is dense enough. In any case, habitable conditions on planet d should be very different from what we encounter on Earth.

Last but not least, the report in Astronomy & Astrophysics suggests that the possible habitability of one of these planets is particularly interesting because of the central star, which is a red dwarf, M-type star. About 75% of all stars in our Galaxy are M stars. They are long-lived (potentially tens of billion years), stable, and burn hydrogen.

M stars have long been considered as poor candidates for harboring habitable planets: first because planets located in the habitable zone of M stars are tidally locked, with a permanent dark side, where the atmosphere is likely to condense irreversibly. Second, M stars have an intense magnetic activity associated with violent flares and high X and extreme UV fluxes, during their early stage that might erode planetary atmospheres. Theoretical studies have recently shown that the environment of M stars might not prevent these planets from harboring life.

M stars have then become very interesting for astronomers because habitable planets orbiting them are easier to detect by using the radial-velocity and transit techniques than are the habitable planets around Sun-like stars.

Both studies definitely confirm that Gliese 581c and Gliese 581d will be prime targets for the future ESA/NASA space mission Darwin/Terrestrial Planet Finder (TPF), dedicated to the search for life on Earth-like planets. These space observatories will make it possible to determine the properties of their atmospheres.

A third study on the Gliese 581 planetary system led by H. Beust and his team study the dynamical stability of the Gliese 581 planetary system. Such studies are very interesting in the framework of the potential habitability of these planets because the long-term evolution of the planetary orbits may regulate the climate of these planets. Mutual gravitational perturbations between different planets are present in any planetary system with more than one planet.

In our solar system, under the influence of the other planets, the Earth's orbit periodically evolves from purely circular to slightly eccentric. This is actually enough to trigger the alternance of warm and glacial eras. More drastic orbital changes could well have prevented the development of life. Beust and his colleagues computed the orbits of the Gliese 581 system and find that the system appears dynamically stable, showing periodic orbital changes that are comparable to those of the Earth. The climate on the planets is expected to be stable, so it at least does not prevent life from developing, although it does not prove it happened either.

Chris Tinney, a member of he world's largest and most prolific team of planet hunters, the
Anglo-Australian, California and Carnegie Planet Searches, thinks that “finding a planet of Earth mass is probably a couple of years away. But…”—and he emphasizes the “but,” pausing for a moment—“there’s always a ‘but.’” As he explains, all of the things they are finding of very low mass are moving in very short orbital periods, which means that they are orbiting close to their parent stars. So although there they are like Earth in terms of their mass and size, these planets are very unlike the Earth in terms of their orbit.

“To find an Earth-mass planet in an Earth-like orbit is just not going to happen with the Doppler technique,” Tinney states. It is simply beyond the technology currently developed. Essentially, it would mean that they would need to be performing measurements 100 times better than any technology is capable of doing.

So does this rule out the possibility of finding a habitable planet?

Not quite. There is a “trick” to planet hunting. Scientists can look for Earth-mass planets in short period orbits around lower mass stars. These types of stars are called M dwarfs and have a mass one tenth the size of the Sun, which means that the velocity signal is ten times larger, and therefore the radius at which the planet must be from the star in order to have water or liquid on its surface is much smaller. For now, it’s Tinney’s opinion that some of the recent reports about habitable planets being discovered “is more hype than reality,” but that the discovery of such planets “will come in due course.”

Posted by Casey Kazan.

Comments

I wonder if they will find us or will we fly to them first. hehe
Chris Jones
http://www.inspirationalexperiences.org

Posted by: Chris Jones | December 18, 2007 at 12:21 PM

If we find life on another planet so what can you honestly assume that we were alone. Than you must regard yourself Pretty high. Imagen all the planet's who have destroyed themselves only to start over of course we will never advance into interstellar travel.

Posted by: Star Searcher | December 19, 2007 at 06:21 PM

Gliese 581 is 20.4 light years away. If by some miracle there is a technological civilization there, and they're capable of picking up and interpreting our radio signals, they're hearing news from 1988, the end of the Reagan administration and the declining years of the Cold War. I wonder if they'd really want to contact us.

Posted by: Brian | January 07, 2008 at 11:58 AM