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 biosphere 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.

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.

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.