Gliese 581: one planet might indeed be habitable (Forwarded)

Astronomy & Astrophysics
Paris, France

Contact persons:

Science:

Dr. W. von Bloh
PIK, Potsdam, Germany
Phone: +49 (0) 331 288 2603

Dr. F. Selsis
LAB, Bordeaux, France
Phone: +33 (0)6 84 00 75 55

Dr. H. Beust
Laboratoire d'Astrophysique
Observatoire de Grenoble, France
Phone : +33 (0)4 76 63 58 99

Press office:

Dr. Jennifer Martin
Journal Astronomy & Astrophysics
61, avenue de l'Observatoire
75014 Paris, France
Phone: +33 1 43 29 05 41

Released: December 13th, 2007

Gliese 581: one planet might indeed be habitable

In April, a European team of astronomers announced in Astronomy &
Astrophysics the discovery of two possibly habitable Earth-like planets.
A&A is now publishing two independent, detailed studies of this system,
which confirm that one of the planets might indeed be located within the
habitable zone around the star Gliese 581.

More than 10 years after the discovery of the first extrasolar planet,
astronomers have now discovered more than 250 of these planets. Until a
few years ago, most of the newly discovered exoplanets were Jupiter-mass,
probably gaseous, planets. Recently, astronomers have announced the
discovery of several planets that are potentially much smaller, with a
minimum mass lower than 10 Earth masses: the now so-called super-Earths
[1].

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 (now known as Gliese 581c and Gliese
581d) were the first ever possible candidates for habitable planets.

Contrary to Jupiter-like 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. However, theoretical
investigations are possible today and can be a great help in identifying
targets for these future observations.

In this framework, Astronomy & Astrophysics now publishes two theoretical
studies of the Gliese 581 planetary system. Two international teams, one
led by Franck Selsis [2] and the other by Werner von Bloh [3], investigate
the possible habitability of these two super-Earths from two different
points of view. To do so, they estimate the boundaries of the habitable
zone around Gliese 581, that is, how close and how far from this star
liquid water can exist on the surface of a planet.

F. Selsis and his colleagues 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. These
limitations also occur when one looks at the Sun's case: climate studies
indicate that the inner boundary is located somewhere between 0.7 and 0.9
AU, and the outer limit is between 1.7 and 2.4 AU. Figure 1 illustrates
the Sun's habitable zone boundaries, compared to the case for Gliese 581
as computed both by Selsis and von Bloh.

W. von Bloh and his colleagues study 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 (that 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. To
compute the boundaries of the habitable zone as illustrated by Figure 1,
von Bloh assumed a CO2 level of 10 bars.

Figure 1 illustrates the boundary of the habitable zone as computed using
both models and, for comparison, the boundary of the Sun's habitable zone.
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 very peculiar climate.

Figure 1 also illustrates that the distance of planets c and d to the
central star has strong variations due to the eccentricity of their
orbits. In addition, being close to the star, their orbital periods are
short: 12.9 days for planet c and 83.6 days for planet d. Figure 1 shows
that planet d might temporarily leave and re-enter the habitable zone
during its journey. 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 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 paper on the Gliese 581 planetary system has recently been
accepted for publication in Astronomy & Astrophysics. In this paper, H.
Beust and his team [4] 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 over
100 Myr 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.

[1] 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.

[2] The team led by F. Selsis (CRAL and LAB, France) includes J.F. Kasting
(Penn State Univ., USA), B. Levrard (IMCCE, France), J. Paillet (ESTEC,
The Netherlands), I. Ribas (CSIC-IEEC, Spain), and X. Delfosse (LAOG,
France).

[3] The team led by W. von Bloh (PIK, Germany) includes C. Bounama, S.
Franck (PIK, Germany), and M. Cuntz (UTA, USA).

[4] The team led by H. Beust (LAOG, France) includes X. Bonfils (CAAUL,
Portugal), X. Delfosse (LAOG, France), and S. Udry (Observatoire de
Genève, Switzerland).

The habitability of super-Earths in Gliese 581, by W. von Bloh, C.
Bounama, M. Cuntz, and S. Franck.
Astronomy & Astrophysics, 2007, vol. 476, p. 1365.
Full article available in PDF format,
http://dx.doi.org/10.1051/0004-6361:20077939

Habitable planets around the star Gliese 581?, by F. Selsis, J.F. Kasting,
B. Levrard, J. Paillet, I. Ribas, and X. Delfosse.
Astronomy & Astrophysics, 2007, vol. 476, p. 1373.
Full article available in PDF format,
http://dx.doi.org/10.1051/0004-6361:20078091

Dynamical evolution of the Gliese 581 planetary system, by H. Beust, X.
Bonfils, X. Delfosse, and S. Udry.
To be published in Astronomy & Astrophysics, 2008. Full article available
in PDF format,
http://dx.doi.org/10.1051/0004-6361:20078794

IMAGE CAPTIONS:

[Figure 1:
http://www.aanda.org/images/stories/PressRelease/PRaa200708/figure.gif
(52KB)]
Illustration of the habitable zone (HZ) boundaries as obtained by the two
teams. The upper part of the figure shows the HZ of the Sun (at its
present age). The red curve shows only the most extreme outer limit of the
HZ. The actual outer boundary is indeed located somewhere between 1.7 and
2.4 AU. The green limits show the boundaries of the photosynthetic zone as
computed with the model by von Bloh et al. The middle part of the figure
shows the limits of the HZ of Gliese 581 computed with the atmospheric
models from Selsis et al. The lower part illustrates the boundaries of the
photosynthetic zone computed with the geophysical models from von Bloh et
al. The boundaries are shown for several possible ages (5, 7, and 9
Gyr-old) of the Gliese 581 planetary system. Following the latest
estimation, Gliese 581 would be 7 Gyr-old. The purple bars surrounding
planets Gliese 581 c and d illustrate the variable distance to the star
caused by the eccentricity of the orbits.


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