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. ****z (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
Geneve, Switzerland).

The habitability of super-Earths in Gliese 581, by W. von Bloh, C. Bounama,
M. ****z, 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/...708/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.