[D66] Extra-solar planet could sustain Earth-like life

[Antid Oto]

Extra-solar planet could sustain Earth-like life
By Bryan Dyne
6 June 2011

Scientists investigating the growing number of extra-solar planets, or
exoplanets, have concluded that the planet Gliese 581 d is the first to be
confirmed to have a possibility of sustaining Earth-like life. A recent study by
a team of researchers led by Robin Wordsworth shows that Gliese 581 d is the
first rocky planet discovered that potentially has atmospheric conditions
allowing for liquid water on its surface.[1]

Speculations of the existence of exoplanets began as early as the 16th century.
Giordano Bruno suggested, based on the Copernican model of the solar system,
that if the Sun is merely a star and if planets orbit the Sun, then the stars
seen in the night sky must also have planetary systems. Isaac Newton made the
same observation two centuries later in the conclusion of his Principia Mathematica.

While the idea that the solar system is not unique in having a planetary system
has been around for centuries, the first definitive confirmation of an exoplanet
occurred in 1992 around pulsar PSR 1257+12. Pulsars are the fantastically dense
remnants of supernova that emit a stream of radiation and rotate very fast and
with great regularity.

Radio astronomers Aleksander Wolszczan and Dale Frail, who were studying the
object, noted a slight anomaly in the rotation, indicating a nearby mass, the
first planet to be discovered outside the solar system. The system is now known
to have three planets, with a fourth unconfirmed.

Using pulsars provides an accurate indicator of a planet’s existence, but this
search method will not find most planets, due to the rarity of pulsars compared
to other stars. In addition, no planet bathed in the radiation of the nearby
pulsar could harbor Earth-like life. However, in the mid-1990s advances in
technology allowed for the indirect detection of the gravitational influence of
planets on parent stars that are not pulsars. In addition, studying periodic
drops in the apparent brightness of stars has become a method of direct
detection of exoplanets.

The Gliese planetary system was discovered from gravitational perturbations on
the parent star. The first planet was discovered in 2005 and the second in 2007.
The third and fourth planets were announced in 2009 and data indicating the
existence of fifth and sixth planets was released in 2010. Currently, however,
the claim of fifth and sixth planets is disputed within the astronomical community.

A large amount of research has gone into determining the habitability of the
planets within the Gliese system. The first criterion that must be met is
whether any of the planets are within the star’s “habitable zone.” This is the
range of orbits at which the surface temperature of the planet produced by the
energy output of the star is capable of sustaining liquid water.

In 2010, a team of astronomers led by Steven Vogt announced that the planet
Gliese 581 g was a planet approximately the mass of Earth and well within the
habitable zone. However, studies since then have brought the existence of the
planet into question, suggesting that the data on 581 g is anomalous.

Gliese 581 d, the third planet discovered in that system, is now considered a
potentially habitable planet. The planet is most likely a rocky world with a
minimum mass of five and a half times that of Earth and twice Earth’s radius. It
is also tidally locked, which means one side of the planet is always facing its
star, much as one side of the Moon is always facing the Earth, or one side of
Mercury facing the Sun.

Atmospheric modeling was the primary tool of Wordsworth’s team. No current
technology is capable of directly analyzing the content of an exoplanet’s
atmosphere. However, certain basic assumptions can be made. It is known that the
most common light elements in the universe, beyond hydrogen and helium, are
nitrogen, oxygen and carbon. These are commonly seen to form molecular nitrogen,
water and carbon dioxide in observed nebula. Using this knowledge, various types
of atmospheres can be hypothesized and tested in simulations. What Wordsworth’s
team discovered is that it is possible for a stable atmosphere to develop that
allows for the presence of liquid water on the side facing the star.

This result is by no means a confirmation of habitability for unprotected humans
on the surface of Gliese 581 d. Many variables in the exoplanet’s geophysics
could create unknown situations that would create uninhabitable conditions, such
as tectonics and potential runaway greenhouse effects.

Additionally, the surface gravity of the planet is known to be approximately
twice that of Earth, which could cause many long-term issues for any human
presence on the surface.

Ultimately, a spectroscopic analysis will be required to discover exactly what
elements are in the atmosphere of Gliese 581 d and in what percentages, before
any consideration could be given to visiting the planet in the far future. While
this is technically possible now, the planned missions to do so have been
canceled due to cost overruns and NASA’s internal politics.

Even if Gliese 581 d turns out to be uninhabitable, much has been and will be
learned in the study of the planet and of the system. Astronomers are
fine-tuning their ability to find multiple planets within a distant solar
system. They can now discover planets close to Earth’s volume and mass. They can
potentially determine the atmospheric composition of a planet 20 light years away.

If Gliese 581 d is indeed capable of sustaining human life, the discovery would
be all the more spectacular. The relative proximity of the system means that it
is only a short hop—at least on the scale of the galaxy—to future colonists
looking to lead humanity to the stars.

Notes:

1. Wordsworth, et al. “Gliese 581d is the first discovered terrestrial-mass
exoplanet in the habitable zone.” The Astrophysical Journal Letters, Volume 733,
Issue 2, article id. L48 (2011). arXiv:1105.1031v1 [astro-ph.EP].

http://wsws.org/articles/2011/jun2011/plan-j06.shtml