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| parallax        =160.91 ± 2.62  mas

| parallax        =160.91 ± 2.62  mas

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'''Gliese 581''', also known as '''HO Librae''', is a red dwarf star around 20.4 [[light year|light years]] away.  It is most famous for the discovery of the [[Gliese 581c|first low mass planet within a star's habitable zone]].  In total, four planets have found thus far orbiting the star, two of which may sit within the [[habitable zone]].

'''Gliese 581''', also known as '''HO Librae''', is a red dwarf star around 20.4 [[light year]]s away.  It is most famous for the discovery of the [[Gliese 581c|first low mass planet within a star's habitable zone]].  In total, four planets have found thus far orbiting the star, two of which may sit within the [[habitable zone]].

Despite being only 20 light years distant Gliese 581 is invisible to the unaided eye. This is due to Gliese 581 being a [[red dwarf]], and thus dim, with an apparent magnitude of only 10.56.

Despite being only 20 light years distant Gliese 581 is invisible to the unaided eye. This is due to Gliese 581 being a [[red dwarf]], and thus dim, with an apparent magnitude of only 10.56.

On April 25, 2007 the same team of astronomers, again using the HARPS spectrograp, announced the discovery of '''Gliese 581 c''' (along with Gliese 581 d).  When announced, Gliese 581 c was the smallest known extrasolar planet around a main sequence [[star]] (until Gliese 581 e was found).  It is the third closest of the four known worlds.

On April 25, 2007 the same team of astronomers, again using the HARPS spectrograp, announced the discovery of '''Gliese 581 c''' (along with Gliese 581 d).  When announced, Gliese 581 c was the smallest known extrasolar planet around a main sequence [[star]] (until Gliese 581 e was found).  It is the third closest of the four known worlds.

The planet is calculated to have at least 5.36 times [[Earth|Earth’s]] mass (maximum of 10.4 times), and is estimated to be 1.5 times the [[Earth|Earth’s]] diameter.  The planet orbits with a mean distance of only 0.07 AU, which takes just under 13 days to complete.  At this distance, Gliese 581 c is [[tidal lock|tidally locked]] to its parent star.<ref name="exo">http://exoplanet.eu/star.php?st=Gl+581</ref>

The planet is calculated to have at least 5.36 times [[Earth|Earth’s]] mass (maximum of 10.4 times), and is estimated to be 1.5 times the [[Earth|Earth’s]] diameter.  The planet orbits with a mean distance of only 0.07 AU, which takes just under 13 days to complete.  At this distance, Gliese 581 c is [[tidal lock|tidally locked]] to its parent star.<ref name="exo"/>

Initially the planet was thought to be inside Gliese 581’s [[habitable zone]], making it the first such terrestrial world outside of our Solar System to be found. This generate speculation about the possibility of the planet being suitable for life as we know it.  However, further research indicated that the planet may be too close to the [[star]], thus causing a [[Venus]]-like runaway greenhouse effect in its [[atmosphere]] that would boil off any [[water]].<ref name="habit">http://arxiv.org/PS_cache/arxiv/pdf/0705/0705.3758v1.pdf</ref>  Gliese 581 d instead is the better candidate for habitability.

Initially the planet was thought to be inside Gliese 581’s [[habitable zone]], making it the first such terrestrial world outside of our Solar System to be found. This generate speculation about the possibility of the planet being suitable for life as we know it.  However, further research indicated that the planet may be too close to the [[star]], thus causing a [[Venus]]-like runaway greenhouse effect in its [[atmosphere]] that would boil off any [[water]].<ref name="habit">http://arxiv.org/PS_cache/arxiv/pdf/0705/0705.3758v1.pdf</ref>  Gliese 581 d instead is the better candidate for habitability.

'''Gliese 581 d''' was discovered alongside and announced at the same time as [[Gliese 581c|Gliese 581 c]], also sitting within the [[habitable zone]] of Gliese 581, and is now the prime candidate for habitability among the known worlds of the Gliese 581 system.  It is the fourth of the four known worlds in distance from its parent star.

'''Gliese 581 d''' was discovered alongside and announced at the same time as [[Gliese 581c|Gliese 581 c]], also sitting within the [[habitable zone]] of Gliese 581, and is now the prime candidate for habitability among the known worlds of the Gliese 581 system.  It is the fourth of the four known worlds in distance from its parent star.

The planet has a mass between 7.09 and 13.8 times that of Earth.  Gliese 581 d has a mean orbit of 0.22 AU, which it completes in 66.8 days.<ref name="exo">http://exoplanet.eu/star.php?st=Gl+581</ref>  In April of 2009, after further studies, the planet was confirmed to be within the habitable zone where liquid water could exist with a greenhouse effect under the right atmospheric conditions.<ref name="habit">http://arxiv.org/PS_cache/arxiv/pdf/0705/0705.3758v1.pdf</ref>  The planet is thought to be too massive to be made only of rock and may have been an ice world at one time, with the abundant ices melting into deep worldwide [[ocean|oceans]] as the planet migrated closer to Gliese 581.<ref>http://www.eso.org/public/outreach/press-rel/pr-2009/pr-15-09.html</ref>

The planet has a mass between 7.09 and 13.8 times that of Earth.  Gliese 581 d has a mean orbit of 0.22 AU, which it completes in 66.8 days.<ref name="exo"/>  In April 2009, after further studies, the planet was confirmed to be within the habitable zone where liquid water could exist with a greenhouse effect under the right atmospheric conditions.<ref name="habit"/>  The planet is thought to be too massive to be made only of rock and may have been an ice world at one time, with the abundant ices melting into deep worldwide [[ocean]]s as the planet migrated closer to Gliese 581.<ref>http://www.eso.org/public/outreach/press-rel/pr-2009/pr-15-09.html</ref>

=== Gliese 581 e===

=== Gliese 581 e===

The discovery of '''Gliese 581 e''' was announced on April 21, 2009. The planet is estimated to have a minimum mass only 1.9 times that of Earth making it the smallest [[extrasolar planet]] in the system and the smallest extrasolar planet known overall.  Of the four known worlds it is nearest to its parent star.

The discovery of '''Gliese 581 e''' was announced on April 21, 2009. The planet is estimated to have a minimum mass only 1.9 times that of Earth making it the smallest [[extrasolar planet]] in the system and the smallest extrasolar planet known overall.  Of the four known worlds it is nearest to its parent star.

The planet has a mean orbit of only 0.03 AU, which it completes in a mere 3 days.  Because of its short distance, the planet is too close to be within Gliese 581’s habitable zone.<ref name="exo">http://exoplanet.eu/star.php?st=Gl+581</ref>  It is unlikely to have an atmosphere because of the high temperatures and strong [[radiation]] it would receive at such a close distance to even a low mass [[star]].   

The planet has a mean orbit of only 0.03 AU, which it completes in a mere 3 days.  Because of its short distance, the planet is too close to be within Gliese 581’s habitable zone.<ref name="exo"/>  It is unlikely to have an atmosphere because of the high temperatures and strong [[radiation]] it would receive at such a close distance to even a low mass [[star]].   

[[Category:Astronomy]]

[[Category:Astronomy]]