Gliese 581 System

Gliese 581 System Web Pages

 * System of the Day Thread
 * Wikipedia
 * Exoplanet Catalog
 * Sol Station
 * Tour

SETI's Observations (1995, 1997)
http://www.space.com/searchforlife/070517_seti_planet.html

Neptunian Planet b Discovered (Nov 2005)
The discovery of a Neptunian planet around this nearby Red Dwarf suggests such planets are common and only now detectable.


 * Press Release: http://www.eso.org/public/news/eso0539/
 * Paper: http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2005A%26A...443L..15B&db_key=AST&data_type=HTML&format=&high=438efdb55325468

Super-Earth Discovered in Habitable Zone (Apr 2007)
''The first terrestrial planet discovered that is potentially capable of hosting liquid water and possibly life. Also the smallest planet discovered to date around a normal star. Another neptunian also discovered further out.''
 * http://planetary.org/news/2007/0425_Most_EarthLike_Planet_Discovered.html


 * First habitable Earth-like planet found! - Extrasolar Visions BBS Thread


 * http://www.nationalacademies.org/headlines/20070510.html
 * http://spaceurope.blogspot.com/2007/05/gliese-581c-exoplanetary-landmark-qna.html
 * http://www.space.com/searchforlife/070517_seti_planet.html

Planet c More Habitable than d (May/Jun 2007)
http://fr.arxiv.org/abs/0705.3758

http://sciencenow.sciencemag.org/cgi/content/full/2007/611/1

No Transits Detected (Jun 2007)
http://www.nature.com/news/2007/070611/full/070611-6.html http://www.cbc.ca/technology/story/2007/06/11/science-super-earth-ubc.html

Planet c Probably Never Inhabitable (Sep 2007)
http://www.abstractsonline.com/viewer/viewAbstract.asp?CKey=%7B983BE5A5-A9D1-47CF-A27F-CD6D753B1CA7%7D&MKey=%7BADDC1E2E-9B5A-4D9F-B9E3-2D6700638A29%7D&AKey=%7BAAF9AABA-B0FF-4235-8AEC-74F22FC76386%7D&SKey=%7B497CF020-85F2-4CD6-85C4-E91F03E3AE74%7D

Smallest Exoplanet e Discovered (Apr 2009)

 * http://www.thecrimson.com/article.aspx?ref=527898
 * http://www.reuters.com/article/scienceNews/idUSTRE53K5FB20090422
 * http://www.philly.com/inquirer/health_science/daily/20090422_A_world_away__two_planets_somewhat_like_Earth.html
 * http://www.abc.net.au/pm/content/2008/s2549990.htm
 * http://astronomynow.com/090421lightexoplanet.html

Bebo Transmissions to the Planet (Aug 2009)

 * http://www.theregister.co.uk/2008/10/09/bebo_interstellar_gumblespurt/

First Habitable Planet g Found (Sep 2010)

 * http://www.space.com/scienceastronomy/habitable-alien-planet-gliese-581g-facts-101001.html
 * http://www.voanews.com/english/news/science-technology/Astronomers-Find-Potentially-Habitable-Planet-104121519.html
 * http://www.time.com/time/health/article/0,8599,2022489,00.html
 * http://www.astronomynow.com/news/n1009/30exo/
 * http://www.space.com/scienceastronomy/exoplanet-possibly-supports-alien-life100930.html
 * http://www.scientificamerican.com/article.cfm?id=habitable-exoplanet-gliese-581
 * http://www.bbc.co.uk/news/science-environment-11444022
 * http://news.ucsc.edu/2010/09/planet.html
 * http://www.santacruzsentinel.com/ci_16206792?source=most_viewed
 * http://www.nytimes.com/2010/09/30/science/space/30planet.html?_r=2&ref=science
 * http://www.washingtonpost.com/wp-dyn/content/article/2010/09/29/AR2010092907706.html
 * http://www.nsf.gov/news/news_summ.jsp?cntn_id=117765&org=NSF&from=news
 * http://news.sciencemag.org/sciencenow/2010/09/astronomers-find-most-earth-like.html
 * http://www.hawaii.edu/news/article.php?aId=3889
 * http://www.ottawacitizen.com/technology/First+potentially+habitable+planet+found+outside+solar+system+NASA/3599573/story.html
 * http://news.yahoo.com/s/afp/20100929/sc_afp/usastronomyplanet
 * http://news.yahoo.com/s/ap/20100929/ap_on_sc/us_sci_new_earths
 * http://www.wired.com/wiredscience/2010/09/real-habitable-exoplanet/
 * http://www.space.com/scienceastronomy/gliese-581-star-system-tour-100929.html
 * http://www.space.com/scienceastronomy/earth-like-exoplanet-possibly-habitable-100929.html
 * http://www.skyandtelescope.com/news/104031014.html
 * http://www.reuters.com/article/idUSTRE68S5XB20100929
 * http://www.newscientist.com/article/dn19519-found-first-rocky-exoplanet-that-could-host-life.html
 * http://www.nasa.gov/home/hqnews/2010/sep/HQ_10-237_Exoplanet_Findings.html
 * http://cosmiclog.msnbc.msn.com/_news/2010/09/29/5202633-alien-planet-looks-just-right-for-life
 * http://news.discovery.com/space/earth-like-planet-life.html

Tour
The fifth planetary system on the tour is one of the most exciting, which is Gliese 581. It is a red dwarf star that contains six known super-earth and Neptunian exoplanets, including the first potentially habitable exoplanet known. Its planets range from epistellar distances to Venus-like distances and from at least 2 Earth Masses to a Neptune-like mass. The system can be compared to the Gliese 876 system, a similarly small BY Draconis variable Red Dwarf with multiple planets. The size of its planets are smaller than those of that system though.

The star itself is 20.3 light years away in the constellation Libra, the scales. This is the fifth nearest planetary system known and the fourth nearest Red Dwarf system. One of the nearest star systems to it is Gliese 570, a triple star system 5 light years away, also in Libra, that contains one orange dwarf, two red dwarves, and one of the coolest known brown dwarves. On October 3 2008, shortly after discovery of Planet c, radio transmissions were sent to the star. The transmission originated from an observatory in the Ukraine and contains 501 messages selected by a contest through the social network web site Bebo. This transmission will take 20 years to arrive and will do so in early 2029. Any responses would return another twenty years from then in 2049. Seti has monitored the stars for signals and has not detected any.

Main Star a
The star is about a third of the mass of the Sun. It is a smaller star than any of the other stars closer than it that harbor known planets, though not by much. The star is about 7 to 11 Billion years old, which is roughly twice as old as the Sun. It is about one percent as luminous as the Sun is. Like the nearer Gliese 876, it is a BY Draconis variable star and also known as HO Librae. These stars vary in brightness as they rotate, with star spots and other activities coming into view. It could emit occasional x-ray flares that would affect any planets around it. Like the other nearby Red Dwarves, this was cataloged by Wilhelm Gliese in 1957.

Planet e
The first planet in the tour is the nearest one to its star, which is planet e, the fourth discovered planet. This is the smallest known planet in the system at about twice Earth's mass. It is also currently smallest mass known exoplanet around a normal star. This beat the record previously held by MOA-2007-BLG-192L b, a frozen microlensing-detected planet around a particularly small star, which is three times as massive as the Earth.

Planet e was discovered in 2009, in the third batch of planetary discoveries. Its discovery coincided with an orbital adjustment of its outermost known planet d was closer in than previously thought and more likely to have liquid water. It orbits about 3% Earth's distance, somewhat closer than 51 Pegasus b to its star. It is not as close as more nearby Gliese 876 d is from its star, however, which is 2% Earth's distance from the Sun and was also once known as the smallest known exoplanet and the first super-Earth discovered.

Neither of the two transiting super-Earths found so far, CoRoT-7b or GJ 1214b, make good analogs for this planet. Both are significantly more massive (about 8 Earth masses). CoRoT-7b is a much hotter place, with temperatures hot enough keep the surface molten from solar radiation, and where silicate sand rains down on the surface. None of these processes likely occur on Gliese 581b. GJ 1214b is at a temperature more similar to Gliese 581, but has a large diameter for its mass, which could indicate that it is a mini-Neptune, a terrestrial with a large atmosphere due to outgassing, or a hot ocean world with water "compressed" by a thick gaseous envelope. Gliese 581 may not be massive enough to maintain an atmosphere capable of inflating the planet in this manner. The exoplanet can likely be considered a super-Mercury. Like CoRoT-7b and Mercury, it likely has little atmosphere, with much of it having escaped the planet. Like CoRoT-7b and GJ 1214b, it is tidally locked to its star and likely has undergone great tidal stress, which causes Io-like volcanism.

Standing on its surface, its star would appear ten times as large as the Sun does from Earth. It would remain in the same spot in the sky all the time, and the constellations of this star's zodiac could be seen passing behind the star on this airless world in regular 3 day intervals. The Neptune-sized planet b would loom as large as a moon would, which can get within five times the distance the Earth is from the Moon. On the eternal night side of the planet, planet b may be the best source of light, which would appear regularly every few days. The planets, which all have perfectly circular orbits and likely perfect inclinations to each other, would perform regular transits with each other and pass behind the star. They would follow the same path through the sky repeatedly as they more along in their orbits.

Planet b
Planet b is the next planet out. Though it is the largest in the system, it was one of the first planets as small as Neptune that was discovered. Its minimum mass is 16 that of Earths, which is in between Uranus and Neptune. It does not dominate the system as Jupiter does the solar system. It is about twice as massive as the second most massive planet, the outermost planet f and composes about 40% of the mass of the known planets around the system. It is the smaller than any of the other "largest" planets that are closer to the sun than it is.

A transiting analog for this planet is Gliese 436 b, the smallest and nearest transiting Hot Neptune. The analog planet was found to have a higher temperature than expected and a larger radius. A layer of "hot ice" compressed by a thick atmosphere and an outer layer of Hydrogen and Helium may account for it. It also was found to have a mysterious methane deficiency and carbon monoxide abundance. That Neptunian is a little larger though, orbits a little further in, orbits a hotter red dwarf, and is thus hotter in general. The discovery of this planet around a star so nearby in 2005 showed that Neptune sized planets are likely common. At the time of its discovery, five dopplar-detected exoplanets had potentially lower masses. It was only the fifth planet found around a Red Dwarf, after the three known planets around Gliese 876 and the one around Gliese 436. Artist concepts of this planet is a blue hot Neptune-like planet.

Planet c
Planet c is the third planet from the star. This planet's discovery was heralded as the first rocky planet found in the habitable zone. Its discovery prompted radio signals to be sent towards the star. While it is in the habitable zone, it is on the inner edge of this zone. It was found that the planet is also susceptible to developing a runaway green-house gas like Venus, but conversely, the outer planet d was now considered at the outer edge of the habitability zone. Once thought to be the smaller of the two, the planet is actually a mass twin of its counterpart on the other side of the habitable zone, planet d.

Its closest transiting analog is GJ 1214b, which is somewhat closer to its star than the habitable zone and about one earth mass more massive. This planet shows that exoplanets like this can get to be quite large and possibly maintain a thick atmosphere. Planet c could be similar, with a large atmosphere. Even if greenhouse gasses caused the temperature to go above the boiling point, a thick atmosphere could force the water into a liquid ocean state.

The planet was also the smallest known exoplanet around a sun-like star, just over 5 Earth masses, though it is now thought to be just under 6 Earths. This beat the two year old previous record held by the first icy super-Earth discovered, OGLE-2005-BLG-390L b, which was found with microlensing. It was thought to have lost its status the following year to Gliese 436 c, which is also nearby and found to be transiting, but this was later retracted. It did lose its status to MOA-2007-BLG-192L b, which would lose its status to another planet in this system, planet e.

Planet g
Planet g was announced as the first almost certainly habitable planet found around another star.

Planet d
Planet d is a large Super-Earth on the outermost parts of the habitable zone. A thick atmosphere may be able to keep it warm enough to make it fairly habitable. While planet c captured headlines as a potentially habitable planet, further studies suggested that planet d was in fact a better candidate for life. After the innermost planet e was discovered, its orbit was refined and it was found to be just inside the habitable zone, and thus less likely to require a large greenhouse to maintain liquid water. After Planet e's discovery, Stephane Udry said "It is probably too massive to be made only of rocky material, but we can speculate that it is an icy planet that has migrated closer to the star. 'D' could even be covered by a large and deep ocean — it is the first serious 'water world' candidate." After the discovery of planets g and f on either side of it, its estimated mass was reduced from seven Earth masses to under six Earth masses, giving it the same mass as planet c on the inner edge of the habitability zone.

Planet f
Planet f is the second largest planet, at about half the mass of Neptune and planet b or 7 times the mass of Earth. It orbits much further away than the other known planets in the system, and is the only one at a distance similar to one of the Solar System's planets. There is enough space between it and the next planet in, d, for perhaps two small undetected planets or an asteroid belt. Though it orbits at Venus-like distances, it is a frigid world where only solid ice could exist. Given its star's weak gravity, its orbit period is larger than the Earths, taking 433 days. It likely has a thick atmosphere and it is uncertain if it has a solid surface or not. It was announced in 2010 during the fourth batch of planets discovered around this star at the same time as the "Goldilocks" planet g was found.