Gliese 581 - an Extrasolar Planetary System Content from the guide to life, the universe and everything

Gliese 581 - an Extrasolar Planetary System

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Updated July 2014

There is not a single star in the sky about which we can rule out the existence of life, even if different from ours.
– Cardinal Nicolo Cusano (1401 - 1464)

There are so many stars in our galaxy that a long time ago astronomers decided to catalogue them. Red dwarf1 star Gliese 581 (also catalogued as HO Librae2), is approximately 20.5 light years3 distant. In galactic terms, that makes it a close neighbour. Gliese 581 is part of the constellation Libra 'the scales', and can be found at co-ordinates Right Ascension: 15h 19m; Declination: −07°43' (epoch 2000) on a star map.

A gas giant planet was discovered orbiting Gliese 581 in 2005. Then, in 2007 a further two planets were detected. They had much smaller masses than the first, and are possibly terrestrial (rocky) worlds, which is a very exciting development in the search for extraterrestrial life. Yet another rocky planet was discovered in April 2009. Two more planetary discoveries were announced on 28 September, 2010. None of them resides within the habitable zone of the star.

The Planets

The nomenclature that has been decided on for extrasolar planet discoveries is to use a lower-case English letter after the parent star catalogue number (or name), commencing at 'b': eg 'Gliese 581 b'. These letters start at b (rather than a) so there is no confusion with binary star classification. The planet designation stays with it regardless of whether subsequent discoveries are made within the same stellar system, and despite the position of any new planet relative to the star. Therefore, the first-discovered planet of Gliese 581 is Gliese 581 b, with Gliese 581 c, and so on, being detected later, even if they have tighter orbits.

Planet b

Gliese 581 b was detected on 22 August, 2005, and the discovery was announced that November. It is a hot gas giant, orbiting approximately every five days at a distance of six million km (0.04 AU).

Planet c

On the treasure map of the Universe, one would be tempted to mark this planet with an X.
– Study team member Xavier Delfosse, of Grenoble University, France.

Gliese 581 c is a terrestrial (rocky) planet; these types are classed as 'super-Earths' as they are rocky worlds bigger and more massive than the Earth. Even though it orbits quite close to its star, the planet was considered to be in the system's habitable zone because the star is a red dwarf, and therefore cooler than our Sun. This was a very exciting discovery for those interested in the search for evidence of life beyond our planet. In 2014 this theory was discounted.

Planet d

Gliese 581 d was discovered by the HARPS instrument (a high-precision spectrograph) on the 3.6-metre telescope at the European Southern Observatory in La Silla, Chile. It's also a suspected terrestrial planet, seven times the mass of Earth.

Gliese 581 c and Gliese 581 d were the first possible candidates for habitable planets ever detected.

Planet e

Yet another planet, Gliese 581 e, was discovered in April 2009. It is a rocky world unfortunately orbiting far too close to the star for life to be considered feasible.

Planet f

The discovery of two more planets in the Gliese 581 system was announced on 28 September, 2010. Planet Gliese 581 f is a terrestrial world orbiting at 0.7AU, considered way beyond the star's habitable zone.

Planet g

The second planet announced on 28 September, 2010 is planet Gliese 581 g, which has been dubbed Zarmina's World by its co-founder Prof Steven Vogt. Planet g is up to four times the mass of Earth and has a circular, stable orbit. One side of the planet has perpetual day, while the other side never sees the dawn of a new day. This is because it is tidally locked to the star, which means there's no planetary rotation. Somewhere around the terminator (the dividing area between the sunlit and permanent night sides), the temperature may be stable enough to be comfortable. Even if the daily temperature is survivable4 there are other factors which would determine whether life would be possible or not.

Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say, my own personal feeling is that the chances of life on this planet are 100 percent. I have almost no doubt about it.
– Astrophysics Professor Steven Vogt of the University of California, Santa Cruz, co-discoverer of planets Gliese 581 f and Gliese 581 g.

Planet Table

The orbital period given in the table below is the time the planet takes to orbit its parent star, which we know of as a year. The mass of the extrasolar planet is compared with that of Jupiter, our Solar System's largest planet, known to astronomers as the 'Jovian scale', and with the mass of the Earth.

catalogue number
Planet mass
(Jovian scale)
Planet mass
(Earth scale)
Orbital period
(Earth days)
Position from starYear of discoveryComments
Gliese 581 b0.0515.65.36#22005Hot gas giant
Gliese 581 c0.0165.35913#32007Super-Earth
Gliese 581 d0.0257.09166.8#52007downgraded
Gliese 581 e0.0061.93.15#12009Super-Earth
Gliese 581 f0.0237.3433#62010downgraded
Gliese 581 g0.01336.65#42010downgraded


The universe is infinite, composed of many worlds and animated by common life and common cause.
–  Giordano Bruno, author of On the Infinite Universe and Worlds (1583)

Some people who are interested in the Search for ExtraTerrestrial Intelligence (SETI) have set up a range calculator page to allow participants to calculate how long radio transmissions from potential sources might take. There is also an image of the original SETI signal (the 'Arecibo Message') devised by Dr Frank Drake5 and transmitted in November 1974 in the direction of M13, the Great Globular Cluster in Hercules. There has been no reply as yet because M13 is 25,000 light years away. Gliese 581 is a lot closer to home but a simple 'Hello' would still take over 20 years to get there.

In 2008, members of the social networking site Bebo were asked to submit messages to send to the Gliese 581 system. Around 500 were chosen, as well as some of the 20,000 tweets by Twitter members, to be transmitted via radiowaves using a radio telescope in the Ukraine. The anticipated arrival time is sometime in 2029 and, if there's anyone there capable of understanding the transmission and they RSVP immediately, we can expect a reply around 2050. If there are astronomers on Gliese 581 g then they will be able to see our Sun in the constellation we know as Cetus 'the whale', which seems appropriate considering the waterworld which resides in the habitable zone of that star.

And Finally...

The Gliese 581 system featured in the script of the 2009 sci-fi film Race to Witch Mountain starring Dwayne 'The Rock' Johnson.

Updated July 2014

Since this article was written, new data discounted the existence of planets d, f and g.

1About ¾ of all stars in the Milky Way galaxy are red dwarf stars.2The designation of capital letters is used for variable stars, ones whose brightness changes over time. HO Librae is a variable of the BY Draconis type.3A light year is the distance light travels in one year, roughly 5.88 trillion miles or 9.46 trillion km.4The temperature during the day shouldn't be too extreme.5American astrophysicist who created the Drake Equation in 1961, and founder of SETI.

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