Gliese 581, and the Habitable Planet

Located 22 light years away, Gliese 581 is a diminutive red dwarf star only a third the mass of our own Sun.

Visually, it appears like many other stars – a pinprick of light on an otherwise black canvas – yet, amid the zillions of stars in the known universe, Gliese 581 stands out as unique.

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>>above: Gliese 581, as imaged by the European Southern Observatory.

More than 800 planets and exoplanets – worlds beyond our own Solar System – are known to scientists in 2013, and between three and six of them accompany Gliese 581 on its tour of the galaxy.

Imaginatively named after the famous letters, b, c, d, e, f, and g, the planets orbiting Gliese 581 are as varied as those in our own neighbourhood; rocky balls, gaseous spheres, and hellish greenhouses.

However, the attention afforded to the Gliese planetary system owes more to its apparent habitability than the colour of its gas giants; at least two of the bodies are (or have been) considered places where life could evolve.

Below, in order of increasing distance from their parent star, are the first three planets in orbit around Gliese 581.

  • Planet One; Gliese 581 e

One of the confirmed (i.e. it definitely exists) planets in the Gliese system, 581 e is the closest of the six bodies to the local star. The planet orbits its diminutive sun every 3.15 days, compared to the 88 days of our Sun’s closest companion, Mercury.


>>above: An artist’s impression of Gliese 581e. Image by tyrogthekreeper at en.wikipedia.

Ironically, in a system routinely associated with life, 581 e is about as inhospitable to critters and bugs as you can get; the planet is a rocky, radiation-blasted furnace. Viewed from the ground, the local sun would burn a hateful behemoth in the sky, ten times larger than the star we see from Earth.

Is it suitable for life? No – unless you happen to be made of asbestos concrete; in which case, fill your boots.

  • Planet Two; Gliese 581 b

Detected in 2005, making it the first exoplanet discovered in the system, Gliese 581 b is the second planet in order from its star. 581 b is likely to be a ‘hot Neptune’; that is, a planet roughly equatable in size to Neptune or Uranus, but located much closer to its parent star than its namesake.


>>above: An artist’s impression of the hot Neptune, Gliese 581b, Image by tyrogthekreeper at en.wikipedia.

Unfortunately, as 581 b does not pass in front of Gliese 581 as viewed from Earth – it does not transit its star – the planet’s composition remains largely unknown to astronomers. It does, however, continue the trend set by 581 e by having a short orbital period; just 5.4 days.

It’s interesting to note at this point that the orbits of the six planets around Gliese 581 would fit inside the orbit of Earth in our own Solar System; in fact, the planet with the longest orbital period, 581 f, at 433 days, would skirt the orbit of Venus if it were somehow transplanted into our Sun’s cosmic garden.


>>above: the comparative sizes of the Gliese 581 and Sun solar systems. Note the orbits of Venus and Gliese 581 f. Public domain image.

So, is 581 b suitable for life? Unlikely – there’s a lot of conjecture surrounding the planet but its proximity to its sun means that it could fall prey to intense solar activity, such as coronal mass ejections, as well as straightforward high temperatures.

Also, if the planet is indeed analogous to Neptune, the lack of a substantial solid surface, coupled with the intense heat and pressures associated with gas giants, could make conditions difficult for life.

  • Planet Three; Gliese 581 c

Gliese 581 c, the third from its star, is the planet that entrenched its system in the public consciousness.

Identified as a super-Earth (a term that indicates size of up to ten Earth masses, and says virtually nothing about habitability), and seemingly located within the habitable zone of its star, 581 c was one of the first planets to harbour some of the conditions for liquid water formation.

Or so it seemed; 581 c’s orbit, which takes 13 days to complete, was later revised to place it closer to its sun, a position associated with greenhouse conditions similar to that on Venus. Ironically, this super-Earth could prove to be entirely unlike its namesake.


>> above: a super-Earth, Kepler 10b, in comparison to its namesake. Image used for size comparison only; the planet on the right is deliberately blank of features, not coloured white. Image by Aldaron at en.wikipedia.

Sara Seager, of the Massachusetts Institute of Technology, intimated that 581 c could be somewhere between an iron-rich cannonball and a gaseous mass of carbon monoxide. Andrew Howard, of the University of Hawaii, was open to suggestion regarding the make up of 581 c.

“Are [super-Earths] really scaled-up rocky, Earth-like planets?” Howard told “Are they water worlds with comet-like compositions? Are they scaled-down Neptune-like planets with rock, water and atmospheres of hydrogen and helium?”

The uncertainty, once again, stems from the fact that Gliese 581 c, like the previous planet, 591 b, does not transit its star and has not been observed directly. Super-Earths seem to be inherently awkward; the mass and radius of only two such planets has been determined by scientists.

However, the idea that super-Earths are miniature Neptunes struggling to rid themselves of a hydrogen shell left over from their creation is a popular one.

So, is 581 c suitable for life? Whether the planet is a Venus-like greenhouse or a diminutive gas giant, the chances of life evolving on 581 c appear to be slim. The former case does present astronomers with an interesting thought experiment  however, as Venus may yet harbour extremophile life in its clouds.

The second part of this post is now available here.


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