Gliese 581 and the Eyeball Earth

Here are the final three planets in the Gliese 581 solar system, a realm of space characterised by its apparent abundance of places that life could call home. If you haven’t read the previous post yet – Gliese 581 and the Habitable Planet – please check it out!


>>above: the orbits of the six Gliese 581 planets compared to that of Mercury, Venus, and Earth.

  • Planet Four; Gliese 581 g

Gliese 581 g is the archetype of the elusive paradise; while initial observations intimated an Earth-like planet in the habitable zone of its star, subsequent investigation revealed only empty space.

The European Southern Observatory would later revise down the number of confirmed planets in the Gliese 581 system from six to four, insisting that the presence of the super-Earth, 581 g, and its distant companion, 581 f, were illusions created by the behaviour of the local sun.


>>above: This artist’s depiction of HD 85512 b could also be a close approximation of the super-Earth, Gliese 581 g. Image by the European Southern Observatory.

Confirming the existence of Gliese 581 g – known informally as Zarmina – would be a landmark discovery for astronomers.

Orbiting 0.146 AU from a cool red dwarf (between the Sun and Mercury in our own solar system), the surface conditions of 581 g could prove conducive to the formation of liquid water and, potentially, complex lifeforms.

Oddly, despite its disputed status, a wealth of measurements exist for the mysterious world. The radius, mass, and even the strength of 581 g’s surface gravity are known to science. The planet is believed to be a super-Earth with a rocky body.

It could have an atmosphere somewhere between that of Earth and Venus, with temperatures in the range of −37 to −12 °C.

It’s worth mentioning here that, in addition to the ‘garden world’ depiction, above, Gliese 581 g could also be the model of a new planet type, the ‘eyeball Earth’. These peculiar bodies are tidally-locked, meaning that one side of the surface always faces the local sun, while the other basks in a permanent night.


>>above: Gliese 581 g as an ‘eyeball Earth’. The planet’s two halves are permanently exposed to the hot and cold, respectively. Note the temperate belt between the two regions. 

This bizarre dichotomy of hot and cold could prove beneficial to life, as alien critters could thrive in the ‘transition zone’ between the two extremes, a realm watered by melting ice on one side, and heated by the warmth of the eternal desert on the other.

Interestingly, eyeball Earths could themselves represent something of a transition between types. “A little bit closer to the star — that is, hotter — they would completely thaw and become water worlds; a little bit further out in the habitable zone — that is, colder — they would become total ice balls”, explained Daniel Angerhausen, of the Rensselaer Polytechnic Institute.

Angerhausen, who is currently leading a research project into planets like Gliese 581 g, added that eyeball Earths, whether “water, eyeball or snowball” are likely to be the first truly habitable planets discovered by astronomers.

So, is 581 g suitable for life? Absolutely. Steven Vogt, a member of the Lick-Carnegie Exoplanet Survey, the University of California team that discovered the planet, believes that life is a certainty on Gliese 581 g, whether it’s an eyeball planet or a standard super Earth: “the chances of life on this planet are 100%. I have almost no doubt about it.”

Of course, the fact that Zarmina may not exist would be a blow to anything currently living on the planet.

  • Planet Five; Gliese 581 d 

The descriptions of Gliese 581 d you can find on the internet conjure up images of blue skies, bright sunlight, gentle rainfall, and an eternal ocean. Unfortunately, the reality is likely to be far more disappointing.


>>above: an artist’s impression of Gliese 581 d. The image appears to depict a super-Earth with a thick atmosphere. Image by Tyrogthekreeper.

Located near the outer edge of its star’s habitable zone, Gliese 581 d is yet another planet in the system identified as a potential harbour for life.

However, with no direct measurements possible, and atmosphere detection still in the realm of science fiction, accurate information for 581 g is scarce. The fact that it may not be there at all (having ‘proposed’ status, as of September 2012) just compounds matters further.

Variously described as a water world, an ice planet, and a small gas giant, a list that highlights just how much of 581 g’s nature is in the realm of imagination, the fifth planet is believed to be a super-Earth around seven times more massive than its namesake.


>>above: a size and orbit comparison chart of the Gliese 581 and Sun solar systems. Image by the European Southern Observatory.

The planet’s size means that it could potentially have an atmosphere of thick carbon dioxide clouds, a trait necessary for any life to take hold; it would likely need a pronounced greenhouse effect to warm the surface and to prevent any liquid water from freezing.

As the planet receives only a fraction of the energy from its sun as the Earth does (Gliese 581 is a red-dwarf star, much cooler than our Sun), the nights may simply be too cold for life to evolve.

So, is 581 d suitable for life? If 581 g is indeed a rocky body, it could be another eyeball Earth, and a potential home for extraterrestrial life. The planet’s position in the habitable zone is favourable but conditions suitable for alien monsters hinge on the density of its atmosphere.

  • Planet Six; Gliese 581 f

Astronomers seem resigned to the fact that Gliese 581 f doesn’t exist, an acceptance that would explain the almost complete lack of information about this enigmatic body.

Located 120 trillion miles away from Earth, the orbit of 581 f is relative to that of Venus in our own Solar System. However, around a star as diminutive as Gliese 581, this planet is a mysterious outsider, with perhaps more similarities to Neptune and Uranus than our local greenhouse.

It is likely to be very cold.


>>above: An artist’s impression of Gliese 581 f as a planet with a thick atmosphere, possibly a gas giant. Image from Celestia.

Exo-planets are usually detected via radial velocity measurements, a procedure by which the spectrum of a star like Gliese 581 is observed for variations over time. Those changes can sometimes be attributed to the presence of an orbiting planet. This method was instrumental in the discovery of 581 f in 2010.

Scientists have since come to the conclusion that solar activity may have produced a false-alarm; in other words, the behaviour of Gliese 581 made it look like a planet was present when, in fact, there was none.

So, is 581 f suitable for life? in August 2013, 581 f remains a member of the Gliese 581 solar system – at least until further investigations can rule it out. However, even assuming that it is a real planet, this lonely sphere is unlikely to harbour life; it may even be a gas giant, akin to Neptune.

So, there you have it.


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