Back in late September 2010, Astronomers made an important discovery that may eventually change the way we view our place in the cosmos.

The story begins with the all together average star Gliese 581 The star is a spectral class M3 main sequence dwarf, not dissimilar to the image on the left.

An artist's impression of an M class dwarf Credit: NASA/Walt Feimer

At roughly 31% the mass of the sun but only 1.3% its luminosity (that is taking all wavelengths into account not  just visual i.e. its Bolometric luminosity).

The star is classified as a variable star due to fluctuations in its brightness over time.

To be more specific it is a BY Draconis variable star as the variability (if confirmed as it currently falls close to the accepted margin for false detection) is expected to be caused by the presence of starspots (the general name for a sunspot that occurs on a star other than the sun) combined with the star’s rotation.

Whilst the star is itself an interesting object, it is what is orbiting it that has caused the media interest beginning in spring 2007.The star has been a target in the search for Exoplanets for some time, the excitement spread when its second planet – Gliese 581c – was revealed to sit just at the inner edge of the systems ‘habitable’ zone.

The term ‘habitable zone’ for a start deeply aggravates me as it means only that the area is neither too hot or too cold, I would prefer it to be known as the ‘temperate’ zone but perhaps that is just me (18 going on a grumpy 60 year old).

Gliese 581 Credit: Digital Sky Survey / ESO

The word habitable implies that the world is suitable for life in every way not just one. Professional astronomers refer to this zone as the ‘Goldilocks zone’ instead as this only refers to the distance from a star that liquid water could theoretically exist on the surface of a terrestrial (rocky) world. Without leading to the assumption that as planet x is y km from its star then it must have life. All this position means is that planet x is in the most likely area of its system for a planet to have liquid water.

The attention soon waned as it became clear that the planet is likely to have a runaway greenhouse effect creating a Venusian world far too hot for life.

The media hype began again with the discovery of the Gliese 581d which sits at the very edge or just outside the Goldilocks  zone and so can be expected to be similar to Mars. Then interest peaked again with the discovery of Gliese 581 e which despite sitting very close to the star was the exoplanet with the closest mass to that of the Earth yet discovered with a minimum mass of 1.94 Earth masses.

The current media extravaganza is centred on Gliese 581 g a planet that sits well within the Goldilocks zone, and within the acceptable mass limits for a stable terrestrial planet meaning that it COULD Potentially be suitable for life. However as I have mentioned the issue is far more complex than just having a planet at roughly the right distance from a star.

The Gliese 581 System. Showing each planet's orbital distance in relation to the projected location of the habitable zone and relative to our own Solar System. Credit: ESO derivative work: Henrykus

Yes could be a life bearing world but there is no proof either way quite yet.

Well as the planet is sitting just 20.3 light years away from us, if it indeed harbours life it would go some way to showing just how common life is likely to be within the universe.

However a spanner may have been thrown into the works.

All exoplanet detections must be confirmed, i.e. by the detection of the planet in more than one dataset. So far Gliese 581 g (and Gliese 581 f, announced at the same time though more mundane as it falls well outside the habitable zone and is expected to be similar to Neptune or a super terrestrial planet) has only been detected in one set of measurements; a set of combined data from HIRES spectrometer on the Keck telescope in Hawaii and the HARPS instrument on the ESO’s La Silla Observatory in Chile.Later measurements taken by HARPS failed to detect the planet, so it may not exist at all.

Though as Martin Rees once said, “Lack of evidence is not evidence for absence.” Who knows the planet may yet be confirmed in later measurements though that’s not quite the end of our story.

During the buzz of attention surrounding the discovery of Gliese 581 c, a radio transmission was sent to the system containing messages selected by users of the social networking site Bebo.The transmission was sent in 2008 and will reach the planet in early 2029 with the potential for a reply from any intelligent life on the planet by 2050.

A member of the Galaxy Zoo Forum (Djj) wrote this limerick for my Object of the Day on the same subject,

“Send a message in radio mime
That we’re here and we’re still in our prime;
Gliese five-eighty-one
Might be sombody’s sun —
We could hear back in forty years’ time!”

I will end this post with one of the more indulgent artist’s impressions of Gliese 581 g:

Gliese 581 g Credit: NASA

For our more technically minded readers, you can obtain the original announcement paper for Gliese 581 g here

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