Science Showdown

Exposing the scientifically Good, Bad and Ugly

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Gliese 581 g “Habitable”?

For the first ever post on Good, Bad and Ugly Science, we look at the recent discovery of Gliese 581 g and some extraordinary extrapolation to certainty about extraterrestrial life.

We are in a golden age. One day very soon (May 2011 is the current estimate), we will discover a planet with the mass of the Earth which orbits its host star in the Habitable or “Goldilocks” Zone, and has a favourable atmosphere, water and maybe even a large moon and a regular diurnal cycle.

There’s just one problem. Just because a planet is in the habitable zone, and has a mass similar to the Earth, doesn’t mean it’ll be at all Earth-like. And we have a really good example of a planet like this: Venus.

The most Earth-like planet we've ever discovered. Apart from Earth.

Venus is extremely similar to Earth. Its radius is 95% that of Earth, its mass, 81.5%. It has a very circular orbit. It has a long year (224 Earth days) and though it’s outside of the Habitable Zone, it’s not that far out (distance from Sol is 0.723 vs 0.725 AU). But with surface temperatures at around 460 °C, Sulphur Dioxide clouds that rain sulphuric acid through at least sections of the atmosphere, and surface pressure 92 times Earth’s, habitable it is not. Remember how similar, yet dissimilar Venus is to Earth when you read anything about “Earth-like” planets.


There seems to be an idea that an Earth-sized planet orbiting a star other than the sun, in the habitable zone, will be, well, habitable. Take for example the latest discovery of the rocky-planet Gliese 581 g. has a brilliant rundown of the Gliese planetary system, which contains several presumably rocky planets with masses 2-8 times the mass of Earth. Phil Plait hits us with the fact here: Possible earthlike planet found in the Goldilocks zone of a nearby star!

Gliese 581 g has an modelled (estimated!) surface temperature of between -31 and -12 °C, which although below freezing, means water could exist in a liquid state in places on the surface. It has a mass of >3 Earths and an orbital frequency of 37 days around its small parent star. Its close proximity to its star also means it is more than likely tidally locked, with one side of the planet always facing its star.

This is an exceptionally interesting. Given the size of the Milky Way, and how quickly we found Gliese 581 g, there could be millions of planets with these “Earth-like” properties.


'New Earth is Discovered' (The Sun)

Astronomers find another Earth …just 20 light years away (The Mirror)

Is this the new Earth? Astronomers discover planet just 20 light years away with similar atmosphere and gravity which has ‘100% chance of life’ - and may contain water (Daily Mail)

Chances Of Finding Life On Planet ‘Are 100%’ (Sky News)

Gliese 581 g cannot be called a habitable planet. It is not a second Earth. It is more than likely not somewhere you could comfortably live (remember the Venus example). It is not somewhere we should send colonisers with all haste. 

We don’t know if there’s an atmosphere. We don’t know if there’s any water. We don’t know for sure that the surface isn’t a magma ocean. We know its mass and orbital distance. That’s it.


But wait! It’s not the media’s fault. In this case one of the researchers, Professor Steven Vogt, interviewed by AP said the following:

We don’t have any direct way to sense if there’s life there. My own personal opinion, it would be hard to imagine that life has not taken hold there.

He has no way to tell, but he’s pretty sure? Facepalm. You can’t say that. As “the expert” your opinion matters. People can’t tell when your opinion is ridiculous.

Here’s the YouTube video:

Apart from the Earth, Venus is the most Earth-like planet ever discovered. Bare that in mind when you hear the next claim of an Earth-like planet being discovered.

  1. scienceshowdown posted this