Finding a Habitable Planet

As the world excitedly speculates about the possibility of life on Gliese 581g, researchers are still trying to confirm the planet's existence.

On September 30, a U.S.-based team of researchers led by Steven Vogt, reported that they had discovered a habitable planet around another star. The obscurely named Gliese 581g supposedly sits at just the right distance – in the so-called habitable zone – from its star to possibly have liquid water on its surface. Think Goldilocks: not too cold, not too hot, but just right.

But two weeks later, questions about the so-called discovery are beginning to surface. Is there really a planet there, or is it just a remarkable coincidence of noise in their data?

Gliese 581g is actually the sixth planet reported in the Gliese 581 system. All the planets are packed in close to the system’s central star (a picture can be seen here). In fact, the reported Gliese 581g takes only 37 days to orbit its star (which means that in Gliese 581g “years,” you‘re ten times older than in Earth years).

So why has the star’s existence proven so difficult to confirm? First, it’s incredibly hard to see planets around other stars. Think of trying to see a glow worm in front of 100 of the world’s brightest lighthouses. This method of detecting stars – “direct imaging” – is in its infancy, although Canadians are leading the way. But of the 492 planets we’ve discovered so far, the vast majority have been found using one “indirect“ technique in particular, rather unromantically known as the “radial velocity method.”

The mutual attraction between planets and stars makes both of them orbit around a common point – the so-called barycentre of the system. For the planet, the orbit is very large, while for the star, it’s tiny. For the Sun-Earth system, for example, the Sun's orbit around the barycentre is only 0.06 per cent the radius of the Sun. However, that small movement is enough to make the star appear to “wobble.” It’s this type of motion that we can detect with high-precision measurements

But if a number of planets orbit around a star all the different tugs act together. The motion of the star thus becomes very complex. While there are analysis techniques to separate each of the tugs, you need good data and lots of it. In Gliese 581g’s case it took observations over 11 years, and a combination of data taken with one of the Keck telescopes and also from the HARPS (High Accuracy Radial Velocity Planet Searcher) instrument in Chile.

The problem of picking out orbits in this data is a bit like listening for a single note in a noisy crowded room. You have to filter out the noise, but you can’t do it perfectly, and sometimes noise will mimic what you are trying to detect.

Last week some of the best minds in the field of extrasolar planets were in Torino, Italy for a symposium of the International Astronomical Union. And the experts asking were asking: “Is there enough data to make the 581g claim?” Members of the HARPS research group, who contributed some of the data used in the Vogt et al study, say they cannot confirm the results of that study in their data. But that doesn’t mean it isn’t there.

Also questions about the certainty of the initial result have been raised. In a paper published in 2008, led by the brilliant Andrew Cumming of McGill University, a test to determine the possible inaccuracy of data was outlined: the “false alarm probability.” Using this number, planet-hunters can estimate the likelihood that their discovered planet is real, or a production of noise in their data. Some experts have questioned whether Vogt et al underestimated the false alarm probability.

Given the prestige associated with finding the first potentially habitable planet outside the solar system it is no surprise that so much hype has surrounded this possible discovery. Vogt even got flak for saying he was “100 per cent certain there is life” on Gliese 581g. That statement, probably given under “duress" of journalists' questioning, does seem inappropriate in retrospect. How can you be certain there is life if you aren’t even 100 per cent sure the planet is there?

For the moment, exoplanet.eu, an official encyclopedia of exoplanets, has placed Gliese 581g in its “unconfirmed” category. There’s little doubt this uncertainty will ensure telescopes will revisit Gliese 581 over the coming months and years. Only more data will solve this problem.

It’s hard to believe only 15 years ago we didn’t know of any planets around other stars. Now we seem to find them wherever we look. But to borrow the language of gold prospecting, Gliese 581g could turn out to be a flash in the pan.