Does Gliese 581 have a planet in the habitable zone?

After Steven Vogt and his team announced the discovery of a planet in the habitable zone of Gliese 581 in late September 2010, other astronomers failed to confirm the discovery. Yet, as Dr. Vogt told EarthSky, the failure to confirm does not mean the planet is not there. Dr. Vogt told EarthSky in October 2010 that he stands by his discovery announcement.

Steven Vogt: Life seems to find a way to thrive. And this planet, as far as we can tell, has all the right conditions for that to happen, so that makes me very optimistic.

Steven Vogt is an astronomer at the University of California, Santa Cruz. In September 2010, Dr. Vogt and his team announced the first planet orbiting in the habitable zone – the zone around a star where a planet might support life as we know it. As of now – in early October 2010 – no other astronomers have confirmed this possible planet – called Gliese 581g. Still, the planet might be there – and if so Dr. Vogt believes conditions are good for it to support life.

Steven Vogt: Life is pretty aggressive in getting a toehold given just half a chance, and it can do it over and over again under the most dire of circumstances.

Vogt believes habitable planets might be common.

Steve Vogt: The Earth is probably not a freak – a place that’s like no other. I mean it’s a very special place, obviously it is. I think there are a lot of places that are similar and maybe even more amenable to life out there in our own galaxy, like the Earth. And that’s a pretty amazing thing to imagine.

A cool characteristic of Gliese 581g – if it exists – is that it always keeps one face toward its star. Because of this, there would be a belt of perpetual twilight entirely encircling the planet, separating the “day” and the “night” sides. Vogt said this twilight zone would be the most hospitable part of the planet for life.

A key instrument for planet searching – the High Accuracy Radial Velocity Planet Searcher, or HARPS, at the European Southern Observatory – which is dedicated to the discovery of extrasolar planets – has not been able to confirm the presence of Gliese 581g. The evidence for it is found in the light of the star itself.

Steven Vogt: We look at the host star that has the planets going around it, and we watch this star wobble due to the gravitational pull of the planets. And we follow that wobble month after month, year after year, watching it repeat over and over again. That movement is caused by the planet orbiting the star.

In other words, though no one has directly seen the planet Gleise 581g, astronomers know it exists because of how the light of its star behaves. By studying the orbits of the other planets in this system, Dr. Vogt and his team believe they can predict the presence of water on Gleise 581g.

Steven Vogt: One of the planets is in a 5.7-day orbit. It’s basically a hot version of our Neptune. And we know that it couldn’t have formed in that close to its star. So we know it would have had to form farther out in the Gliese 581 system and migrated in. And we think that all of these planets migrated in together. That means they would have all formed out at place beyond the so-called “snow line” or “ice line” where things like water exist in icy form. So they would have carried their water in with them.

October 18, 2010

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