Gliese 581 g: A potentially habitable planet

One interesting news item at the end of last month was the announcement of the discovery of a planet in the habitable zone of a star other than the Sun. This was exciting news for many reasons, though not too surprisingly a lot of journalists (and even the discoverers themselves) went a little overboard in their descriptions of what had actually been found, and even when they didn't, many readers (judging from comments some made on the articles) weren't quite able to grasp all the details. Many other people (particularly those who didn't do more than glance at the articles and those who only saw the news mentioned on TV) no doubt ended up with a completely mistaken understanding of what had been found. I wouldn't be surprised in the least if there are some people who think the astronomers actually found alien life or even an alien civilization. This is far from the case, though the discovery is an important milestone in the search for alien life nonetheless.

Gliese 581 is a red dwarf star located about 20 light years from our solar system. As interstellar distances go, this is not very far; the closest star other than the Sun is 4.2 light years away, while the center of the galaxy is about 25,000 light years away. In fact most of the stars that we can see in the sky without any optical aids are farther away than Gliese 581. We can't see Gliese 581 without a telescope, however, since, like all red dwarfs, it is very faint and cool (the closest star other than the Sun, Proxima Centauri, is also too faint to be seen by the naked eye).

In 2005, a planet was discovered around Gliese 581, and since then five more planets have been found, including this latest one, giving it the third most known planets of any star (the Sun being first). What makes this latest planet, designated Gliese 581 g, special is the fact that it orbits in the middle of the star's habitable zone. This refers to the distance from the star at which a planet might be expected to have surface temperatures above 0 degrees Celsius but below 100 degrees Celsius, and thus could potentially have liquid water on its surface. The habitable zone varies for each star, being closer in for small, cool stars like Gliese 581 than for stars like the Sun. Gliese 581 g orbits at a distance of 0.146 AU from Gliese 581. One AU (Astronomical Unit) is equal to the average distance between the Earth and the Sun, and so 0.146 AU is a distance of about 22 million kilometers. It takes Gilese 581 g only 37 days to orbit its star, compared with a little over 365 days for Earth and 88 days for Mercury, the planet closest to the Sun. Since it is so close to its star, one might think that it would be inside the habitable zone, making it too hot to support life, as Mercury and Venus (though only barely in the latter's case) are in our Solar System. But as noted, the habitable zone for Gliese 581 is much closer to the star than ours is to the Sun, since it is much cooler. In fact, another planet in the same system, Gliese 581 d, is 0.218 AU from the star and orbits in 67 days, still much less than Mercury, and yet is believed to be on the outer edge of the habitable zone (like Mars in our Solar System).

Though hundreds of planets have been discovered outside our solar system, only one previous discovery was clearly in the habitable zone of its star, the planet 55 Caceri f. However, this planet is a gas giant similar to Neptune or Saturn in our Solar System, and so as it is not thought to have a solid surface, it couldn't have liquid water or Earth-like life (on the other hand, if it has a large enough moon, the latter could have liquid water and life). Gliese 581 g is believed to be a “super-Earth”, a large rocky planet with a mass of 3 to 4 times that of Earth and a diameter of up to twice Earth's. So it is the first Earth-like planet that is situated right in the middle of its star's habitable zone (other than the Earth itself), and that's what makes the discovery exciting.

On the other hand, it is important to remember that there is a great deal that we don't know about this planet. Though it is in the habitable zone, we don't actually know that it even has water in any form, nor do we know if it has an atmosphere. This latter point is important, as an atmosphere is necessary to retain heat and bring the planet's surface temperature above the freezing point of water. This is also the case on Earth, as without the greenhouse effect caused by our atmosphere the planet would be permanently frozen. Given Gliese 581 g's large mass in comparison to Earth it seems probable that it should have an atmosphere, possible one thicker than Earth's, but we don't know this for certain.

Also worth noting is Gliese 581 g's rotation period. Since it is relatively close to its star, it is probably tidally locked, like the Moon is with respect to Earth, so that its day is equal to its year in length and one side of the planet is constantly turned towards the star (and the other side is always turned away). This means that one side of the planet would be very hot and the other side very cold, though an atmosphere (if there is one) would ameliorate this to some extent. So it would actually be the band around the planet on the edge between the day and night sides (the “twilight zone”, so to speak) that would have the greatest potential for life. On a planet this size, this would still be a fairly large area, but it's not quite the same as an entire planet that is habitable. (Though it has also occurred to me to wonder if the tidal lock might not be more like that of Mercury, which rotates 3 times for every 2 revolutions about the sun, rather than a 1:1 lock; if so, the temperature extremes would be somewhat less. But perhaps the tides in this case make a 1:1 lock certain – I don't know the physics and math well enough to check this myself).

I should also point out that we haven't actually seen this planet, or for that matter the vast majority of other 500 or so known extrasolar planets. There are no pictures of it, even pictures showing it as a tiny dot. This is because it is so dim compared to its parent star that the light from the latter makes the planet invisible, even to our best telescopes. Only very large planets which orbit at a significant distance from their stars and yet are hot (and so emit a lot of infrared radiation) can actually be imaged directly. So how do we know the planet exists? As a planet orbits a star, its gravity tugs on the star, causing it to move towards or away from us. This causes a slight shift in the light of the star (due to the Doppler effect). Analysis of these shifts in the star's light, tiny though they are, allows astronomers to detect planets and even determine certain of their characteristics (such as mass, distance, and period of revolution) fairly precisely. Of course this takes repeated observations so that there is sufficient data. In fact recently another group of astronomers were unable to detect Gliese 581 g in their own analysis of the data, though apparently they only used one of the data sets that the discoverers use, and the latter have stated that both data sets are necessary to detect the planet with any reliability. But while we can still say the planet probably exists, this is a reminder of how little we actually know about it.

Of course given all the above caveats, it should be clear that we don't really know for certain that Gliese 581 g is habitable, even though it is in the habitable zone, much less whether it actually has life. It has the greatest potential for having some type of life of any planet we've discovered outside the Solar System – that's what everyone is excited about – but it doesn't necessarily have any life, let alone intelligent life (and even if it is habitable for life "as we know it", it might not be habitable for us -- at least without a lot of work). In fact, there are several places in our own Solar System other than Earth which are as likely to have life based on what we know now (namely Mars and Europa), though if we were able to confirm that Gliese 581 g had water and an atmosphere it would have to be considered a more likely candidate, at least for larger life forms (it's highly unlikely that Mars has any life larger than microbes, if that, and while Europa might have larger life forms, we know too little about how life gets started and evolves to be confident of this, since on Europa any life would have to have formed under the ice that covers the moon – unlike on a watery Gliese 581 g, where it could live on the surface).

Unfortunately it will be some time before we know anything more about Gliese 581 g, other than confirming its existence. To actually detect and analyze the planet's atmosphere, we would need better telescopes, such as the proposed Terrestrial Planet Finder. Unfortunately, this has so far been a victim of insufficient funding for NASA (yet another example of why NASA should be getting more money, as I have argued before). But another reason Gliese 581 g is significant is simply that it shows there are Earth-like planets in the habitable zones of stars other than the Sun, and so we can probably expect to find more. After all, there are more than a hundred billion stars in the galaxy, and even within the relatively close range of 100 light years there are thousands (though such stars are only close in a relative sense; none of our current spacecraft can travel even a few light years in less than a few centuries). Many of these stars resemble the Sun much more closely than Gliese 581, which as noted above is relatively small, faint and cool, and so any planets they have in the habitable zone would not have the tidal locking problem. Even if Gliese 581 g doesn't have life, or if it only has very simple life, we now have to consider it more likely that there is a planet out there somewhere with more, and that we will find it eventually.