What exactly does the word ‘habitable’ mean? The question comes to mind because of two things, the first being the media buzz over Gliese 581g, now widely described as the first potentially habitable planet we’ve found. The second is Paul Davies’ presentation yesterday at the International Astronautical Congress in Prague, where Davies was careful to differentiate between ‘habitable’ and ‘inhabited.’ More on the latter in a moment. Let’s look first at this outstanding find, two new planets in the Gliese 581 system discovered through the unflagging efforts of the Lick-Carnegie team.

A World in the Zone?

The beauty of Gl 581, of course, is not only that it has yielded a storehouse of planets (six known at present), but that these worlds are on nearly circular orbits, and several have caught our eye re habitability before. The current buzz seems a bit tamer than the one that greeted the announcement of Gl 581c, at the time thought to be capable of sustaining liquid water on its surface, although later research has shown that the planet is much more likely to be a Venus-like heat trap. Gl 581d, too, has its supporters, who argue that given the right atmospheric composition, temperatures there might be sufficient to sustain life.

But Gl 581g looks to be the most promising of the bunch, with a mass between three and four times that of Earth, a rocky world with enough gravity to hang onto an atmosphere and surface temperatures that average between -31 and -12 degrees Celsius. That’s cold, of course, but actual temperatures here are going to depend on where you happen to be, the planet being tidally locked to its star. I like what Stephen Vogt (UC-Santa Cruz) said at the press conference announcing the finding. Vogt talks about ‘eco-longitudes,’ meaning that on a tidally locked world like this, there will be temperature zones that follow the longitudes, from very hot at the center of the star-facing side to freezing on the dark side. Let me quote him briefly:

“If you are in the most comfortable place on this planet, which would be on the terminator, you would see the star sitting on the horizon. an eternal sunrise or sunset, depending on whether you are an optimist or a pessimist. Over billions of years, there would be stable zones where the ecosystem stays at the same temperature. You have ‘eco-longitudes,’ so that if you have evolved to like hot zones, you move a bit toward the star side, and if you like cold zones, you move toward the shadow side. And it stays like that more or less forever, so there are a lot of different niches for different kinds of life to evolve.”

The ‘goldilocks’ metaphor is being used all over the place to describe this planet’s orbital placement, a comparison Vogt was quick to exploit (this from a UC-Santa Cruz news release):

“We had planets on both sides of the habitable zone–one too hot and one too cold–and now we have one in the middle that’s just right… Any emerging life forms would have a wide range of stable climates to choose from and to evolve around, depending on their longitude.”

Tidal Lock and Open Questions

I’ll never get over the fascination of places like this, where the sun never moves in the sky and patterns of light and shadow are permanent, so that to move through what we consider a day-like cycle of light and dark would require physically moving along the surface of the planet. The terminator region between dark and light may be the most habitable region, though our modeling of atmospheres on such worlds is an ongoing thing, and it was only in the late 1990s that we began to realize, thanks to work by Manoj Joshi, Robert Haberle and team at NASA Ames, that a tidally locked planet might sustain regions stable enough to support life. Other questions, particularly that of solar flares on frequently active M-dwarfs, remain open.

Vogt talks about 238 observations with a precision of 1.6 meters per second being required for the radial velocity observations that found this world, and in the news conference, he pulled out a telling analogy to explain their precision. Use exquisitely sensitive calipers to measure a 6-inch ruler. Now stand the same ruler up on its end and measure it again. It will be infinitesimally shorter because it shrinks under its own weight, the effect of gravity. That tiniest of all changes is about on the order needed to find a Gl 581g amongst the data. It’s superb work, combining HIRES spectrometer (which Vogt designed) observations with published data from the Geneva group, and it is the result of fully eleven years of study of this interesting star.

The Nature of Habitability

But let’s get back to that question about habitability. First, Vogt is of the opinion that finding GL 581g this early in the planet hunt (with some 500 exoplanets now found) is a telling feat:

“If these [habitable worlds] are rare, we shouldn’t have found one so quickly and so nearby. The number of systems with potentially habitable planets is probably on the order of 10 or 20 percent, and when you multiply that by the hundreds of billions of stars in the Milky Way, that’s a large number. There could be tens of billions of these systems in our galaxy.”

Encouraging indeed. But I’m also reminded of Paul Davies’ comment yesterday in Prague. Davies was addressing the change in opinion in the scientific community, which has in the past fifty years moved to support the idea of extraterrestrial life. In the paper on which his presentation was based, Davies writes “…to profess belief in extraterrestrial life of any sort, let alone intelligent life, in the 1960s and 1970s, was tantamount to scientific suicide. One might as well have expressed a belief in fairies.” But all that has changed with new discoveries, leaving many scientists convinced of life’s ubiquity. Davies takes a more cautious view.

We might, for example, call a planet ‘habitable’ because it can support liquid water at the surface. But is this enough? Again from the Davies paper (and all the italics are his):

Water does in fact seem to be abundant in the solar system and beyond, so (it is reasoned) life should also be abundant. Unfortunately this simplistic reasoning confuses a necessary with a sufficient condition. To be sure, liquid water is necessary for life (at least as we know it), but it is far from sufficient. The reason life on Earth inhabits almost all aqueous niches is because Earth has a contiguous biosphere, and life has invaded those niches; it has not arisen there de novo.

Davies is concerned less about life’s ability to adapt to extreme conditions than about the likelihood of its formation in the first place. Indeed, this is the question that so confounds our pulling meaningful information from the Drake Equation, in his view. He goes on:

Another reason given for the current optimism about life beyond Earth is the dawning recognition that life can survive in a much wider range of physical conditions than was recognized hitherto, opening up the prospect for life on Mars, for example, and generally extending the definition of what constitutes an “earthlike” planet. But this at most amounts to a factor of two or three in favor of the odds for life. Set against that is the exponentially small probability that any given complex molecule will form by random assembly from a soup of building blocks. In short, habitability does not mean inhabited. It is natural that we should concentrate on the habitable planets in our search for life – by the “keys under the lamppost” principle -but at this stage we cannot put any level of confidence – none at all – on whether such a search will prove successful.

Cause to Celebrate Nonetheless

So there you are, a cautionary note in an otherwise celebratory morning (and Davies, of course, wasn’t aware of the Gl 581g news when he wrote this). But even as I understand Davies’ position, I have to say that my own mood is much more upbeat. We have much to learn, but what an achievement this discovery is, showing our capability of finding planets of near-Earth mass, planets likely to have atmospheres, and surface conditions that could sustain water in liquid form. Whatever really exists on Gl 581g, it’s a fact that this discovery points toward near-term discoveries of planets much more like our own, around G and K stars at distances where tidal lock is not an issue. We still have no ‘second Earth’ but it’s only a matter of years (perhaps even months) before we find one.

And let’s not forget the second planet in the discovery announcement. No question of habitability here, but we should note the existence of Gl 581f, at a minimum-mass seven times that of the Earth, and orbiting at 0.758 AU with a period of 433 days. Gl 581g is going to get the lion’s share of attention, but we’re moving into the era of characterizing entire planetary systems and that in itself is worthy of celebration. It will ultimately teach us much about planetary formation and help us refine the strategies we need to bag that first unquestionably ‘Earth-like’ planet.

The paper is Vogt, Butler et al., “The Lick-Carnegie Exoplanet Survey: A 3.1 M_Earth Planet in the Habitable Zone of the Nearby M3V Star Gliese 581,” accepted by The Astrophysical Journal and available as a preprint.