G is for Goldilocks

Analysis / by Lee Billings /

Gliese 581g is the most promising habitable world astronomers have found so far. But the chances of finding life there are vanishingly slim.

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Alas, as far as anyone can tell, Gliese 581’s planets don’t transit; they were discovered via wobbles. Every bit of information about them beyond their orbits and their estimated masses is based entirely on guesswork. This paucity of hard data has led the system down the “Goldilocks” path before. In 2007, Swiss astronomers trumpeted their discovery of Gliese 581c, a planet with a 13-day orbit at the inner cusp of the star’s habitable zone. But subsequent theoretical analysis suggested the planet was likely too close to the star and too hot for life. Another planet discovered by the Swiss team, Gliese 581d, has a 67-day orbit just at the outer edge of the habitable zone. It’s conceivably habitable, and has also been lavished with popular attention, but it’s rather big and cold, closer to the mass of Neptune than Earth. These two planets, one too hot, one too cold, bookend Gliese 581g, similar to how scorched Venus and freeze-dried Mars respectively lie inward and outward from the Earth around the Sun.

But just because a planet is within an order of magnitude of Earth’s mass and in a habitable orbit doesn’t mean it’s actually a place where anything could live. Discussing the Gliese 581 system in 2009, David Charbonneau, a Harvard astronomer who specializes in transiting planets around red dwarfs, summarized the associated uncertainty. “Since they don’t transit, we have no idea whatsoever what these planets are made of,” Charbonneau says. “We don’t know their true masses, and we certainly don’t know their density, since we don’t know what their size is.”

The only obvious way to gain this information about Gliese 581g would be to build a specially designed space telescope that could filter out starlight to gather the faint photons reflected or emitted from the planet itself. Even conservative cost estimates for one such observatory, NASA’s Terrestrial Planet Finder (TPF), soar into several billions of dollars. Without strong and sustained public support for its construction, support that currently does not exist, it’s hard to see how TPF or anything like could be built before, at the earliest, the 2030s. And still, according to Sara Seager, a planetary scientist at MIT, the telescope would quite possibly not be up to the task of imaging Gliese 581g. “It would be tricky,” Seager says. “The angular separation between the star and Gliese 581g is only about 24 milli-arcseconds, but designs for TPF are optimized for imaging at 65 milli-arcseconds. This planet is about a factor of three too close to its star for that.”

For comparison, a single grain of sand held at arm’s length has an angular diameter of about 150 arcseconds. That’s more than 6,000 times larger than the angular separation between Gliese 581g and its star as seen from our solar system. There is a not-insignificant chance that we will never see an image of this planet. But if we ever do, what would it look like?

Future Worlds

Imagine, for a moment, standing on the surface of Gliese 581g, and you’ll quickly appreciate just how unearthly it is. Its sun, though much smaller than our own, would loom far larger in the massive planet’s sky due to its close proximity, and light would filter down through a thick atmosphere to fall upon a landscape flattened by the world’s stronger gravitational field. But half the globe would never see the sun at all; tidal forces raised by the nearby star would quite likely have sapped the planet’s rotational energy until it spun once for every orbit, so that it perennially showed the same face to its star. One hemisphere would be bathed in light, sputtered by solar flares and harsh ionizing radiation, while the other would be forever shrouded in darkness. Perhaps only the thin ribbon of twilight encircling the planet from pole to pole would be hospitable. Driven by the temperature difference between the two sides, high-altitude winds would whip around the planet in an eternal circulating storm. Or, if the atmosphere somehow lacked sufficient amounts of greenhouse gases like carbon dioxide, it could conceivably freeze out into a massive ice cap on the dark side, rendering the entire planet uninhabitable. A global ocean could act as a heat reservoir, preserving the atmosphere, but only if sufficient amounts of water were somehow delivered to the planet during or after its formation. Any way you slice it, this isn’t exactly home sweet home; you wouldn’t want to live there.

“We might want Gliese 581g to have substantial amounts of liquid water on its surface,” says Caleb Scharf, the director of Columbia University’s Astrobiology Center. “But this is a planet formed in a very different type of system around a red dwarf star, so we have no idea whether it’s a wet world or not. Maybe it’s like Earth with lots of silicates, but it could easily be different, with more carbon and fewer metals—which would alter its geophysics, not to mention the potential types of chemistry at its surface.”

This, more than anything else, could be the most sinister surprise that a planet like Gliese 581g holds for astronomers seeking signs of life. The vagaries of planet formation around red dwarfs and other stars could possibly result in worlds with elemental abundances quite alien from our own, composed almost entirely of water, or graced with carborundum mountains and hydrocarbon rains, or even sheathed in endless annealed plains of pure iron. No one really knows whether life could arise on such bizarre planets, or what its exotic biochemistry would look like across the light-years if it did. There is a distinct risk that astronomers will one day stumble across signs of life beyond the solar system, only to fail to recognize them.

Faced with those uncertainties, and barring fundamental breakthroughs in the understanding of life’s origins on our own planet, astronomers have no other choice but to simply keep looking. “Life is such a complicated process to predict that as observers we should really just go out and measure, and not worry too much about whether it’s likely or not in any particular place,” Charbonneau says. “But, ideally, we should lavish attention on any planet where we could imagine life conceivably existing.”

At present, however, the research trend is shifting away from focusing on individual planets or even individual planetary systems. New automated planet-finding telescopes on the ground and space-based surveys like the Kepler mission are promising a bounty of many hundreds, more likely thousands, of new worlds—too many for the entirety of the world’s astronomers to thoroughly study in all their lives. “Because there were fewer planets known in the recent past, we’ve become really good at focusing on individual objects,” Seager says. “But we’re now moving into the realm of deep statistics, drawing on larger sample sizes to learn new things. This may not be as attractive to the public as studying individual objects in depth, but for answers about planet formation, orbital dynamics, and the evolution of planetary systems, that’s where we have to go.”

In the end, the most remarkable thing about Gliese 581g, this strange planet we may never see, is a matter of statistics: Gliese 581 is the 117th nearest star to our Sun. Of the other 116, less than half have been seriously investigated for wobbles that could indicate planets, and only 9 have been as thoroughly combed for small, rocky worlds as has this fertile red dwarf system. Expanding outward to the next few hundred closest stars, only about 10 more have been deeply searched for habitable planets. The survey is vastly incomplete, only spanning a few hundred out of the Milky Way’s hundreds of billions of stars, yet even so, counting our own world, two more or less habitable planets are already known.

As Vogt and Butler conclude in their paper, “either we have just been incredibly lucky in this early detection, or we are truly on the threshold of a second Age of Discovery.” And that, more so than illusory certitude and self-centered visions of other worlds just like our own, is something worth getting excited about.

Lee Billings is a staff editor for Seed. He likes space.

Originally published October 1, 2010

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