Gliese 581g: Coolness in Context

Potentially habitable exoplanet discovered

Words: Maggie Koerth-Baker – Art: Lynette Cook

There is a planet in orbit around a red dwarf star, roughly 20.5 lightyears from Earth.

It's larger than our home world. But not by much, relatively speaking. Computer models predicted that it was probably a rocky planet, or at least one covered in oceans. No gas ball floating in this patch of space. When astronomers announced the discovery, the planet became a media sensation, thanks to its location in the habitable zone—an orbit far enough from its star that water wouldn't boil away, and close enough that it wouldn't freeze. Dubbed a "Goldilocks planet", experts referred to it as the "X" marking a home for alien life on a treasure map of the Universe.

The experts turned out to be wrong (link). Within two months, new calculations showed the planet was closer to its star than everyone had thought. Goldilocks was decidedly too hot. That was in 2007. The planet: Gliese 581c.

The first half of the story was replayed last week, as a team of astronomers with the Lick-Carnegie Exoplanet Survey (link) announced the existence of Gliese 581g—a planet they now think harbors the sanctuary its neighbor 581c turned out to be too sweltering to hold. This discovery is important, don't get me wrong. Gliese 581g is no outer-space equivalent to Ida the Overhyped Monkey Fossil (link). But you can't separate coolness from context. And the context is this: The first paper is not the last word. The initial findings, for any scientific discovery, are always subject to change.

I asked Vogt about that [100% chance of life] quote. He says it's come back to bite him, but he stands by what he said. At least, as long as the proper context is understood.

A Sure Thing?

Gliese 581g, which rhymes with "Lisa," really is a big deal. We're talking about a planet that, at first glance, seems to be roughly Earth-sized—which means there's a good chance that it has a rocky, solid surface—and is located in an orbit that would give it a reasonably survivable temperature range. In fact, it's pretty much smack in the center of that habitable zone, which means that it would take large errors—much larger than those that doomed Gliese 581c—to knock it out of the running.

All of that adds up to Gliese 581g being the best place we've yet found to look for extraterrestrial life. But that's quite a bit different from saying we're likely to find life there, say astronomers outside the Lick-Carnegie team. For one thing, the whole point of astrobiology is figuring out whether Life As We Know It is the same thing as Life, said Jim Kasting, Ph.D., a geoscientist with Penn State University. (link) Based on our limited view of the Universe, it's reasonable to assume that Life goes along with rocky planets that have an atmosphere and water. But we don't know that's true. We study places like 581g to test the hypothesis.

Second, Gliese 581g is in the habitable zone, but that doesn't mean we know it's habitable. You have to remember that nobody has actually seen this planet. Any time we're talking about the discovery of distant worlds, what we're usually talking about is evidence collected using a technique called radial velocity, said Manfred Cuntz, Ph.D., associate professor of physics at the University of Texas at Arlington. (link)

"You don't see the planet. What you're seeing is the way the planet's gravity causes wobbles of the star," he said.

The indirect evidence can tell you a lot, but not whether there's water or an atmosphere. That all makes the money quote that goes with this story a little hard to swallow. You've probably heard about this. Steven Vogt, Ph.D., professor of astronomy and astrophysics at the University of California Santa Cruz (link) and lead author on the paper announcing the discovery of Gliese 581g (link), told a press conference audience, "My own personal feeling is that the chances of life on this planet are 100 percent."

I asked Vogt about that quote. He says it's come back to bite him, but he stands by what he said. At least, as long as the proper context is understood. Namely: This is just his opinion, a gut feeling. It's not what the peer-reviewed paper said. In fact, there's no way to support that statement with evidence. But when Vogt looks at the evidence he does have, he sees something exciting. The way I understood him, "100%" was just his way of expressing enthusiasm over how much more likely life on this planet was compared to what we've found before.

That said, given that the setting was a press conference and not a dinner party, it's easy to understand Kasting's assessment, "That was kind of a silly statement."

Star Power

One thing is increasingly clear: Gliese 581 will be remembered as a special little star. Besides 581c and 581g—outside our mythology-rich solar system, the naming of interstellar bodies takes a turn for the well-organized and prosaic(link)—astronomers have found four other planets in its orbit, and one of those, Gliese 581d, is also a potential candidate for habitability. 581g, though, is a much stronger candidate.

It's not necessarily an issue of other stars lacking potentially habitable planets. Frankly, we haven't studied enough other stars to really know. But Gliese 581 has a couple of special features going for it. First off, it's created a nice neighborhood for us to poke around in. Most stars of Gliese 581's type are prone to sunspots and solar flares, Kasting said. That activity not only makes it difficult for astronomers to get clear measurements and find planets, it also makes it more likely that any planets close enough to the star to have water will have had their equally important atmospheres fatally disrupted by solar shenanigans. Gliese 581, in contrast, is quiet and calm. So it's relatively easy for scientists to pick the planets out of the chatter, and more likely that any atmospheres that formed are still there.

Second, Gliese 581 seems to have a larger percentage of low-mass planets than other stars we've studied, Vogt said. That's important, because the big boys—usually balls of gas, like Jupiter, or ice, like Neptune—don't fit with what we know about the conditions necessary for life.

Gliese 581 is important because it combines good conditions for planet-hunting with a family of planets that are more likely to be Earth-like.

Research Paper as Soapbox

Besides the new planet and its star, the paper itself was also on the unique side. Normally, scientific journal articles are rather dispassionate, all facts and figures. But this paper was chock full of editorializing—both on the importance of collaboration between research teams, and on the need for funding for a dedicated, land-based, planet-finder telescope. Neither is really a bad goal, but the effect was weird. As Manfred Cuntz pointed out, the peer review process usually catches, and deletes, that kind of thing.

Turns out, this was Steven Vogt, being all self-expressiony again.

"I wrote the paper, and I take all responsibility for that," he told me. "I have this 15 minutes of soapbox, so I said it. And the [peer-review] referee had no problem with it. Hopefully somebody will listen."

Anytime you discover a new planet, let alone one like Gliese 581g, you know the resulting research paper will be read by a lot of people. Vogt decided to take the opportunity to make a couple of statements.

First, he said, the world of planet finding has been a particularly contentious one, driven by competition between the Lick-Carnegie team, and one based in Geneva, Switzerland. It's a quiet sort of conflict these days, but it got out of hand for many years, Vogt says, and the wounds are still there on both sides. He wanted the world to know how important Swiss data was to his team's discovery of Gliese 581g. Sort of a spoon full of sugar to make the medicine go down.

"I'm putting the Swiss on notice that this is hard work and we have to get past the stage of, 'Our data is perfect and yours isn't and nah-nah-nah,'" Vogt said. "In the paper, I said we have to work together on this. That helping each other is the best way to find the truth."

The other big issue was money. The other researchers I spoke with agreed that funding was a problem for their research, especially, Kasting said, funding to build a dedicated telescope. Right now, a research team like Vogt's gets only 15 days a year to collect data at a sort of time-share telescope that's used by a lot of different researchers to study a lot of different things. That means it takes a long time to gather up enough data points to successfully ID a planet like Gliese 581g—11 years, in this case.

"Everybody has their own agenda about getting money for next step," Vogt said. "There's all this attention on new techniques as if what we have today isn't good enough. You don't need sexy new technique. You need a big telescope that just does this, every night. Nothing else. Then you'll get these things [planets] pouring out of the sky."

This article just covers some context about Gliese 581g that I haven't seen talked about much elsewhere. There's a lot of cool details about the planet that I've left out—like, say, the fact that it's probably tidally locked, with one side always facing Gliese 581, and the other side always facing space. To read more about the planet, I recommend combing through these links:

The Astrophysical Journal—Original research paper written by Steven Vogt • [pdf] link.

NASA press release on Gliese 581g • link. Science Monitor article on Gliese 581g discovery • link. Q&A on Gliese 581g • link.

A Message From Earth: Project that sent radio signals to Gliese 581c in 2008 • link.

Images showing position of Gliese 581 in the constellation Libra • link.

Extrasolar Planets Encyclopedia: Technical Details on Gliese 581 • link.

Seed Magazine: G is for Goldilocks link.