Does Gliese 581g exist?

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Jan 18, 2011
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Back in September 2010, astronomers announced the discovery of a remarkable and exciting planet: it was three times our mass (high, but far closer to Earth conditions than the super-Jupiters usually found) and orbiting in the "Goldilocks zone" of its star... which meant that it could possibly have liquid water on its surface! This achingly earth-like planet made a major buzz, and in fact I used its characteristics to estimate that there could be billions of Earthlike planets in our galaxy.

But there's just one small, really eensy-teensy problem: the planet may not exist. But it also might. Maybe.

We're still early in the game here, and there's a lot going on... but it's worth peeking a bit deeper. There's science here, and math, and even some interesting media jiggery-pokery.

Finding planets

We know of more than 500 planets orbiting other stars, and astronomers have a diverse set of tools to find them. The first were discovered by what's called reflexive motion (a nice animation of this is on the site); as a planet orbits a star, the planet's gravity tugs on the star, causing a tiny Doppler shift in the starlight. This is a very small and difficult thing to measure, but techniques improved vastly in the 1990s, and most planets have been discovered this way. The success of this technique has been confirmed by other methods, too, including planetary transits, when the orbiting planet passes in front of the star from our viewpoint, and blocks a little bit of its light. Several planets detected using reflexive motion were confirmed by subsequent transits. We know the method works.

But like any technique, things get fuzzy when you push it. Gliese 581 is a red dwarf star a mere 20 light years way; it's one of the closest stars in the sky to us. Two different teams of astronomers, one Swiss and one American, have observed the star for a long time, and they both confirm the existence of four planets around the star (more on that in a sec). But one of the teams (Steven Vogt and Paul Butler) claimed they found two more planets: Gliese 581 f and g, with the latter being the planet in question.

Odd planet out

Almost immediately, the planet was called into doubt; the Swiss team re-examined their data and could not be absolutely certain that Gliese 581 g was there, but still gave it a thumbs-up at the 90+% level. That's not too bad.

Interestingly, not too long after the announcement I was at a meeting with several astronomers, and one noted that Vogt's team made a big assumption: all the planet orbits were circular. If in fact one of the planets had an elliptical orbit it could set up a false-positive, making it look like another planet was there when it wasn't. According to Vogt this turns out not to be the case; I contacted him and he let me know that orbital ellipticity was one of the characteristics they modeled as a variable. In other words, their computer model made no assumptions about orbit shape, but in fact the best fits in the end were circular orbits.

Still and all, there have been some questions about the planet's existence, and I've been holding back from posting until something happened. Well, something did: Philip Gregory, an emeritus astronomer with the University of British Columbia, has analyzed both data sets using sophisticated statistical techniques, and he concluded that Gliese 581 g almost certainly wasn't real. In fact, he says the odds of it being a false alarm are 99.9978%!

So which is it? Is it 90+% certain to be real, or 99.9978% certain it isn't? Let me be up front with you: I don't know. Gregory analyzed the data using Bayesian analysis, a method of looking at the statistical certainty of a set of observations. This is fiendishly complex in practice and to be honest is not something I'm familiar with. However, in his paper, Gregory himself claims that Vogt and Butler underestimated the amount of noise in their data. Vogt disputes this, saying that Gregory adds noise to their data rather arbitrarily. I'll admit that it seemed odd to me that Gregory would add noise the way he did, but again I'm no expert.

Vogt also notes that how you run the computer model will change whether or not you find the planet. This part interests me, because I've run into similar situations myself. If you tell your computer that one of the planets (in this case, Gliese 581 d) has a highly elliptical orbit, then Gliese 581 g disappears: when you calculate the statistics, it's far more probable that the planet does not exist. But if you keep Gleise 581 d's orbit circular, Gliese 581 g can be seen in the data. These two different assumptions lead to two different solutions, where one has Gliese 581 g in it and the other doesn't.

Which one is right? Vogt claims 581 g exists. I won't go into details (the math gets a bit hairy) but basically he claims that statistically speaking, his solution fits the data batter then Gregory's.

He said/He said

Well, that's science! Two people disagree, and they make their cases. Vogt's disagreements with Gregory's methods are reasonable, in that he can make his case scientifically and mathematically. He may not be correct, but that's a matter to be hammered out using science and peer review. Given that the claims are pretty specific (methods used, input parameters, statistical measurements), I think this will work itself out pretty rapidly.

However, the media got involved, and then things got a bit sticky.

I was tipped off to this matter with a link to the (Australian) ABC site which wrote about this disagreement. The following passage, I'll admit, made me cringe a little. Note that the HIRES data are the observations by Vogt and Butler, while HARPS is from the other, Swiss, team:

Dr Steve Vogt says he and his colleagues "stand solidly" by their original findings.

"I have studied [the paper] in detail and do not agree with his conclusions," he says.

Vogt is concerned that Gregory has unfairly manipulated the HIRES data.

"By doing so, he finds a solution that is more consistent with the HARPS data only," he says.

OK, yikes. The word "manipulated" is pretty loaded. It's easily interpreted as meaning the data are somehow being changed unfairly, and on purpose.

But then I saw an article in the Toronto Star that said this:

The revelation Gregory put forward is being dismissed by Vogt, who was quoted by the Australian Broadcasting Corporation as saying Gregory “manipulated” the numbers.

Egads. That made me cringe a lot. Note this is a second-generation quote; the Star was using something written in the ABC article. The Star continued with this:

"Vogt is not familiar with the Bayesian techniques so he might assume that I am manipulating the data. I attribute that to a lack of awareness on his part," said the soft-spoken Gregory.

Oh my. Well, to me the use of the word "manipulate" would be pretty accusatory in this context coming from a scientist when discussing the work of another, and this is why I initially contacted Vogt. He sent to me the email he sent to the ABC, and the word "manipulate" is nowhere in it. To a layman his email would be strongly worded, but as a scientist I see him attacking Gregory's work, not the man himself. What he said wouldn't draw any surprise at all were it said at a scientific conference, for example.

But the Star article actually got a response from Gregory about the "manipulation". That line I quoted above is a bit loaded, in my opinion, right down to the adjective "soft-spoken" used to describe Gregory. It's almost as if the media were playing up the contention between the two men, trying to frame the story as being personal (with one scientist the aggressor, and the other the defender) as opposed to just a scientific difference of opinion.

Again, I strongly suspect that if Vogt and Gregory got together (or when Gregory's paper goes through the review process; it's been submitted but not peer-reviewed yet) this would all get figured out pretty quickly.

[UPDATE: As I was putting the final edits on this, Wired posted a pretty good article about all this.]

To g, or not to g?

So, does Gliese 581 g exist? I can only form an opinion right now based on what I've seen, and I don't like to speculate over much. However, Vogt has good rebuttals to the opposing claims, and the Swiss team of astronomers does seem to back him on the existence of the planet.

What we really need are more and more sensitive observations. That's going to be the rule and not the exception as we move forward in looking for earth-like planets. They're small, and move slowly, and make themselves very difficult to detect with our current hardware. But progress moves on, and whether Gleise 581 g exists or not, finding another Earth orbiting another star is only a matter of time. Count on it.

Related posts:

- Possible earthlike planet found in the Goldilocks zone of a nearby star
- How many habitable planets are there in the galaxy?