Tag: Steve Vogt

Does Gliese 581g exist?

By Phil Plait | January 18, 2011 2:46 pm

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 Astrobio.net 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?
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CATEGORIZED UNDER: Astronomy, Piece of mind, Science, Top Post

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