Gliese 581g: now you see it, now you don’t

What happened to Gliese 581g? One minute, the discoverer of the “Goldilocks” planet (not too hot, not too cold…) promised a 100 percent chance of life in another solar system, and the next, scientists weren’t even 100 percent sure the planet existed. Though the Americans who announced Goldilocks in late September stand by their discovery, a team from Switzerland says the planet just isn’t there.

But how is it possible to debate the existence of a rock three times the size of Earth? How can scientists point their telescopes at the same patch of sky and see different things?

Actually, we can’t see Goldilocks with our eyes, explained Nader Haghighipour, a member of the American team at the University of Hawaii at Manoa.

At more than 100 trillion miles away, the Gliese 581 solar system is much like our own — with several planets orbiting a large central star. Stargazers on Earth can see Gliese 581’s “sun” using a regular telescope, but astrophysicists must use indirect measurements to detect its planets.

When a large planet circles a star, its mass and gravity tug the star outward a little. By measuring how far and how fast the star wiggles, planet hunters can infer the presence of a planet. After they identify the strongest signals and conclude they’re caused by a planet, they mathematically remove that planet’s effects so they can search for more planets. In so doing, they analyze successively weaker signals.

Building from four already-confirmed planets in the Gliese 581 solar system, the American team analyzed the weakest signals available and concluded there were two more planets there. One of them, the team announced, was in a region of the solar system where temperatures were not too hot and not too cold for liquid water to form. That meant it could be just right for life to develop. The “Goldilocks planet” was reported in news outlets around the world, exciting astrobiologists and UFO-seekers alike into speculation that perhaps we aren’t alone in our corner of the galaxy.

Others are more skeptical.

“Even in the [Americans’] published data, after taking out the first four planets, what’s left looks like noise,” said Joseph Catanzarite, an astrophysicist at NASA’s Space Interferometry Mission, where he hunts for habitable planets.

That was exactly the conclusion of the Swiss team, which couldn’t find Goldilocks using a new data set either.

If Goldilocks exists, she appears to move her central star by one meter per second. Problem is, instruments measuring the shifting of stars can only register a bump of about one meter per second. The planet’s signal is so close to the instrument’s threshold that it may or may not be noise.

Haghighipour, though, said the main disagreement was in the way the groups analyzed the data. “They weren’t able to see our planet because it was masked by their assumptions,” he said.

The Swiss assumed elliptical orbits for the first four planets of the solar system. Conversely, the American team modeled the orbits as circular. According to some scientists, this puts them at greater risk of reporting false positives.

“It’s like taking a circle out of an oval,” said Sara Seager, a planetary scientist at the Massachusetts Institute of Technology. “Some of the oval gets left behind and the residuals can act as weak planetary signals.”

Everyone involved agrees that more data will overcome resolution problems and determine the shape of Goldilocks’ orbit, as well as rule out other possibilities like a false positive due to chance.

For now, many planet hunters and astrophysicists seem to believe the American team raised a false alarm and false hopes.

“The question isn’t quite settled but it’s close—it looks like the planet isn’t there,” said Catanzarite. “But it’s a good thing that someone reported it. It’s going to stimulate people to keep looking, and it’s going to make sure that whoever reports the next habitable exoplanet will have their ducks in a row.”

But it’s not over ‘til it’s over, and the members of the American team have a spotless track record for finding planets so far.

“In 15 years of exoplanet hunting, with over hundreds of planets detected by our team, we have yet to publish a single false claim, retraction, or erratum,” said Steven Vogt, lead investigator on the American side. “I stand by our data and analysis.”

So it’s back to the telescopes for both teams, and for other groups trying to resolve the debate. It’ll take a few more years’ worth of data before scientists know for sure whether Goldilocks is a real planet or just a fairy tale.