Gliese 581g: A Black Eye for Believers in Habitable Earth-like Exoplanets
Back in 2010 there came exciting news of the discovery of the first habitable exoplanet, just 20 light years from Earth, circling the star Gliese 581. The star was said to be orbited by four planets. One of those, Gliese 581g, was both in the galactic habitable zone (being so close to Earth, this followed logically) and in its system's own circumstellar habitable zone. It was said to be rocky, some three times more massive than Earth, potentially harboring liquid water, and thus, hypothetically, life.
NASA released an image, "Planets of the Gliese 581 System," an "artist's conception" that made Gliese 581g look like quite the garden spot. It was all but ready to be profiled as a vacation destination in Sunset magazine.
At the time, and later, we offered some reasons for skepticism. See Jay Richards's comments here at ENV, "Science Reporters Should Quit Crying 'Life!'" In a podcast for ID the Future, Jay's co-author on The Privileged Planet, astronomer Guillermo Gonzalez, was similarly discouraging of media hype.
Personally, given the ubiquity and propensity of life to flourish wherever it can, I would say, my own personal feeling is that the chances of life on this planet are 100 percent.
Well, we were wrong about one thing. Of all the reasons for doubting there is life on Gliese 581g, we missed the most decisive: The planet doesn't exist. Science reports: "Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581."
From the Abstract:
The M dwarf Gliese 581 is believed to host four planets, including one (GJ 581d) near the habitable zone that could possibly support liquid water on its surface if it is a rocky planet. The detection of another habitable-zone planet -- GJ 581 g -- is disputed, as its significance depends on the eccentricity assumed for d. Analyzing stellar activity using the Hα line, we measure a stellar rotation period of 130 ± 2 days and a correlation for Hα modulation with radial velocity. Correcting for activity greatly diminishes the signal of GJ 581d (to 1.5σ) while significantly boosting the signals of the other known super-Earth planets. GJ 581d does not exist, but is an artifact of stellar activity which, when incompletely corrected, causes the false detection of planet g.
So actually that makes two nonexistent exoplanets, Gliese 581g and Gliese 581d, that turn out be the result of glitches, misreadings of "stellar activity," nothing more. The same popular science media that boosted the existence of these planets are now forced to swallow hard and explain to readers the disappointing news.
Not to worry, though. Over at The Daily Beast, Matthew R. Francis spins the revelation as confirming how wonderfully science, and therefore by implication science reporting, "works":
It's sad in a way to say goodbye to the first rocky exoplanet in a habitable zone. However, since 2010 researchers have found a number of other worlds unplagued by the same sorts of uncertainties. Let Gliese 581g, the exoplanet that wasn't, be a reminder that science works.
So when the system screws up and gets everyone excited about a nonexistent Earth-like planet that's certain to host life -- not just likely to do so, but "100 percent" positive -- and then is compelled to backtrack just four years later, that actually demonstrates how competently the system functions. And you know what, I think a lot of these guys actually believe that.