Another Green World: a Lurker’s Guide to Gliese 581g

“The Gliese 581 system exerts an outsize fascination when compared to many of the other exoplanetary systems that have been discovered to date.”

That quote comes from the paper that started it all, “ Planet in the Habitable Zone of the Nearby M3V Star Gliese 581”and that is probably its least controversial statement. Since late September, the news of a possible “second Earth” has spread and is now resting comfortably in that space where it is an item of casually conversation; a vague, “oh yeah that thing”, sort of thing. After all the fuss, what remains says more about our shared mythologies of the future and ourselves than it does about the stars.

Briefly, back to the stars. If you are interested in being on top of this one, the original paper is the best place to start. It represents the results of 11 years of radial velocity (RV) measurements of the Gliese 581 system. However, despite its very readable style, it is an involved, technical review of data from two observatories. This data, when combined, revealed an additional two planets for Gliese 581. One of these planets, 581g, is possibly only slightly larger than Earth and sits well within the system’s “habitable zone” ( the area around a star where the average temperature allows for liquid water to form). For those who would like a simpler but accurate account of the report, the first mention of the paper by Science Daily back on Sept. 29, 2010 is as good as anything that has been published since.

Before going into the  speculations about Gliese 581g, one caveat. The existence of 581g and its less famous sibling 581f  have yet to be confirmed. If a subsequent re-analysis of the data or a review of another set of observations yield a different result, 581 g and f will vanish as surely as if their primary star went nova and their letters will be reassigned to planets that show up with better credentials.

The team that has posited these two new worlds are aware of this but optimistic enough to posit a further point to their discovery:

Confirmation by other teams through additional high-precision RVs would be most welcome. But if GJ 581g is confirmed by further RV scrutiny, the mere fact that a habitable planet has been detected this soon, around such a nearby star, suggests that η⊕ could well be on the order of a few tens of percent, and thus that either we have just been incredibly lucky in this early detection, or we are truly on the threshold of a second Age of Discovery.

A bold statement even in italics but what is this value “η⊕” ?  Other than looking like an inscrutable tatt for some equally inscrutable Goth chick, it does have a distinct meaning:

In recent years, the parameter η⊕ has been minted by the NASA community to aid in evaluating and planning for space missions that seek to discover habitable planets. The official definition of η⊕ is given by the Exoplanet Task Force Report (Lunine et al. 2008) as: “The fraction of stars that have at least one potentially habitable planet. The Task Force defines a potentially habitable planet as one that is close to the size of the Earth and that orbits within the stellar habitable zone”.

Glieses 581g fits this to a tee so long as you say “potentially”. Size matters as does an orbit in the habitable zone but that’s not enough to go any further than “potentially”. There’s the matter of Gliese 581 itself.

What we have here is a red dwarf star: class M. These stars make up 76% of the main sequence stars in our local neighborhood. They remain stable for trillions of years; lots of time for life to evolve. Their radiation output is low, about 10% of the Sun’s, but planets have been found inside their habitable zones. Gliese 581 may have at least three such worlds. However, these zones are very close to the red dwarf which results in two effects. First, although stable, even red dwarfs can emit flares which are powerful enough to rip away a planet’s atmosphere at such close range. This only has to happen once to make a planet uninhabitable. Another effect of being too close to a star is the fact that the orbiting planet, like our Moon, will be tidally locked with one side perpetually facing the star and the other side in eternal night.

A temperate strip could exist along the planet’s “twilight zone”; the terminus between day and night. This area would receive only a very strained light from a red sun made even redder by hanging in a frozen sunset (or sunrise, your choice). Is there enough time, even on the longest lived of any possible world, for life to develop an alternative to photosynthesis and bring the atmosphere into some state other than being a kinder and gentler Venus?

The team leader of the group that “discovered” Gliese 581g, Prof. Steven Vogt in an interview with Annalee Newitz for io9, put forth his ideas on what conditions would be like on this “second Earth” which he has nicknamed Zarmina, after his wife. He’s very positive about 581g’s potential as a habitable world and cites that there are computer models for tidally locked worlds that don’t end up as a global cyclone. But when Newitz asked if it would be possible to use spectrometry to see if 581g had oceans, Vogt pointed out, that with our current techniques we can’t do that with planets that don’t pass directly in front of their primary star. It was almost less than moot but it brings up a key matter in just how much we can ever know about  some these exoplanets without actually going there.

And here’s where the fun starts. The idea that 581g might harbour life or provide humanity with a backup world if we foul this one up jumped up pretty fast.

The new found world even got its own Facebook page but its founding fans picked the wrong planet. They linked it to Gliese 581c,  a  5 times Earth gravity world on the habitable zone’s inner edge that had its heyday back in 2007. It  was the target of a Message from Earth radio signal sent by the Ukraine Space Agency in 2008. It appears that none of the 500+ fans on the Facebook page are  aware of the mistake. Hey..at 20 lightyears what’s a few million miles.

Dave Goldberg at his website A User’s Guide to the Universe dusted off his calculations for a trip to Alpha Centauri and remodelled a trek of 20 lightyears to the Gliese 581 system in the article, Full Impulse on My Mark…Engage! Here  he actually worked out the fuel costs for an anti-matter driven starship. Matter/Anti-matter reaction spacecraft are pretty much the Holy Grail of  plausible future spacecraft propulsion. Back when Gene Roddenberry was doing the research for Star Trek in the mid-1960’s that’s the answer the boys at the Rand Corporation and NASA gave him when he asked them what it would take to get the Enterprise up to warp speed. And it hasn’t changed. At almost  100% energy efficiency, anti-matter is the best fuel but how much does it take? A lot: 530 times the weight of your cargo for a twenty light year voyage.

Undaunted by mere physics, the writers and artists at io9 weighed in with posters celebrating the 100th anniversary of the Earth colony on Zarmina and speculations that maybe the Zarminians have already begun colonising their own solar system or are being invaded by their neighbors from either side of the habitable zone. The Gliese 581 system is starting to look about as action packed as the one in Firefly. Briefly there was also the suggestion to name this twilight zone world “Gloaming“.

Gloaming or Zarmina; for any designation to stick it will have to be passed by the International Astronomical Union which is presently more concerned about the planet’s existence than what to call it. As I was finishing this article the first serious challenge has been issued by a team at the Geneva Observatory lead by the discoverer of Gliese 581e, Michel Mayor. The article from PhyOrg is worth a look but starts with a very long review of the claims made for 581g before getting to this first criticism of the initial paper. The short of it is that Mayor’s team where unable to find any evidence in the data for 581g’s existence that leads them to see it as anything more than random “measurement noise”. An artifact, an illusion: will Zarmina join the canals of Mars, or the planets of Bernard’s Star that Peter Van deKamp reported in the 1960’s  but later turned out to be due to a flaw in his observatory’s telescope? To say that someone’s results are little better than chance makes them sound like some “psychic” or spoon bender.

If Mayor’s criticism holds then the hypothetical planet will have existed for less time than it’s 39 day long year. I don’t see Vogt and his team just packing it in so the arguments surrounding 581g will probably play out over the next few years. How interested will the news media be in the fine points of this debate?

Looking at all of the hype and speculation I’m left with more thoughts and questions than facts and answers. The desire for another world is as old as regret and people will bend their entire lives and those of others to enter an imagined paradise. The costs of going beyond our Earth are great maybe greater than we can pay. To travel to the nearest worlds of our own Solar system it may be necessary to change ourselves in ways that may take us past whatever we have previously called human. When we finally meet the aliens, will we have become as alien to ourselves as they are to us?