G is for Goldilocks

Analysis / by Lee Billings /

Gliese 581g is the most promising habitable world astronomers have found so far. But the chances of finding life there are vanishingly slim.

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Artwork by Lynette Cook

On Wednesday, based on observations from large telescopes in Chile and Hawaii, astronomers announced that the Earth has, if not a twin, at least a planetary cousin. It’s called Gliese 581g, so named for the small, dim star some 20 light-years away that it circles in a 37-day orbit. One of its co-discoverers, Steven Vogt of the University of California, Santa Cruz, has proposed a more memorable appellation: Zarmina’s World, after Vogt’s wife.

In the arcane accounting of astronomy, the “g” of Gliese 581g indicates that the planet is the sixth to be discovered in this particular star system, but for the public it has quickly come to signify something else: Goldilocks, a planet placed “just right” in its star’s habitable zone. Of the nearly 500 planets known beyond our solar system, nearly all appear to be gas-shrouded giants like Jupiter or Neptune, and most are either too hot or too cold to harbor life-giving liquid water. In contrast, with only three to four times the mass of Earth, Gliese 581g is probably mostly made of rock, and is at the proper distance from its star to have lakes, seas, even oceans of water upon its surface. If confirmed by follow-up observations, Gliese 581g will be the most promising potentially habitable planet discovered so far. From Roswell saucer-heads to eminences of the astronomical community, the newly discovered planet is the stuff that dreams are made of—at least until something better comes along.

If all this was fuel for a wildfire of speculation, then a statement from Vogt was the spark that lit the blaze. At a press conference organized by the National Science Foundation, Vogt was careful to note that he was an astronomer, not a biologist, but then said he thought that “the chances for life on this planet are 100 percent.” Another leader of the discovery team, Paul Butler of the Carnegie Institution in Washington, quickly backpedaled onto firmer ground, reiterating that solid data indicates “the planet is the right distance from the star to have water and the right mass to hold an atmosphere.” Nevertheless, Vogt’s “100 percent” quote has caught fire around the world, leaping from the understory of news articles into the canopy of blog postings, with Twitter updates and Facebook groups forming a self-feeding firestorm. Some researchers bask in the sudden heat and light, throwing on the occasional log; others run for buckets and hoses.

In all likelihood, a sizable chunk of people now believe that Gliese 581g is a place quite like Earth, and that astronomers will soon confirm life’s presence there, if they have not already. Unfortunately, this belief is almost entirely wrong. On the bright side, the truth is much more interesting, though it may now be only a candle in a conflagration.

Red Dawn

Gliese 581 is a red dwarf star, only a third as massive as the Sun and a hundredth as bright. But what red dwarfs lack in mass and luminosity, they make up for in longevity and prevalence. Massive stars are rare and fleeting, burning out in only tens or hundreds of millions of years. A star of middling mass like the Sun may shine for ten or twelve billion years. Lightweight red dwarfs sip their nuclear fuel, parsimoniously fusing protons together to faintly glow for practically forever. There are red dwarfs steadily shining today that were probably born not too long after the Big Bang. Some of those stars will still be shining when the Sun, and the Earth with it, are but a memory, and they will endure for many hundreds of billions of years after that. Life on planets around red dwarfs would have vastly longer to emerge, evolve, and grow than life on our own planet. It may be that stars like our Sun are actually quite poor incubators, overshadowed in the fullness of time by these smaller, dimmer nurseries for life.

That efficiency comes at a price: Red dwarfs shine so weakly that any planets could only be habitable if they huddled very close to the star’s nuclear fire. The orbits for Gliese 581’s inner five planets would comfortably fit within the orbit of Mercury around our Sun. The system’s sixth planet, its largest, orbits further out like a miniature Jupiter, but still at less than our Earth-Sun distance. In their discovery paper, to be published in The Astrophysical Journal, Vogt, Butler, and their co-authors write that Gliese 581’s planetary architecture, with its circular orbits and small close-in planets shepherded by a larger one, is “eerily reminiscent” of our own, albeit shrunken.

Their size and pervasiveness nearby are what make red dwarfs attractive targets for planet-hunters. Most of the Sun’s neighboring stars are red dwarfs. Generally speaking, the closer a star is to us, the easier it is to study. And the closer a planet is to its star, the easier it is to detect.  Because red dwarfs are so small, the periodic gravitational wobble induced by any orbiting planets around them is easier to see. Red dwarfs are also advantageous for another search method, which looks for planetary transits, the dimming of a star as orbiting planets periodically cross its face, casting shadows toward Earth. Since red dwarfs are so faint and tiny, a transiting planet creates a proportionally greater dimming for them than for larger, brighter stars.

Wobbles are a relatively indirect way to detect planets, and only provide a planet’s orbital parameters and an estimate of its mass. Since transits are actually the silhouettes of planets, they yield a planet’s diameter as well as its orbit, from which its density and composition can be inferred. By carefully studying starlight that filters through or reflects off a transiting planet, astronomers can even learn about its atmospheric composition—crucial information for determining whether or not the planet is habitable. NASA’s James Webb Space Telescope, launching in 2014, should be able to observe the atmospheres of potentially habitable transiting planets around nearby red dwarf stars, and might even be able to detect chemical tracers of life like water vapor and oxygen. To planet-hunters, red dwarf systems are not so much stars as abundant, pendulous low-hanging fruit. Finding and studying them lies at the heart of planetary astronomy’s immediate future.

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