Habitable Planet Gliese 581g Is A 'Ribbon World'

Gliese 581g lies right in its star's habitable zone, where it liquid water can exist. It is one of six planets orbiting Gliese 581, a red dwarf just 20 light-years from Earth.

“The threshold has now been crossed,” said astronomer R. Paul Butler of the Carnegie Institution of Washington, one of the planet’s discoverers, in a press briefing Sept. 29. “The data says this planet is at the right distance for liquid water, and the right mass to hold on to a substantial atmosphere.”

The new planet is about three times the mass of Earth, which indicates it is probably rocky and has enough surface gravity to sustain a stable atmosphere. It orbits its star once every 36.6 Earth days at a distance of just 13 million miles.

That means the planet has a blazing-hot daytime side, a frigid nighttime side, and a band of eternal sunrise or sunset where water — and perhaps life — could subsist comfortably. Any life on this exotic world would be confined to this perpetual twilight zone, Steven Vogt says, but there’s room for a lot of diversity. [Astronomer Steven Vogt of the University of California, Santa Cruz, who designed some of the instruments that helped find the planet.]

“You can get any temperature you want on this planet, you just have to move around on its surface,” Vogt said. “There’s a great range of eco-longitudes that will create a lot of different niches for different kinds of life to evolve stably.”

One reason that this is a big deal is that stable, hydrogen-burning, M dwarf stars make up about 75% of all stars in the galaxy; therefore the probability that there are life-sustaining planets has just improved.

Update: One reader questioned why it was stated in my source article that "gravity dictates" that the planet is tidally locked. Good question! I looked in the scientific paper that announced the discovery (referenced below), and here is the relevant quote:

Gliese 581g is likely to have evolved to a spin-synchronous configuration, leading to one hemisphere of the planet lying in perpetual darkness. Joshi et al. (1997) presented three-dimensional simulations of the atmospheres of synchronously rotating planets in the habitable zones of M dwarfs and concluded that such tidally-locked planets can support atmospheres over a wide range of conditions, and despite constraints involving stellar activity, are very likely to remain viable candidates for habitability. Joshi (2003) presented a more sophisticated three-dimensional global atmospheric circulation model that expanded on the previous work of Joshi et al. (1997) and evaluated the climate of a spin-synchronous planet orbiting an M dwarf star. The results of that study reinforced the conclusions of Joshi et al. (1997) that synchronously rotating planets within the circumstellar habitable zones of M dwarf stars should be habitable.

Also, the following statistics for Gliese 581g were cited, that it is "minimum-mass 3.1 M⊕ planet orbiting at 0.146 AU with a period of 36.6 days," showing that it is much closer to its star than Mercury is to our sun. Apparently, this characteristic has a close correlation with spin-synchrony.

Here is a short quote from a 2007 article in Astrobiology:

While low mass stars have much longer lifetimes, their luminosity is so feeble that any planet would need to be nestled very close to the star to permit the possibility of having a surface temperature conducive to liquid water, which seems to be the sine qua non of life as we know it. In such small orbits [0.1–0.35 astronomical units (AU) for an M0 star, and closer still for smaller stars], any planet would become tidally locked to the star.
(From A Reappraisal of the Habitability of Planets Around M Dwarf Stars)

Comments from knowledgeable astronomers are invited. And thanks for asking, Murgatroyd666.

End update.

In his 1952 novel Foundation and Empire, Isaac Asimov called a planet like this a ribbon world:

Radole was a small world – and, in military potential, perhaps the weakest of the twenty-seven. That, by the way, was another factor in the logic of the choice. It was a ribbon world – of which the Galaxy boasts sufficient, but among which, the inhabited variety is a rarity for the physical requirements are difficult to meet. It was a world, in other words, where the two halves face the monotonous extremes of heat and cold, while the region of possible life is the girdling ribbon of the twilight zone.

Such a world invariably sounds uninviting to those who have not tried it, but there exist spots, strategically placed – and Radole City was located in such a one. It spread along the soft slopes of the foothills before the hacked-out mountains that backed it along the rim of the cold hemisphere and held off the frightful ice.

The warm, dry air of the sun-half spilled over, and from the mountains was piped the water-and between the two, Radole City became a continuous garden, swimming in the eternal morning of an eternal June.

Via Wired; also, read the scientific paper that announced this discovery at A 3.1 M⊕ Planet in the Habitable Zone of the Nearby M3V Star Gliese 581. Thanks to Winchell Chung for the tip and the reference on this story (you can also follow him @Nyrath).

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