Gliese 581 is a diminutive red-dwarf star located in the constellation Libra at a distance of about 20 light years. You can’t see it with your eyes---you’ll need a really good telescope. It’s also one of the most exciting stars in the sky right now.
Now, the sky is about full of diminutive red-dwarves. In fact, the majority of the stars within 30 light years of us happen to be them. They’re just there—they burn long. They live long. And they’re dim. They’re so dim that if our star had a red-dwarf companion out past Pluto, we’d barely see it if we saw it at all, and yes, there is a hypothesis that the Sun does indeed have a red-dwarf companion that has an orbit of roughly 26 million years. If it exists, it may have been spotted by WISE which scanned the skies in the infrared, whose data set is publically available and being analyzed by research teams all over the world.
In 2007 the first of what we now know is probably six planets were detected around Gliese 581. Now, they weren’t directly imaged. Doing that is difficult although, it has been done, several times. Rather, they were detected via radial velocity. As planets orbit a star they tug on it. These tugs are detectable, and from there, one can tease out a great deal of data, including a planet’s mass, its orbital period, and its orbital eccentricity. Perhaps even now, somewhere in the galaxy, alien astronomers are viewing a region of space that includes our sun and noting the presence of perhaps 4 large worlds (those worlds being Jupiter, Saturn, Uranus, and Neptune). Perhaps they’ve even teased Earth out of their data set. But I digress.
The planet detected in 2007 doesn’t really have a name. We call it Gliese 581 c. It’s probably a hot, unpleasant place. Its minimum mass is 5.6 Earth Masses. It hugs its sun so closely that its year is two weeks long. It’s likely tidally locked, its surface roiled by steaming, blazing, horror-storms of fire.
Other worlds orbit Gliese 581, at close range to their star. The most exciting one though, is the one labeled “g.” It’s also the most controversial.
Does it Exist?
In 2010, news broke that planet “g” was discovered, and it orbited within Gliese 581’s habitable zone. A habitable zone is the zone around any star where water can exist in liquid form. Life as we know it here on Earth requires water, and it’s a fair assumption that this will be replicated across the universe.
Planet g was not the first planet detected within the habitable zone of a star. At 55 Cancri, a sun-like star located about 40 light-years from us, a large, Saturn-sized planet was detected orbiting roughly where water would be liquid. What’s exciting about Planet g is that it’s fairly small. In fact, its mass at minimum just 2.2Earth masses.
However, the planet’s existence was in doubt from the beginning. Only one team of planet-hunters detected the world. Other Planet-Hunters could not detect the characteristic “pull” that would signify that it existed.
On October 11, 2010, an astronomer from the Geneva Observatory's HARPS project announced at the IAU Symposium 276 that their team had not been able to confirm the existence of either planetary candidates Gliese 581 "g" or "f" based on only their own, expanded but smaller dataset of 180 observations over 6.5 years. Simulations based on their data have shown that the probability that the radial-velocity variations can be produced "by chance" because the noise is not negligible. This is because the signal amplitude of "g" and "f" are very low and close to the level of the measurement noise (Leslie Mullen, Astrobiology, October 12, 2010; and Rachel Courtland, New Scientist, October 13, 2010).Basically put, the planet was just a data artifact. It isn’t real. I was very sad after that news broke.
New research (from the same research team) suggests that planet “g” is indeed real.
Now the original discoverers of Gliese 581g, led by Steven S. Vogt of UC Santa Cruz, present a new analysis with an extended dataset from the HARPS instrument that shows more promising evidence for its existence. The new analysis strengthens their original assumption that all the planets around Gliese 581 are in circular and not elliptical orbits as currently believed. It is under this likely assumption that the Gliese 581g signal appears in the new data.Actual paper can be read here. Obviously, debate will continue as to whether it exists or not.
“This signal has a False Alarm Probability of < 4% and is consistent with a planet of minimum mass 2.2M [Earth masses], orbiting squarely in the star’s Habitable Zone at 0.13 AU, where liquid water on planetary surfaces is a distinct possibility” said Vogt.
Based on the new data Gliese 581g probably has a radius not larger than 1.5 times Earth radii. It receives about the same light flux as Earth does from the Sun due to its closer orbital position around a dim red dwarf star. These factors combine to make Gliese 581g the most Earth-like planet known with an Earth Similarity Index, a measure of Earth-likeness from zero to one, of 0.92 and higher than the previously top candidate Gliese 667Cc, discovered last year.
Now, what kind of place is planet “G”? Probably habitable, but not very nice for human life.
It swings about its star in just 37 days. If it exists, one face likely faces its star at all times, and one away. This is called tidal locking. The best example? Our moon. Now, it’s possible that interactions with the star system’s other planets have forced a slow rotation. Perhaps it’s in resonance with the other worlds such as for every three rotations, it orbits its star two times. This is why Mercury isn’t tidally locked with the Sun. But we don’t know this, from our far vantage point at 20 light-years away.
With its minimum mass of 2.2M, it possibly is 1.3 to 1.5 times larger than Earth if it’s a rocky planet, and gravity at its surface is 1.1 to 1.7 times that of Earth’s. If we could go there (again, assuming it exists), we’d feel a lot heavier, for starters. Walk around long enough here, and you'll get thighs that can choke a horse and a butt you could bounce quarters off of.
What kind of weather would this place have? Not very nice weather. It’s been speculated that a planet that doesn’t rotate would have a hellacious “Hot-Pole” and a dark and arctic “Cold-Pole.” Early simulations in the days before computers speculated that the atmosphere would freeze out completely. Later simulations suggested that a dense enough atmosphere would not freeze out at all, and that while the planet wouldn’t rotate, the atmosphere would via heat transfer. Essentially, the surface would be ridiculously windy, as hot air blew around the planet from the Hot Pole to the Cold Pole, and then back to the Hot Pole. It’d be a lot more complicated than that depending on the presence of liquid oceans and continental features such as mountains and plains, but that’s basically the idea. Both the Hot and Cold Poles would not exactly be habitable, but where the sun hangs just above or below the horizon might be temperate and nice, comparatively (this is called the terminator). This planet, being larger and denser than Earth, could have a comparatively thicker atmosphere. That atmosphere would pack a hefty punch when it got moving.
Now, if the planet isn’t tidally locked, but rotates every 3 times for every 2 orbits, then things get pretty darn weird and alien. You’d have a sunrise that’d take days to happen. On some days, the sun would retrograde (move backwards) through the sky. The planet would rotate much too slowly for a Coriolis Effect, so perhaps there wouldn’t be any hurricanes or cyclonic storms as we know them. But perhaps the rising sun would also be accompanied by a great deal of rain, or snow turning to rain. Planetary local noon might be very hot. Seasons wouldn’t exactly matter either—or seasons would be synonymous with where a specific location is within the planetary day. Local noon might equal local summer. Sunset might come with heavy rain, changing to snow, as the sun sets, an event that’d take days. Local midnight might equal the depths of local winter. In short, just like Denver. (Kidding!)
Barriers to Life
Red-Dwarves have a bad habit. They flare. A lot. And big ones.
Star-flares throw off a great deal of radiation. A planet close in could be completely sterilized. Now there are models that indicate that a thick enough atmosphere can absorb the radiation (ours does), thus maintaining its habitability. Flares, however, could over time erode away the atmosphere of such a world, orbiting close in. Gliese 581 is known to be variable, but it isn’t known to be violently variable like our sun’s nearest known stellar neighbor, Proxima Centauri. That’s a plus in “g’s” favor.
Another barrier is the type of light. Red-dwarves emit far more infrared light than they do visible light. Plants require visible light to perform photosynthesis, and using Earth as a dataset, we know that planets require a set range of temperatures to perform photosynthesis. It’s possible that plant-analogue life may develop that utilizes infrared light, but we don’t know that just yet. Perhaps the plantlife isn't green at all.
There will be much more debate over Gliese 581 g over the coming months as more data gets analyzed by different teams around the world. This is indeed an exciting time for planetary discovery. The Kepler telescope discovers new worlds every month and the knowledge that our solar system isn’t unique seems quite firm. The stars are full of worlds and perhaps life too. The only thing that makes me sad is it seems unlikely we’ll ever visit them. But if we could, tomorrow, I’d be the first to sign up to go.
What do you think? I've offered a lot of speculation on my part, now I want to hear yours.