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A Gliese 581 Primer

Gliese 581c is a planet recently discovered outside our Solar System. What makes it so interesting is that it is by far the most Earth-like yet discovered. The key factor making it Earth-like is that it may harbour liquid water. It orbits within the Goldilocks Zone of its star, not too hot, like baked Mercury and not to cold like icy Mars, but right in the middle. Couple that with the fact that it is practically spitting distance away, makes it far and away the most interesting target for the search for life.

Gliese 581 c is one of about 235 exo-planets discovered in the last fifteen or so years, with more are being discovered every moment. Most known exo-planets are very large - Jupiter-sized. This is not an indication of how frequent large planets are or how rare small planets are, it is simply that we don't have the technology to detect smaller planets yet.

Or at least, we didn't...

Where is Gliese 581?
Gliese 581 is, astronomically speaking, just down the street. Sol's closest neigbour, Alpha Centauri, is 4.3 light years away. Gliese 581 is 20.4 lights years away. In a galaxy containing 500 billion stars, only 87 are closer to us than Gliese 581.

It is a rather diminutive red dwarf, Type M (Sol is Type G) and radiates at 3480K. About 5 billion years old, it is about the same age as Sol.

How long would it take to get there?

Gliese 581 is close enough that, at least in principle, we will be able to make a journey there in the future. We would want to have our craft accelerate at the just right speed so that the occupants felt an artifical gravity. We woulld accelerate at 1g for half the journey, then turn ourt ship around and decelerate for the second half of the journey to arrive at Gliese stopped. Doing this, the craft would spend most of its journey at very near the speed of light. This would mean that the actual trip would take just slightly longer than it would take light to arrive there - 22.6 years.

But what's really interesting is that the relativistic speed of the craft would slow time down for the occupants. They would experience a journey of a mere 6.1 years. If they were to take some quick samples and then turn around and head honme, they would arrive at an Earth had had aged 45 years, yet the travellers would have only aged 12 years; they would be younger by 33 years.

Can I see it?
At Magnitude 10.5* Gliese 581 is much too dim to be seen by human eyes. But if you wanted to look in the right direction, or with a telescope: Find the Big Dipper. Follow its tail/handle in a curve around to the very bright star Arcturus. Continue about the same distance along the curve to the next very bright star Spica. Arcturus and Spica are two of the brightest stars in the sky (-0.4 and +1). If Arcturus and Spica form the base of an equilateral triangle, Gliese 581 forms the apex of the triangle right a the tip of the constellation Libra.

For ambitious types who want to try their hand with a telescope:
Right Ascension: 15h 19m 26.8250s Declination: -07° 43' 20.209"

*Dimmer stars have larger numbers. We can at best see stars as dim as 6. Sirius is -1.42.

What is it like compared to our solar system?

Gliese is like a toy version of our own Sol - about 1/3 the mass, and only about 1/80th as bright.

Likewise the whole planetary system is scaled down. Its outermost planet is well inside our own innermost Mercury's orbit. Their "years" follow suit. Gliese d's year is only 84 (Earth) days long, Gliese c's year is 13 days and Gliese b grazes its parent star only 6 million km away and completes its orbit in a breakneck 5.4 days.

In Gliese's system, all but Gliese d falls within an area called tidal locking zone. This is the area within a certain radius of the star where gravity has such a powerful effect on the bodies that their rotations are slowed until they stop with one face always facing their parent body. This is why Earth's Moon always only shows one face to Earth. In our Solar system, only Mercury is within the tidal locking zone.

Normally, this would put all the planets far too close to their parent and uncomfortably warm. But Gliese puts out much less light and heat than Sol. Gliese's habitable zone or "Goldilocks" zone (not too hot, not too cold, but just right to support liquid water) is much closer. Gliese c falls smack in the middle of this zone. The mean temperature on Gliese will fall between 0C and 40C.

(Note Mercury's eccentric orbit, ranging between 43 and 70 million km from our sun)

Size-wise Gliese's planets are all pretty much larger than Earth and smaller than Uranus/Neptune. Gliese c is about 1.5x the diameter of Earth and about 5x the mass.

Gliese c has very much the same density as Earth. It is not so large as to be a gas giant like any of our Solar System's outer planets.

What would it be like living on Gliese 581c?
The first thing you'd notice upon setting down on Gliese is that you'd feel like you're carrying someone on your back. A 150 pound man would weigh about 330 pounds on Gliese 581c. c is about 5x the mass of the Earth, which would make a man weight 5x his Earth weight. But the fact that c is 1½x the radius of Earth partially cancels this out.


Funny thing about c, its temperate zone is very different from Earth's. Because Earth rotates on its axis it is heated fairly evenly all around. It's warmest areas ar those that receive the most direct sunlight - around the equator. But Gliese c does not rotate; it is tidally locked with its sun, so that one face is always in daylight and one face is always in night. The daylight side will be baked and the night side will likely be so cold that some of the atmosphere will freeze out into snow and ice. Rather than a horizontal ring, the temperate zone forms a ring from pole to pole.


Gliese c's sky is unique. The "sun" is a deep red and its disk is five times the diameter of Earth's sun. While Sol's disc is only about half to a third of your pinky finger at arm's length, Gliese's disc is about twice the width of your pinky.

But its sun does not move in the sky, on Gliese you will never get lost, you will always be able to orient yourself to the Sun, which will always be near the horizon (depending on where you live, it may be above the horizon or below the horizon).

Gliese' daylight is dimmer than Earth's. The larger size of its sun's disc is more than cancelled by the fact that it is 1/80 as bright, making for a daytime sun that is about 1/3 as bright as Sol.

Couple that with the deep red colour, and the nape-of-horizon position, and a typical Gliese day is very much like an Earth sunset.

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Why is it called Gliese and how do you pronounce Gliese anyway?

In 1969 Wilhelm Gliese created the Catalog of Nearby Stars - all known stars within 25 parsecs (107.5 light years). Gliese 581 would presumably be the 581st entry. Being German, Gliese would be pronounced Glee'-zuh