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What is Gliese 581?
Gliese 581 is a small red dwarf star in the constellation Libra. Its spectral type is M3 D and it is roughly 20 light years away, putting it relatively close to the Earth. The star was officially catalogued in 1957 in the Gliese Catalog of Nearby Stars, which was a survey of hundreds of stars within 20 parsecs of Earth. Gliese 581 is actually the 87th closest star to the Earth. M type stars are the smallest stars in the universe that still fuse hydrogen into helium in their cores. Gliese 581 has a radius 1/3 that of the Sun's. The low mass means that the hydrogen at its core is fusing much more slowly, putting out much less energy. Gliese 581 itself has a temperature of 3,480 Kelvins. When you combine a star being very small and not radiating much light, its total luminescence becomes extremely tiny. Gliese 581 puts out a little more than 1/10th of one percent as much light as the Sun.
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Planets around Gliese 581
When astronomers look for planets orbiting distant stars, they actually observe the movement of the star itself. When a planet orbits a star, the force of its gravity tugs on the star, causing it to wobble slightly. Observing the size and the speed of this wobble allows us to determine the size and the orbit of a planet. Because Gliese 581 is a small M type star, planets orbiting it will have an accentuated affect. Using this method, astronomers have found at least 4 planets orbiting Gliese 581, and possibly as many as 6.
A specific distance from Gliese 581's surface is what astronomers call the Goldilocks Zone, where the temperature is hot enough so ice melts, but not so hot that it boils. Since Gliese 581 is so small and puts off so little energy, this Goldilocks Zone is much closer than most other stars. In April 2007 astronomers discovered a planet inside this Goldilocks Zone; Gliese 581c. Using Kepler's Laws, the mass of this planet was calculated to be around 5.8 times that of the Earth, and a radius one and a half times as large. This implies that the planet is a rocky planet like the Earth of Mars rather than a gas planet like Jupiter or Saturn. This means that Gliese 581c is the first exoplanet that has the possibility of sustaining life. On top of that, another planet, Gliese 581d, was discovered to sit on the colder end of the Goldilocks Zone. Its mass is even bigger, pulling in at 7.7 Earth masses.
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Orbits: Zina Deretsky/ National Science Foundation/ NASA
Gliese 581: NASA http://obswww.unige.ch/~udry/Gl581_preprint.pdf