Gliese 581

Gliese 581 is an M-class red dwarf star that is not far from the Earth. It is 22 light years away from the Solar system. This makes it the 89th closest known star to the Sun. It is also known as HO Librae, GL 581, or GJ 581.
Astronomers have discovered some planets that orbit Gliese 581. The astronomers agree that four of the planets are real, but some think there are two more planets.
Gliese 581 has a radius, and mass, about a third that of the Sun. Its estimated temperature is 3,498 Kelvin (4125 Celsius, 7457 Fahrenheit). Its luminosity is 0.01205 times the luminosity of the Sun.
People on Earth can see Gliese 581 in the constellation Libra. The star's apparent magnitude is 10.57. This means that a person on Earth would need to use a telescope to see Gliese 581, because the star is not bright enough for the naked eye to see.
Planets.
In the 2005, scientists found a planet orbiting Gliese 581. The scientists, led by Stephen Vogt, used a method called Doppler spectroscopy to find the planet and some facts about the planet. They said that the planet, named Gliese 581b, has a size close to the size of Neptune (the eighth planet in our solar system). They found that 581b weighs 16.6 times as much as the Earth, and that its orbit around GL 581 lasts for 5.366 days.
After this study was published, more scientists started to look for other planets that orbit Gliese 581. In the 2007, another group of astronomers said that they found two more planets. These two planets were named 581c and 581d. For planet c, the scientists measured a mass of 5.06 times the Earth's mass. They found that the planet's orbit lasts for 12.931 days. For planet d, they found a mass of 8.3 times the Earth's mass, and an orbit time of 83.4 days.
In 2009, the first group of astronomers found a fourth planet, named 581e. They measured the planet's mass to be 1.94 times the mass of the Earth, and measured the planet's orbit to be 3.149 days long. The astronomers also studied planet 581d again, and made more accurate measurements for that planet. They found that the planet has a mass that is 7.09 times the Earth's mass, and that its orbit lasts for 66.8 days.
Possibility of more planets.
Vogt's group of astronomers said that they have found two more planets orbiting Gliese 581, named 581f and 581g. They published their findings in 2010. For planet 581f, they found a mass of 7.0 Earth masses and an orbit length of 433 days. For the other new planet, 581g, the group measured a mass that was 3.1 times the mass of the Earth and an orbit that was 36.6 days long.
After the Vogt group published their report on the fifth and sixth planets, some other astronomers published opposing results. In 2012, these other scientists said that planets 581f and 581g did not actually exist. Their report stated that the Vogt group's new findings were false positives. They looked at the original Doppler spectroscopy data again, and found no signs of the two new planets.
However, the Vogt group published even more research that supported the two new planets. Later in 2012, they studied new Doppler spectroscopy experiments and found stronger signs of planets 581f and 581g. They also simulated the two orbit models that the two groups of scientists have proposed. Vogt's group found that the proposed system with four planets was unstable, meaning that if there were only four planets orbiting Gliese 581, the system would have broken a long time ago. On the other hand, they said that the system with six planets was much more stable.
Today, there is still no consensus about whether or not planets 581f or 581g exist.
Orbit models.
Two orbit models for the Gliese 581 system are being considered currently. An orbit model is a scientific model that astronomers create as a hypothesis of how a star's planets go through their orbits. One has four planets with eccentric orbits, and the other has six planets with circular orbits.
In the first model, some scientists propose that only the four planets 581b, 581c, 581d, and 581e orbit Gliese 581. None of the planets orbits in a circle. Instead, their paths around the star are ellipses.
In the second model, the six planets 581b through 581g orbit around Gliese 581. Here, each planet has a circular orbit around the host star.
Comet belt.
In November 2012, scientists at the European Space Agency said that they found a belt of comets orbiting around Gliese 581. Our solar system has a similar comet belt called the Kuiper belt.
Habitability.
Astronomers think that Gliese 581's habitable zone goes from approximately 0.05 AU (astronomical units) to 0.24 AU away from the star. This means that planets orbiting between these distances could be the right temperature to let organic life (life as we know it on Earth) survive.
If the six-planet model is correct, then at least two of the planets might be habitable. In other words, humans might be able to live there. In this model, the planet 581d orbits at the outside edge of Gliese 581's habitable zone, and the planet 581g orbits in the middle of the habitable zone. If the atmosphere on 581g is like Earth's atmosphere, the planet's temperature would be between 236 and 261 Kelvin (-37 and -12 Celsius, -35 and 10 Fahrenheit). This is only a little lower than Earth's temperature.
Even though these two planets might be the right temperature for life to live, other issues could make their environments hostile. Planet 581g might be stuck in an orbit resonance, where the planet spins at the same rate that it orbits around the star. This would mean that one side of the planet always faces the star, just like the same side of the Moon always faces the Earth. If this is the case, the side that faces the star would be too hot for people to live, while the other side would be too cold. Planet 581d might also be caught in an orbit resonance.