The planets discovered in the Gliese 581 system (located only 20.3 light-years away) have generated much excitement in the scientific community as this system appears to have at least one habitable planet. Until recently, two planets were viewed as potential habitable planets as they orbited on the outskirts of the star's habitable zone, however now a new planet has been discovered which lies almost at the center of the habitable zone making it, officially, the first extrasolar Goldilocks planet.
Let's take a look at the Gliese 581 system and examine the possibility of life in that system. Gliese 581 is a red dwarf which, as the name suggests, is a small bright red star. Its mass and radius are both about 0.3 that of our Sun. This is a size and appearance comparison between the Sun and Gliese 581:
The star is also cooler and less bright than our own Sun, as its luminosity is just 1.3% that of the Sun. This means that the Goldilocks region is at a distance from the parent star which is much smaller than in our own solar system, since a planet that would have the same average temperature as Earth would need to be closer to the red dwarf. Fortunately, because Gliese 581 is so small, the planetary system has formed very close to the star.
So what are the planets of Gliese 581?
There are 6 known planets in that star system, we will talk about them in order of discovery.
The first planet to be found is Gliese 581 b, a planet comparable to Neptune in size and mass. This would indicate that it is a gas giant (like Jupiter, Saturn, Uranus and of course Neptune) and even though this is uncertain it is highly probable. It is very close to its parent star, orbiting at about 6 million kilometers and completing an orbit in about 5.4 days. By comparison, Mercury, the closest planet to the Sun in our own solar system, is at a distance of 58 million kilometers from the Sun and completes its orbit in 88 days. Gliese 581 b is the second closest planet to its parent star.
The second planet discovered is Gliese 581 c. This discovery led to significant media coverage of the event since this planet appeared to be a rocky super-Earth, with a mass 5.3 times that of Earth, orbiting inside the star's habitable zone. As mentioned in previous posts, the habitable zone is the area around a star where the conditions are such that liquid water can exist. It is also known as the Goldilocks zone, since it's not too hot and not too cold. Finding such a planet is a monumental discovery, as it is the best candidate for alien life. Unfortunately, although Gliese 581 c is in the habitable zone, it's at the inner edge, in the too hot area, and although the predicted surface temperature is Earth-like, it was calculated only based on radiation coming from the parent star. Taking into account the atmosphere, which could contain greenhouse gases (like carbon-dioxide) the temperature rises to the point where it becomes uninhabitable. Therefore, this planet is very similar to our own Venus since if we calculate the temperature on Venus based only on radiation from the Sun, we get a value of about 34.25 °C. However, the actual value is 463.85 °C, due to the planet's 96.5% carbon-dioxide atmosphere which has caused a runaway greenhouse effect (an extreme form of global warming). While the excitement of finding a potential habitable extrasolar planet was short lived, this discovery attracted astronomers' interest for the Gliese 581 system. Gliese 581 c is the third planet from it's star.
The next planet to be discovered was Gliese 581 d, the fifth planet in the star system which is also on the outskirts of the habitable zone, but unlike Gliese 581 c, this planet lies on the outer edge of the Goldilocks zone, in the too cold region, similar to our own Mars. It is 5.6 times more massive than Earth and so it is also a super-Earth planet. Regarding how the planet looks like, it is believed to be covered by a large and deep ocean. The planet is too far away from it's star to receive enough heat for liquid water to exists, however if it's atmosphere has greenhouse gases they could raise the temperature just enough to make liquid water possible.
Gliese 581 e is next on our list, the closest planet to it's star. This planet orbits further in than the habitable zone and is way too hot to support life. It has a minimum mass of 1.7 Earth masses, making it the smallest extrasolar planet discovered around a normal star, and the closest in mass to Earth. It is believed that the planet does not have an atmosphere, since it is so close to it's star that stellar winds would "blow" the atmosphere into space and it is also believed that it experiences intense volcanic activity as a result of tidal heating. Tidal heating is when an object is so close to a massive planet/star that the differences in the intense gravitational field generated by the massive body, create friction in the crust of the orbiting object, causing it to heat up. We have an example of this in our own solar system: Io, a moon of Jupiter is so close to Jupiter that it experiences tidal heating and as a result has massive volcanic eruptions.
Now comes the crown jewel of the Gliese system, the planet discovered on September 28th 2010, Gliese 581 g. Why has this planet generated so much excitement? Because it is well inside the habitable zone and almost at its center. In our own solar system, our home Earth lies between the orbits of Venus, the too hot planet, and Mars, the too cold planet. Well, in the Gliese 581 system, we have something very similar: Gliese 581 g lies between the too cold Gliese 581 c and the too hot Gliese 581 d making it the first Goldilocks planet ever discovered. The planet is tidally locked to its parent star, what this means is that one side of the planet always faces the red dwarf and one side is always dark. So there, a year (the time it takes a planet to orbit its star) is precisely equal to a day (the time it takes a planet to complete a rotation around its axis) and is about 36 Earth-days. It is believed to have a mass 3.1 to 4.3 times that of Earth, and a radius of 1.3 to 2.0 that of Earth. Assuming it is a rocky planet, it's surface gravity is expected to be between 1.1 and 1.7 times Earth's, meaning that walking on the planet's surface would be similar to walking on the surface of Earth, except you'd feel slightly heavier. It is also predicted that its atmosphere would be denser than Earth's.
What would life on such a planet look like?
It is believed that plants would be black in color. This is due to the fact that red dwarfs emit light mostly in the red and infrared part of the spectrum, so plants would need to absorb more of it in order to make photosynthesis. Therefore, they would have a black color, since black absorbs all light, making these plants more efficient at absorbing light than plants on Earth. These plants would probably exist only on the side of the planet facing the star, since the "dark side" would never receive any light and therefore plants undergoing photosynthesis would have no chance of surviving there. Even though the plants are more efficient, they would be less productive in the dim red sunlight, thus producing less oxygen than their counterparts on Earth. This means the atmosphere would have less oxygen and that potential animals that might exist would be smaller in size (it is believed that dinosaurs were so large because Earth's atmosphere had a higher concentration of oxygen back then). Also concerning animal life, it is possible that animals would be able to see in the red-infrared part of the spectrum since this is the most abundant form of radiation emitted by the star.
Weather models predict fierce winds blowing continually toward the night side and permanent torrential rain at the center of the dark side. Also, the red dwarf's variability could lead to serious climate changes. Red dwarfs are far more active than our own Sun, with solar flares and coronal mass ejections popping up fairly often. This can have a serious impact on life on the planet. It could prevent the existence of complex life, or complex life could develop forms of protection and dwell only in shielded places such as caves or deep under water.
Other problems include large periods of inactivity for the red dwarf, which could lead to ice ages. Many scientists believe that these variable conditions could render the planet uninhabitable. Still, the fact that the planet is within the habitable zone leaves the possibility open and we will only be able to know for sure when we send a probe there to investigate.
The last planet in the Gliese system is Gliese 581 f, a large planet (about 7 Earth masses) located at the edge of the solar system. This planet is probably a large terrestrial planet or a gaseous Neptune-like planet, and is far too cold to support the existence of liquid water on its surface.
Discovering such a complex star system like Gliese 581 so early in field of extrasolar planet finding, raises hopes regarding the discovery of other systems with the potential to support life. An abundance of systems like Gliese 581 doesn't necessarily imply intelligent extraterrestrial life and even complex life seems improbable, but even the existence of the most simple organisms would be a monumental discovery. Such an abundance would increase the chances of finding another intelligent species. Whether that has a positive or negative impact on our civilization remains to be seen.