Goldilocks and the Three Planets: the Story of Gliese 581G and What is Meant by Habitability

I've received many questions over the last few days, regarding the exciting news about the newly-discovered planet, which for the time being goes by the not so exciting name of Gliese 581G. This name comes from the name of the star that the planet orbits, Gliese 581, a red dwarf star about 1 percent as bright as our star, the Sun, and located approximately 20 light-years away. As far as I can tell, I'm being asked about the planet, because I'm an aerospace student and the detection of planets around stars other than the Sun is one thing that astrophysicists do. While aerospace engineers do not look for planets, but we're very interested when new worlds are discovered, because they could turn out to be places where life forms have taken hold, and Gliese 581 certainly is in this category. At the same time, since I'm not a physicist, I tend to explain things without using a great deal of mathematics, and most readers may appreciate this.

Now actually several planets have been detected around Gliese 581 and they are named alphabetically, planets A, B, C, D..., but this does not correspond to their relative distances from the star. Thus, planet G actually orbits outward from planet C and inward from planet D around Gliese 581. While all three of these planets orbit closer to Gliese 581 than Mercury does to the Sun, they illustrate what both astrophysicists and astrobiologists term the Goldilocks zone. Everybody knows the story of Goldilocks and the three bears; for those of you with young children, it may be fresher in your memory. The first bowl of porridge is too hot, the second bowl is too cold, but the third one, the baby bear's porridge, is just right, not too cold, not too hot. While I have no better idea what porridge is than I had when I heard the story as a child, it is an extremely useful analogy. When we talk about the Goldilocks zone, we mean the range of distances from a star, where a planet can orbit and have moderate temperatures, thus allowing for water to be stable in the liquid state.

For a star as bright as our Sun, the Goldilocks zone turns out to run roughly from the orbit of Venus to the orbit of Mars, with Earth orbiting at a distance intermediate of our two planetary neighbors. And so, Earth, Venus, and Mars, all orbit within the Goldilocks zone of the Sun. While the atmospheric pressure on Mars is much lower than it is on Earth, it is such that at the surface it is possible to have transient liquid water at certain times of the day, at certain times of the year. It also is possible for water to exist as liquid for long periods of time on Mars, provided that it is very salty water -a scenario that has been proposed to explain the observations that have been made in recent years, suggesting that water flowed on the surface of the Red Planet fairly recently, even though the atmospheric pressure at the surface is no more than 7 millibars. Analysis of meteorites that were catapulted from the Martian crust and made their way to Earth shows high levels of various salts in the rock, and so this hypothesis of briny seas and rivers on Mars of the past really makes a lot of sense. And if there was water, then probably life arose on Mars and still exists there today, since life is pretty tenacious and evolves to take advantage of changing environments.

As for Venus, it too is in the Sun's Goldilocks zone, albeit at the inner edge. While the surface temperature on Venus is something like 900 degrees, this is not because the planet is a little bit closer to the Sun than Earth is. Rather, it is because of an immense greenhouse effect, resulting from the thick Venusian atmosphere, which is loaded with carbon dioxide, much more than Earth's is. If similar amounts of carbon dioxide were released into our atmosphere, Earth too would become uninhabitable at the surface, though theoretically it is possible that Venus could harbor microorganisms somewhere it its cloud layers, where the air is cooler. Most people are aware that Mars is much colder than Earth. This is partly because it is further away form the Sun, but as in the case of Venus, it also has to do with the atmosphere. Although the Martian atmosphere is mostly carbon dioxide, because t