The latest announcement making the headlines is for GJ 667Cc. The claim is that this is the 4th extrasolar planet found in another star's habitable zone.
The GJ 667 System
First, a breakdown of the name. GJ stands for the Gliese & Jahreiß Catalog of Nearby Stars. This is the 667th star in that catalog. The uppercase C refers to the fact that this star is in a triple system: there's GJ 667A, GJ 667B, and GJ667C. And finally, the lowercase c refers to the second planet discovered in the system (the first would be GJ 667Cb, there is no 'a' planet).
GJ 667, located about 22 light-years away, is a triple system formed by a pair of closely separated K stars and a more distant M-type companion. K and M are spectral types and indicate the effective temperature at the surface of these stars, this in turn is modulated by a star's mass and age. The M star (GJ 667C) is the least massive of them all and is at least 230 astronomical units (AU, the distance between the Earth and the Sun) away from the closer pair. I had recently mentioned how binaries and multiples can hinder planet formation (see here), this is yet another example of the case where the separation between the stars is large enough that planets can still form.
One other interesting observation is that the system is 'metal poor'. What this means is that, compared to the Sun, the system has far less heavy elements (oxygen, carbon, iron, etc). These are the building blocks of planets and you expect to see more planets when you have more of these elements.
The Habitable Zone
A key concept in extrasolar planet research is that of the habitable zone. The idea is that if we are to search for life, we should search for the conditions that permitted life to develop on Earth. The most important thing for life on Earth is liquid water. Hence, all we have to do is look for systems that can host liquid water. This is why so much emphasis is given to the exploration of Mars and Europa. Both of these worlds in our own solar system may have liquid water beneath their surfaces.
For extrasolar planets, however, the situation is trickier. We don't know the characteristics of the planet, and the atmosphere is of utmost importance in determining the surface temperature. For example, Venus could be argued to be in the habitable zone, but with a surface temperature of 460 °C (860 °F), it clearly cannot have liquid water on its surface. One easy approximation, though, is to assume that the planet has an atmosphere comparable to that of the Earth's. If the Earth were moved closer to the Sun, the surface temperature would be higher and, eventually, all water would evaporate. In the opposite case, if we move Earth farther from the Sun, the temperature drops until all the water freezes. The region where the water can be liquid on the surface defines the habitable zone. The edges are rather fuzzy, since atmospheric physics plays an role in controlling the temperature.
We can apply the same criteria to other star systems. In this case, though, we have to take into considering the type of star. Hot stars will emit more light and have more distant (and broader) habitable zones. Cool stars, like GJ 667C, will have habitable zones that are closer and narrower than the Sun's. The estimated habitable zone for GJ 667C extends from 0.11 to 0.23 AU. For comparison, Mercury's orbit around the Sun is about 0.3 AU.
|Habitable zone comparison for stars of different luminosities.|
An international team led by Guillem Anglada-Escude of the Carnegie Institution of Washington just recently put out a Letter on the Astrophysical Journal regarding their results (you can check out the paper here). I remember seeing the paper this morning, but didn't read the author list. Now I realize one of my colleagues here at Universidad de Chile is among the authors. There's also a pair of authors from the Pontificia Universidad Catolica de Chile, so it's no surprise that it's all over the news in Chile (for example here and here).
The study involved using a new set of software to re-analyze some radial velocities. Most planets discovered to date have used the fact that a planet will cause the star to shift back and forth relative to us. This will induce a small variation in the radial velocity of the star that we can detect. The bigger the planet and the closer to the star, the greater the variation and the easier we can detect it.
With this technique, the team finds planets orbiting with periods of 7.2 and 28.15 days. There's also a suggestion of a third planet (at 75 days) and a linear trend that could suggest a giant planet in a much larger orbit.
GJ 667Cc is the 28-day planet. For this system, that corresponds to a distance from the star of about 0.123 AU. As mentioned above, this is within 0.11 to 0.23 AU, the habitable zone for this star. The planet is considered a 'super-Earth'. This is because the estimated mass is about 4.5 Earth masses. This is much larger than the Earth's and presumably the atmosphere will be a lot thicker. This can pose some problems, but we don't actually know any details of the atmosphere. Even if the planet itself is problematic, you can't rule out moons in orbit around it. These would also be in the habitable zone (though they could be too small to have an atmosphere and liquid water).
|Orbital configuration of the GJ 667C planet system in comparison to our Sun's. The habitable zone, and it's possible extent, is illustrated. Note the different distance scales for both systems. Credit: Guillem Anglada-Escude et al. (2012)|
Can the planet truly have life? Unfortunately, we are in no place to answer that question. We still need to pin down the parameters of the system and we would still have no idea on the atmospheric properties of the planet. In addition, I had previously mentioned that low mass stars are notorious in producing large flares of high-energy radiation that can be detrimental to life (see here). The activity level of the star is not mentioned in the paper, but, if I recall correctly, planet searches tend to focus on the more quiescent stars. Even if it's quiet now, it may have been more active in the past when it was younger (the system is at least 2 billion years old).
Other Habitable Worlds
The other candidate habitable worlds being listed in these press releases are GJ 581d, HD 85512 b, and Kepler 22b. GJ 581 has a controversial planet (g) that lies neatly in the habitable zone, but different researchers argue whether the planet actually even exists. The d planet, however, is not disputed and lies just inside the outer edges of the habitable zone. All of these planets are super-Earths. That is, they are several times the mass of the Earth. This is not surprising as the more massive a planet is the easier it is to detect. I'm sure that as our methods improve we will find smaller, more Earth-like, planets.