Astronomy: New Potentially Habitable Planets

Artistic depiction of the view from Gliese 667Cd. Credit: ESO/M. Kornmesser

The recent news is that 2 more planets in the Habitable Zone of the star Gliese 667 (aka GJ667) have just been discovered. These are GJ667C f and e. I had previously written about GJ667Cc, another habitable world in the same system. It appears that this star system could host 3 planets that, if the conditions are right, could harbor liquid water on the surface of the planet.

These planets join a growing list of potentially habitable planets.

List of potentially habitable planets. Credit: PHL @ UPR Arecibo

The figure above shows a dozen planets thought to be most likely to be habitable. Note that a few, like Gliese 581g, are unconfirmed.

The planets are ranked by the "Earth Similarity Index" or ESI, which I recently learned is an index that measures how similar a planet is to the Earth. It compares the radius, density, surface temperature, and escape velocity to that of Earth's and values close to 1 are very good (ie, Earth-like). I was surprised to see escape velocity among the parameters (since it depends on the mass and radius) given that the density, which also folds in the mass and radius, is already considered.
Note that despite the high temperature on the surface of Venus, the mass and radius are so similar to the Earth that the ESI is close to 0.8. However, Venus is not in the habitable zone as it's too close to the Sun.

This is an exciting time for planetary research in astronomy as more and more planets are being discovered and we are starting to probe planets whose conditions may be favorable for liquid water and ultimately, we hope, for life.