exoplanet - Are there any common characteristics of habitable planets?

I think I will answer your question from two vantage points. Firstly, what do we have to measure about a new exoplanet to consider it potentially habitable? Keep in mind that in detecting these 1000 or so new exoplanets, we've only gone so far as to show that they (statistically) exist and to measure some very very basic properties of them. It is on these very basic properties that we make our judgement of habitability and further research and observation is necessary to confirm these suppositions. When a new exoplanet is announced, we generally know the following information, which we use to determine habitability:

1. Distance from the star. This is a crucial piece of the puzzle because it tells us generally how hot or cold the planet is. We presume that life cannot exist on planets which are boiling hot or freezing cold. This information can be ascertained from either the radial velocity method or the transit method.


2. Size of the planet. Note here, size generally refers to diameter or radius. This is important because we want to generally know what type of planet we've detected. Gas giants are, well ... giant and inhospitable to life as we know it. We want to make sure the planet we've found is small enough that it is likely terrestrial. Although size is not a guarantee that it is terrestrial, its a piece of the puzzle. We can measure size by the transit method, however there is some degree of uncertainty to this process because we need to know the size of the host star to a good degree of accuracy, which is not always possible.


3. Mass of the planet. This is a crucial component in determining habitability. Size alone may tell us that a planet is what we call a "Super Earth" or "Sub Neptune" in that it is slightly larger than our own planet. The problem with that is, if the planet sits on the size border between Earth and Neptune, how are we to know whether it is rocky or gaseous? This can be resolved by looking at the mass, which is ascertained by the radial velocity method. The mass combined with size, gives us density and density is a surefire way to know if the planet is made of dense material or light gases.

That's really about it for basic information gleaned from simple planet detection methods. If you have distance from star, size, and mass (all of which you can get from the RV and transit methods) then you can presume that the planet is Earth-like and at a good temperature. That's enough to claim potential habitability, but not enough to actually show it can support human life. For that, you need to investigate the planet further and show it meets a variety of other conditions. As of know, some of those conditions are measurable with enough work, some aren't. These conditions may include, but are not limited to:

1. An atmosphere with water in it. We have the technology to measure this fact, but I'll say that it is very very difficult to do so and I don't know if anyone has really successfully shown water exists in an exoplanetary atmosphere (some have convincing evidence though). Generally, the process to do this is to measure the radius of the planet as a function of wavelength. Different wavelengths of light will refract through the atmosphere differently (just like in a prism) and so you measure a different radius of the planet (radius being the physical size of the planet plus the extent of the atmosphere) at different wavelengths. From this information, you can model what the atmosphere must be made of to cause the radius to change depending on wavelength. The issue with this process is that is requires removing all possible false detections and having a great signal to noise which is very hard.


2. A magnetosphere. We believe this is a necessity for life. Every second of every day our planet is being bombarded with high energy particles from our Sun. Most of these particles are harmlessly deflected by our magnetosphere and so we're protected. Planets without this protection would have a hard time hosting life as that life would be destroyed by radiation (or at least life like us). There is currently no way to detect whether a planet has a magnetosphere, although I believe there are potential ideas on the chalkboard.


3. The presence of a moon. This one is a bit iffy. Some people will argue our Moon has been instrumental to our development and for life in generally. The argument generally goes that the Moon has helped stabilize our tilt and thus kept our seasons and global weather well regulated. While we might be able to exist on a planet without such conditions, some believe life cannot begin on such a harsh planet without that regulating moon. There is also the benefit that the Moon has shielded us from a large number of impacts by taking the blow itself. Again, we could live on the planet, but the lack of a moon might not foster growth of life naturally. Moons, at this time, are out of our reach to detect.

There are numerous other factors for what helps define whether a planet is habitable, be it by us or in hosting new life. The list could go on and on, but these are the major points I think.