NASA has a huge task at hand and in trying to best decide where to look and what to look at; I can only hope they aren’t ‘throwing the baby out with the bath water!’ Perhaps not the best saying to use, but you get the point. I commend NASA on their methodology and ranking BUT and this is a big but, they also need to account for the possibility that life doesn’t always follow ‘our’ rules.
Why do I ring this up? The James Webb Space Telescope is due to be launched in October of 2018. Because this telescope (which is the next iteration of the Hubble) has many advantages that the Hubble did not, we will be looking at the Universe in a whole new way! You are able to see distant objects best if viewed in infrared. (The object’s light is pushed from the UV and optical into the near-infrared.) As our interests have developed, we have looked farther out into the Universe. While Hubble has some infrared capabilities, the Hubble usually views the Universe at optical and ultra-violet wave lengths. Webb on the other hand will mostly look at the Universe in infrared.
Another major difference is that the Hubble circles the Earth at about 17,000 MPH. (The ISS circles it at about 17,150MPH) whereas the James Webb will reside near (not in) L2 – the second LaGrange point in a halo orbit. (That is a 3-d orbit located near either the L1, L2, or L3 La Grange points.) The Second La Grange point combines the gravitational pull of the Sun and the Earth and also will allow a spacecraft to complete the orbit of the Sun in the same time as the Earth does. A halo orbit can best be described as a controlled drift in the vicinity of the La Grange point. AND because it will be in close company with the Sun, the Webb Telescope has some awesome shades. (It has a sunshield - a membrane of aluminum and silicon and is made of polymid.)
Now the point that I am taking my time in getting to is that because NASA understands that it will be expensive to ‘book’ time on the telescope, they are following a ‘habitability index for transiting planets. Since the total to date of exoplanet candidates is over 4000 and confirmed ones at 1000, knowing which planet to focus on first when looking for signs of life is important. Previously the habitable zone carried the necessary filter but now a habitability index has been developed to further narrow it down. Interesting that being considered is “habitability” which suggests we could live there, not those factors which might be attractive to other species. Two of the factors being considered are: rockiness of the planet as they are more Earth-like, called “eccentricity-albedo degeneracy” - the energy reflected back to space from its surface — and the circularity of its orbit, which affects how much energy it receives from its host star.
While it certainly makes sense to look for planets that would be more habitable to us, if we are truly looking for life consider the many extremophiles. Actually, just consider one: the Tardigrade or Water Bear or would you prefer the Moss Piglet. While not able to take advantage of its adaptability as it goes dormant in several situations, it will none-the-less survive them intact. Tardigrades have been found in hot springs, on top of the Himalayas, under layers of ice, in ocean sediments and remain active as long as they can remain moist. In outer space – they can survive without any little spacesuits or atmosphere. In fact, they were the first known animal to survive being sent into orbit and exposed to solar radiation. (FOTON-M3 mission – when they returned to Earth they were rehydrated and after 30 minutes some were okay others had produced off-spring and their progeny survived.
So if we are going to search for life, the habitability index is good, but make sure included is also some outliers – cause one never knows what form life will take or how it will survive!