Catalog of Habitable Planets
The idea of what makes a planet “habitable” is one of temperature, distance from its parent star, composition, etc. It’s not really one of utility, i.e. “actually being able to go live there”.
The search for exoplanets via NASA’s Kepler project and others has perhaps not discovered life-rich alien worlds (the equipment they use couldn’t do that if they tried), but it has reinforced the idea that there are a TON of planets out there. Planets, like or unlike our own, are very common.
The Planetary Habitability Laboratory at Puerto Rico’s Aricebo observatory (the big crater telescope from Contact) put together this catalog of potentially habitable known exoplanets ranked by their “Earth Similarity”. Visit the link for a load more details.
As our techniques and equipment advance, this list will surely grow by leaps and bounds. It doesn’t mean that any of them will harbor extraterrestrial life, but it reminds us that while the scientific characteristics of our home might not be uncommon, the ability for one species on Earth to search for others like it surely is something special.
When a planet is assessed based on its size, composition, density, temperature and orbit velocity; it is given a score from zero to one (one being identical to earth). This measurement is the ESI (Earth Similarity Index).
Giving the humongous number of galaxies in the universe (an estimate of 100 Billion), each galaxy would have 100 Billion stars of its own, each star would have half a dozen or a dozen of planets; do the math, you get the idea!
Here’s where the ESI comes in play. Scientists need a way to filter out discovered planets in the vast outer space, on a mission to find that one other, or many others, habitable planets. Therefore, compiling such catalog is hardly an achievement, because a time will come when scientists have to come up with new measurements to filter out the planets listed in that catalog once again, based on whole new criteria to compile another catalog that shares the same fate with its predecessors; and the cycle starts over, again and again.. in order to cover a whole bunch of complex criteria, as complex as life on earth.. depending on equipment and technique advancements of course.