Scientists have developed a way of tracking the habitability of planets where life may exist outside of Earth. And so where science was once hoping to discover life on other worlds by chance, a new system that categorically evaluates each new prospective world studied according to a series of criteria and then assesses the likelihood of each world actually harboring life has been developed.
The new system to be published in December in the journal of astrobiology solves this problem by instituting an Earth Similarity Index (ESI). The mystery of other worlds and whether they might have life either that we understand well or not has been at the heart of all planetary research performed since the inception of astronomy. So which are these planets most likely to have life? And are any of them within our grasp now? While not all of them are actual planets, a number of them are with other places including moons.
Of course the most habitable planet we know at the moment is Earth with an Earth similarity index of 1. Next in line is the recently discovered Gliese 581g with an index of 0.89, meaning it is the most earth-like planet of those in the known universe. The distant Gliese 581d comes in third with an Earth similarity index of 0.74 with Gliese 581c coming in fourth with an index of 0.70. But not all of the planets similar to Earth are quite so remote. We have a number of planets in our own solar system worthy of the list including Mars, which earned itself a habitability index of 0.70 just beneath Gliese 581c. Strangely enough Mercury also found itself on the list with an Earth similarity index of 0.60 - making it one of the more surprising additions to the list. A bit further away was HD69830 d with an index of 0.60 orbiting the orange dwarf star HD69830. Though HD69830 d is one of the more Earth-like planets, it is thought to be mostly gaseous like Neptune meaning if life does exist there it is unlikely to be life like anything we have ever seen before. 55 Cnc c is listed just beneath HD 69830 d with an Earth similarity index of 0.56 as it orbits 55 Cancri c - a sun very similar to our own with an estimated five planets orbiting it with 55 Cancri c estimated to be near the sun's habitability zone. The final two on the list are the Earth's moon with an Earth similarity index of 0.56 and Gliese 581e just beneath it with an index of 0.56.
Strangely enough, several of the planetary hopefuls did not appear on the list of potentials including Titan, Saturn's moon with not only a stable atmosphere but also strong evidence of liquid pools on the moon's surface. This is partially because the Earth Similarity Index is supplemented by a second list, the Planet Habitability Index. The list will likely be updated as additional measurements and theories are made, but for the moment this list of the ten most Earth-like discovered bodies in our galaxy will serve as evidence that the universe around us is constantly living with the possibility that life could spring up eternal around us.