What We Are Looking For and How Do We Find It?

     When it comes to planets outside our own, to put it precisely we are looking for another Earth. For planets nearby, we can see what they are made of and though there is some possibility to colonize them, it would be nearly impossible to make them like Earth.
     The main ingredients that come into building a suitable planet are as follows: there needs to be an atmosphere with a large amount of oxygen and not to many toxins, there needs to be habitable land that can support plant life preferably, some sort of harvestable structural resource to make a settlement, and water that exists in multiple phases, though it is fine if there is no ice. Beyond these there are a few subtle features like a differentiated and molten core to support a magnetosphere and a differentiated atmosphere, but if the planet has all of the features from the first list, it is likely that the other main features are accounted for. What we are looking for for these is a planet within the "Habitable Zone," an area around a star where its not too hot and not too cold.
     So how do we know where a habitable zone is. Mostly it depends upon the star, for example the following picture illustrates the difference in the habitable zones of our own fair system and the heavily populated Gliese 581 system.

     Notice how the zone is much closer to the star since the star is significantly closer to the star than it is in our own system.

     As for finding a planet with actual atmosphere and water, it is currently impossible unless the system is extremely close (and with the discovery of new brown dwarfs, there may be some very "local" systems). It will be up to future technology to determine which worlds have dry land, which have oceans, and which ones we would feel perfectly at home on.

     For now I will also leave this amazing Sagan video to highlight the importance of this wonderful planet we are part of.

May The Stars Shine Brightly In Your Skies!