The closest that many has come to explore is the red planet for being near to habitable. Mars is still being explored by NASA in the hope that one day it can be liveable. One problem however, since Mars used to be like earth and due to its core solidifying and losing it's magnetic poles, then the problem here lies. After it lost it's magnetic poles, it's atmosphere lost its pressure and water just vaporized out of the atmosphere so currently in some ways, it is the dry planet and the first thing that the scientist needs to figure out is that, out to terraform Mars by slowly spraying the atmosphere with Carbon Dioxide to form the atmosphere. Well, it is not easy, the planet is big and it may take a lot of centuries. I guess, Nasa could built these green house like structures to house the different habitats that lives there. Then water would be a concern. The next shipment to go into Mars could take seven years.
Scientist I guess are constantly searching for another planet to maybe send humans to one day, but it would not be an easy task as it takes lots of resources, research and man power to do it. Hopefully, one day, Aliens come to our planet and guide us there with their disc shaped advanced technological space crafts. Hopefully there aren't underground Aliens that live underground.
There are planets which are too cold like pluto and too small. Then there are planets that are too hot which are planets that are near the sun. I guess, Mars could be the next best thing. Why not scientist just spend more time improving our planet. This is the best thing we have at the moment. =)
Only time will tell. But I think it might be possible. By now humankind is not even close to being able to settle on another planet. Imagine what technology would be needed in order to do that.