celestial bodies in our solar system, maybe Phobos is not a celestial body!
Other than Mars' size and being about forty percent less dense in composition than Earth, Mars could be transformed into a habitable planet for Humans. The process of 'terra forming' could create an atmosphere conductive for life as we know it. If carbon dioxide generators were infused throughout the planet's surface, over a period of several hundred years there could be enough bulk to the atmosphere to cause a runaway greenhouse effect. Eventually this greenhouse effect would allow the ice to melt and slowly create an atmosphere capable of growing plant life, which in turn would produce an oxygen rich atmosphere, possibly capable of supporting mammals. This is just a hypotheses and may not even be practical, but who knows for sure as man's knowledge advances.
THE JOVIAN PLANETS
Scientist and astronomers assume (incorrectly) that all solar systems are formed as our solar system was formed. The belief is that gas giants form outside of all terrestrial planets because of their lighter composition. Truth of the matter is, many solar systems have gas giants inside the orbits of terrestrial planets (in the habitable zone), making our solar system not typical in the Universe. If all solar systems were formed as ours were, there would be many more habitable planets in the Universe.
The first gas giant is Jupiter, the fifth planet from the Sun. Jupiter is so massive that its mass is more than twice that of all the other planets combined, more than three hundred Earths could fit inside of it. With that being said, Jupiter also has the largest magnetosphere in the solar system (other than the Sun). The magnetosphere is so large in fact that at times the planet Saturn is within Jupiter's gravitational influence.