Liquid water. All else is incidental.
Differentiation occurred in other inner planets, besides Earth.
Maybe, although we haven't found any planets with life-supporting conditions on them yet.
In this galaxy, only Earth for humans. Mars may once have had life, but it is not habitable to humans. Astronomers and scientists have found planets somewhat similar to Earth, but they may not have all the conditions necessary for humans. We still know very little about these planets.
No person has ever 'explored' any planet, besides Earth. Several people have been to the moon. Unmanned vehicles have been to Mars. Mankind has only 'explored' other planets visually.
The four inner, rocky planets (also known as the terrestrial planets) are; Mercury, Venus, Earth and Mars.Mercury, Venus, Earth and Mars
none
earth is the only planet with water, and is the only planet to sustain life but they are looking at mars because it is said to be another planet besides earth that may be able to obtain life
Differentiation occurred in other inner planets, besides Earth.
mars
most likely
the one that paul comes from
Yes.
Yes. Besides earth there are seven other major planets and 5 recognised dwarf planets.
You need safe living conditions in other planets besides the Earth, so it may still take a good while.
no because there is no oxogen in space
to have something to look at when you can see them