Earth is basically sandwiched in between two very similar planets, Mars and Venus, making these two the obvious next step in our journey out into space. The three planets make up a perfect Goldilocks scenario - Earth, at the moment, is "just right", so we need to figure out which is the next best thing. Although Venus is much more similar to Earth in terms of size and mass, its surface is a death trap due to its thick atmosphere and rampant global warming, and is roughly 467°C on a good day. Mars, in contrast, is about -55° C but, despite this, makes a good candidate for Earth Mark II. A Martian day is very similar to on Earth at 24 hours, 39 minutes and 35.244 seconds. It also has an axial tilt very similar to Earth's meaning that the seasonal variations would be roughly the same, although the years are twice as long (this does mean that your Martian age is half of your Earth age, so that's a bonus). The presences of water ice and even running water on the surface of Mars makes it a compelling choice as all we would have to do is melt it, rather than import or synthesise our water supply. It is thought that Mars used to be very similar to the Earth, but its lower gravity and lack of magnetosphere meant that it lost most of its atmosphere a long time ago. This also means that it is unshielded from the dangerous solar radiation streaming out of the sun. These are issues that we could potentially overcome in order to colonise the Red Planet, but more on that later.
