For the first time in human history we know of planets around other stars and many of those other planetary systems look quite different from our own. Many have a planet like Jupiter, or even bigger, nearest to the Sun. If we are to understand why this is the case, and how likely it is that there are Earth-like planets elsewhere, we need to better understand how planets form.
Many of the other solar systems have massive Jupiter like planets close to their Sun, closer even than Mercury. Many scientists now believe that these gas giants could not have formed there. Rather, they must have began out where our Jupiter is, and moved inwards, scattering the smaller planets with their powerful gravity as they went. Why is it that our Jupiter and Saturn did not migrate inward? We are trying to learn more about our outer solar system by sending probes there. We sent Galileo to Jupiter, Cassini is at Saturn right now, and New Horizons is on its way to Pluto even as you read this.
Microbial life forms have been discovered on Earth that can survive and even thrive at extremes of high and low temperature and pressure, and in conditions of acidity, salinity, alkalinity, and concentrations of heavy metals that would have been regarded as lethal just a few years ago. These discoveries include the wide diversity of life near sea–floor hydrothermal vent systems, where some organisms live essentially on chemical energy in the absence of sunlight. Similar environments may be present elsewhere in the Solar System.
The possibility of finding life elsewhere is for many people the most compelling reason for humankind to explore beyond the Earth. We believe that liquid water and carbon are required for life to arise and thrive, as well as a source of energy. Many places in the Solar System provide these, at least for a time; not only planets, but also some moons and even certain comets. But for life to arise we presume that a hospitable environment must be more than just transient.