Big Questions for Mars Exploration Rover - Spirit 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. 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.