Where do we look for life in the Universe? The first place to look is in what is called habitable zones.
The habitable zone around the Sun is the region where conditions are right for liquid water to exist.
If we count the Sun as the only source of energy, the habitable zone looks like a ring around the Sun.
Rocky planets with an orbit within this ring may have liquid water to support life.
The habitable zone is not something static; it moves as the Sun changes and evolves.
A long time ago, Mars probably belonged to the habitable zone since the Sun was brighter during its early age.
As the Sun ages its luminosity will grow again and the habitable zone will migrate outwards.
The habitable zone around a single star looks similar to the habitable zone in our Solar System.
The only difference is the size of the ring. If the star is bigger than the Sun it has a wider zone,
if the star is smaller it has a narrower zone.
It might seem that the bigger the star the better.
However, the biggest stars have relatively short lifespans, so the life around them probably wouldn't
have enough time to evolve. The habitable zones of small stars face a different problem.
Besides being narrow they are relatively close to the star.
A hypothetical planet in such a region would be tidally locked
That means that one half of it would always face the star and be extremely hot,
while the opposite side would always be facing away and freezing.
Such conditions are not very favorable for life.
Habitable zones may exist around wide binary stars
, which are, as the name suggests,
a system of two stars orbiting around each other. Considering habitable zones around
binary stars is important, because only 30% of stars in our Galaxy are single stars like the Sun.
If the two stars are far enough apart, the planet close to one of them and far from the
other may have a stable orbit in the habitable zone.
According to one theory, the habitable zone of the Milky Way
also looks like a ring.
All the stars inside this ring make up what is called the corotation circle
with approximately the same speed as the galaxy spiral's arms, so they rarely will cross.
Passing through the spirals is dangerous, because the matter is denser than the rest of Galaxy.
That means that we have to expect more cosmic catastrophes like "comets showers" when planetary systems cross.
The other dangerous zone is the Galactic center, because of the hypothetical black hole and
the higher concentration of the stars.
Some moons might also be fit for life. Recent discoveries show that a few of
the Jovian moons
have subsurface oceans.
These oceans on otherwise freezing bodies are formed by tidal heating
which is the frictional heating of a satellite's interior caused by the
pull of the parent planet and neighboring satellites.
If we assume that the development of simple life is possible in such an environment,
we should expand the habitable zones to spherical shells around giant planets,
where the rocky and icy satellites may have stable life supporting orbits.