mercury badge: name and stats
Chart of relative sizes of planets
Gliese 581 d HD85512 b
Gliese 581 d
HD85512 b

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 which rotates 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 gravitational 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.
Solar System's habitable zone

Currently only the Earth falls in the Solar System's habitable zone. It starts behind the orbit of Venus at 0.95 a.u. and ends before orbit of Mars at 1.37 a.u.

medium sized yellow star

The habitable zone will exist only around the star with the appropriate mass and chemical composition. These are the stars that have the mass between 0.5 and 1.5 solar mass with approximately same age as the Sun. If they are significantly older they will have less heavy elements.

Alpha Centauri

Alpha Centauri is the closest star system that may have habitable zone. The two brightest stars Alpha Centauri A and B form a wide binary. They orbit around each other with 79 years period and mean distance between them is 23 AU. The third star Proxima is so far way that is difficult to determine if it really belongs to the system.

The Milky Way Galaxy with locations of the Sun and the Perseus Arm highlighted.

Our Sun is located about 25,000 light years way from the Galactic center and 1.95 KPc away from the Perseus spiral arm, the nearest arm to us. Some existing hypotheses are trying to link mass extinction of life forms on Earth with the Solar System passing through the galaxy's spiral arms.

tidal locking

The locking of one astronomical body to another, so that one side of the smaller object is always facing the same way during its orbit around the larger object.

binary star

A two-star system, where the individual stars orbit their common center of mass, often appearing to revolve around each other.

Milky Way

The galaxy to which the Solar System belongs. It is a barred spiral galaxy and home to 200-400 billion stars.

corotation circle

The area inside the disk of a spiral galaxy which rotates around the galaxy's nucleus at the same rate as the galaxy's spiral arms.

Jovian moons

The moons of Jupiter, of which there are 64 known.

gravity

One of the fundamental forces of physics; gravity is responsible for the attraction of bodies on the surface of a planet to the mass of the planet.