The conditions in space are hostile to life and travelling in spacecraft is difficult and expensive.
So scientists are using different methods to see if there is life on planets other than the Earth.
Space probes can explore other planets without needing astronauts.
Some of the missions undertaken by space probes include:
The Earth's atmosphere contains about 21 per cent oxygen as a result of photosynthesis by plants and single-celled organisms.
If we found evidence of oxygen in the atmosphere of another planet, it could indicate the presence of life forms.
It is possible to detect oxygen and other gases on other planets by studying the light reflected from planets.
It is thought possible that alien civilisations, capable of transmitting radio signals, may exist.
The Search for Extra-Terrestrial Intelligence (SETI) is a programme that uses radio telescopes to look for non-natural signals coming from space.
Space probes and landers are also looking for extra-terrestrial life.
There are photographs of channels on Mars that may have been created by flowing water.
Landers touch down on planets and take a soil sample, which is then analysed for evidence of life.