One of the most common unanswered questions scientists find themselves asking is 'Is there life on other planets?' Since the first famously documented UFO sighting in 1947, the idea of extra-terrestrial life has been debated almost non-stop. The subject has inspired many TV programs, such as The X-Files, and films (Mars Attacks, Independence Day, and the Men in Black, Alien, just to name a few). Scientists have come up with many new ideas and ways of trying to either prove or disprove the existence of life elsewhere. Mars is a very similar planet to earth in relation to size and atmosphere. Seeing the similarity one would assume to start searching there for life.
At the end of the 19 th century, an American named Percival Lowell built himself an observatory to study Mars in specific detail when its orbit was closest to Earth. At this time it had recently been suggested that the planet had a system of channels on the surface, present from the evaporation of flowing water. Looking through his telescope Lowell became convinced he could see a network of artificial canals. This led him to believe that there were intelligent beings on Mars who had built these canals. However, spacecraft have now visited Mars and found that there is no evidence of water at all. It is now thought that the lines he could see were the combination of Lowell's overactive imagination, and scratches on the lens of his telescope.
We are now searching one of Jupiter's moons, Europa, as this seems to be the next likely place to hold life. It is seen to be more likely, however, that we will find less intelligent life in one of two different ways: It may be possible for us to obtain material from another planet or moon or star from elsewhere in the Solar System. Spacecraft may be able to visit these bodies and, for example, use a robot to collect material for examination. This may be examined on site, or brought to Earth to be investigated in laboratory conditions.
They could be tested for things such as evidence of fossilized organisms. Another, possibly slightly far-fetched hope is that we may find simple organisms like bacteria actually living on the desired planet. These ideas spanned from the discovery of rock on our planet that originated from Mars; knocked from the planet when a comet collided with it. In 1996 a group of scientists created conflict by claiming that they had found evidence of fossilized bacteria in one of these rocks, but other scientists disputed this idea. The other possibility is that we can examine the atmosphere of other likely planets or moons. The planet Earth is largely made up of oxygen (20%) due to the presence of photosynthesizing plants producing oxygen as a waste product.
If an indication of oxygen appeared in a different planetary atmosphere, it would have a high chance of holding life forms on it. To learn about different atmospheres it isn't mandatory to visit the planet. We can find this information by looking at the light spectrum it emits.