The idea that there might be life on other planets is an idea that has intrigued the human race for many years. With reports of UFO sightings and claims of alien abductions it could be argued that we are not alone in the universe, but how likely is it that we are alone in the universe?
In order for a planet to sustain life it is believed that it needs to lie in the zone around a star called the habitable zone. This area is thought to be where temperatures on the planet are suitable for there to be H20 (water) on the surface in liquid form. H20 is thought to play a vital role in creating life as a solvent for biochemical reactions.
However this only relates to life as we know it, while carbon based life needs to be placed in a certain area of the solar system to create life this would not necessarily be true if other life forms such as hydrogen or helium based life forms were to evolve on a planet.
Assuming our search for life is simply for carbon-based life forms then you can argue using probability that it is incredibly unlikely that we are alone in the universe. We have currently found, using the Hubble telescope and other deep space probes that there are around 100 billion galaxies in the universe, if each of these galaxies has 100 billion stars in it, that means there are 10,000,000,000,000,000,000,000 stars in the universe.
When looking at stars other than our own astronomers have already found stars close to ours with planets orbiting them. It is believed that stars with planets orbiting them are not unusual phenomena. Even if the chances of this are one in a million which, judging by recent findings is a very conservative estimate that would mean there are 10,000,000,000,000,000 other solar systems in the universe
If you then estimate that the chances of one of these solar systems contains life is again one in a million that means there are 10,000,000,000 (ten billion) planets in the universe which are capable of producing life.
Recent studies, however, suggest that the planet Jupiter plays a key role in sustaining life on a planet. It is widely believed that these larger planets protect smaller planets from collisions by interstellar objects. It is thought that up to 10% of stars in the Orion nebula have Jupiter sized planets, which means the chances of extra terrestrial life out there might be quite low.
Scientists are constantly looking out into the universe and finding new planets, some are remarkably earth-like, however so far the ones they have found have been deemed too hot to contain liquid H20. The simple fact is that we don't know exactly what is out there in the universe, every day new planets are being discovered and we simply don't know if there is life out there.