Lets also assume that it is very rare among planets to be Earth like. Again only 1 in a million, and this is very conservative because we are assuming that only earth like planets are suitable for life. So by simply dividing we get ten billion. But for more scientific approach we can consider Drake's equation. This equation basically states the number of communicating civilizations in our Galaxy, which depends on a number of factors which when combined yield a habitable planet where life can develop and grow to a certain level of technological advance. So, by his estimate the number of communicating civilizations in our Galaxy is 10,000. .
There are up to 50 billion planets like earth in our own Milky Way galaxy. The chances that none of those planets support life are so slim that they can be basically discounted. Many of the stars that have been observed contain rock and iron rich material, which is connected to them by gravitational force of orbiting planets. This is another indication that life may be common in our galaxy. There are many planets like earth orbiting those stars and the probability that there is life similar to ours is greater then it was before we considered this evidence. .
There are few conditions that are important when we are considering the possibility of life on other planets. Those include temperature, water and atmosphere. Also the planet has to be in a suitable habitable zone, which means that it has to be in small range distances from the star where temperature will be suitable, water may exist in liquid form and atmosphere will be retained. .
The search for life in our solar system reached its highest peak with successful landing of the two Viking spacecrafts on mars in 1976. They contained materials to look for biological processes, photosynthesis and respiration. Although metabolism was found, they failed to detect any presence of organic molecules.