Question

How many planets are believed to have life on them?

Answer 1

Hint: You may be very familiar with the various missions intended on finding these planets that can sustain life. When you read the question clearly, you will realize that we are supposed to give the number of the planets that have life on them not the ones with mere possibility to sustain life. Hence, answer accordingly.

Complete answer:
In the question, we are asked how many planets are believed to have life on them. This is a much abstract question. Believes can be anything as it varies from person to person. As a science student, you should look for proof to confirm any of these beliefs. However, one should also keep an open mind to accept the possibility of the existence of a planet other than the one on which we are living that has life on them.
You may be well aware that the major part of our universe is left unexplored due to a variety of reasons. So, we are unaware of the existence of another planet that has life but it may or may not exist.
Now, NASA major missions are eyed at finding such exo-planets and also the possibility of life sustainable on other known extraterrestrial objects.
We could quantify the number of planets that are habitable in our galaxy which would be around 5 billion.
So we could conclude that our planet earth is the only known planet that possesses life.

Note:
Now you may think of the factors that qualify a particular planet to be habitable. Some of these characteristics are: it should be orbiting at a right distance from a host star in order for the liquid surface water to be present. Also, various geophysical and geodynamical aspects, atmospheric density, radiation type and also intensity along with the host star’s plasma environment are qualifying factors.