SPACE

what is the main idea of the text 

it's teaching us abut Atrobiology it's the study of life in outer space. Astrobiology scientists look for conditions needed for life like liquid water a good temperature or the presence of oxygen. Some possible places to find life in the solar system are moons of Jupiter or moons of Saturn. These places may have liquid water.  They may be too cold to support life on their surfaces but if they have liquid water the water must be warm enough to support life.  Astrobiologists think that there may be a liquid water ocean under their surfaces. and about Extraterrestrial life that is about life that is not from the planet Earth. It is alien life. It is possible that planets like ours exist and that life  might start there also. So far none has been found though it is possible that life once existed on Mars. Searches have been made for signals from extraterrestrial life-forms. No signals have been received. This is not a new idea. Quite a few scientists have written about their guesses on the existence of other planets like ours with the  idea that what happened on Earth could also happen there. One thing we do know is that many other star systems do  have exoplanets. and last but not least is exoplanet is a planet from outside our own solar system. Estimates of the number of exoplanets in the Milky Way ranged from 17 billion to 144 billion. One estimate studied planets seen by the Kepler space observatory. Among them are 461 Earth-size planets at least four of which  are in the habitable zone where liquid water can exist. One of the four named Kepler 69c is 1.5 times the size of the Earth and  around a star like our own Sun – about as near as we have to finding an Earth 2.0 

Astrobiologists think there are at least 100 billion planets similar to our in our galaxy an average of at least one per star. There are  also planets that orbit tiny stars and free-floating planets that orbit the galaxy directly just as the stars do. It is unclear whether either  type should be called a planet.