How can we estimate the number of Earth-like planets in the Galaxy?


I'm doing a science project and i had to read a science fiction book and find the facts in it and it said that 100 billion stars in our galaxy and 10 percent of those starts have planets and maybe one percent of them have earth-like conditions. Is that true? And how can you estimate the number of stars with planets with earth-like conditions?

We do not have a very complete picture of how planets form around stars and how Earth-like planets form, although the basic idea is that they form from a disk of matter around a young star.

There have been a few observations of such disks around young stars and we know of about 100 extra solar planets (none are Earth-like).

From the basic idea and the small number of planets we have found we can make an educated guess at the number of planets that are really out there and the fraction of those which might be Earth-like. The numbers you quote sound OK, but should be taken only as a rough estimate for the reasons I gave above.

You might notice that these small fractions still give an estimate of 100 million Earth-like planets in our Galaxy alone, so the main point that you should draw from this is that since there are such a large number of stars in the Galaxy, even if it is extremely unlikely that a given star will have an Earth-like planet around it, there could be a large number of them overall.