Free Astronomy Magazine September-October 2021

5 SEPTEMBER-OCTOBER 2021 ASTRO PUBLISHING By considering stars slightly smaller to slightly larger than the Sun, we find estimates ranging from 0.02 to over one potentially habitable planet per system. It is difficult to determine which of these two ex- treme values is closest to reality, as the smaller planets are less easy to detect, especially with the most pro- lific method used by astronomers - transits across their stellar discs. A planet comparable in size to the Earth, seen as it transits in front of a star similar to the Sun, reduces the star’s brightness by just about 1/10,000, a value that is close to the detection limits of the small space telescopes currently used in these types of observations. Being able to observe planets even a little smaller than the Earth does not, however, mean being able to make a reliable projection of the number of existing Earth-sized planets. In fact, we know