There is a common belief among astrophysicists and other scientists that studying the universe has revealed our own planet as something less than special. The reasoning is as follows: Earth, long assumed to be stationary and unmoving, is just one of many planets orbiting our sun. Our sun is nothing more than a regular, nondescript star, one of hundreds of billions found within the Milky Way. The Milky Way itself is just one of an estimated 2 trillion galaxies strewn across the expanse of our observable universe. As our own insignificant home, Earth, is teeming with life, including intelligent and technologically innovative human beings, wouldn’t it be reasonable to infer that whatever is common here is plentiful throughout the universe?
According to this default assumption, the same ingredients found here—elements, molecules, and various favorable conditions—can be found practically everywhere we look. The same physical rules that apply here are no different elsewhere in the universe. Given all the stars, planets, and chances for life that surely exist within our galaxy and beyond, we’ve mostly stopped asking whether life exists beyond Earth. Instead, we now ask how common it may be.