Nicolaus Copernicus taught us in the 16th century that we are nothing special, in the sense that the Earth on which we live is not at the center of the solar system. This realization, which embodies a principle of mediocrity on the astrophysical scale, has become known as The Copernican Principle.
In the centuries that have passed since Copernicus’s discovery, it seems that the Copernican principle has significantly gained strength through a series of steps that have demonstrated that our place in the cosmos is of lesser and lesser importance.
First, astronomer Harlow Shapley showed at the beginning of the 20th century that the solar system is not at the center of the Milky Way galaxy. It is in fact about two thirds of the way out. Second, recent estimates based on searches for extrasolar planets put the number of Earth-size planets in the Milky Way in the billions. A good fraction of those are even in that “Goldilocks” region (not too hot, not too cold) around their host stars, that allows for liquid water to exist on the planetary surface. So we are not very special in that respect either. Third, astronomer Edwin Hubble showed that there exist galaxies other than the Milky Way. The most recent estimate of the number of galaxies in the observable universe gives the staggering number of two trillion.
Even the stuff we are made of—ordinary (baryonic) matter—constitutes less than 5 percent of the cosmic energy budget, with the rest being in the form of dark matter—matter that does not emit or absorb light (constituting about 25 percent), and dark energy—a smooth form of energy that permeates all space (about 70 percent). And if all of that is not enough, in recent years, some theoretical physicists started speculating that even our entire universe may be but one member in a huge ensemble of universes—a multiverse (another scientific concept we ought to get used to).
It seems, therefore, that the Copernican principle is operating on all scales, including the largest cosmological ones. In other words, everybody should be aware of the Copernican principle because it tells us that from a purely physical perspective we are just a speck of dust in the grand scheme of things.
This state of affairs may sound depressing, but from a different point of view there is actually something extraordinarily uplifting in the above description.
Notice that every step along the way of the increasing validity of the Copernican principle also represents a major human discovery. That is, each decrease in our physical significance was at the same time accompanied by a huge increase in our knowledge. The human mind expanded in this sense just as fast as the expansion of the known universe (or the multiverse). Copernican humility is therefore a good scientific principle to adopt, but at the same time we should keep our curiosity and passion for exploration alive and vibrant.