More Evidence for the Copernican Principle

18 November 2010

Thursday


The world is usually more complicated than we realize; there are almost always further layers of reality to discover. A widely reported discovery of a particular exoplanet made me aware of another layer of complexity in the world. An article in Science, subsequently reported on Science Daily and the BBC, described a particular exoplanet, i.e., a planet outside our solar system. Exoplanets are not news anymore, since hundreds have been discovered. We know that planets are plentiful in our galaxy. Now, it seems, we can even get a glimpse of planets in other galaxies.

I have been aware for some time that, in the long term history of the universe, galaxies collide, and the larger galaxies swallow up smaller galaxies. I have mentioned this in relation to the supermassive blackholes that reside in the center of spiral galaxies (in Appearance and Reality in Cosmology). It is pretty well certain that, in the distant future, the Milky Way and the Andromeda galaxies will meet in a slow motion collision, and at some time the supermassive black hole in the center of the Milky Way will be swallowed up by the even more massive black hole at the center of the Andromeda galaxy (or maybe they will end up orbiting each other). What I learned today, and what I hadn’t thought of previously, is that our galaxy has already swallowed up smaller galaxies in the distant past.

As it turns out, the Milky Way is surrounded by stellar “streams” that are the remnants of galaxies that have had the misfortune to run into the Milky Way galaxy, and were torn up and largely absorbed by the Milky Way. There is a list of stellar streams on Wikipedia. Not all galaxies have supermassive black holes at their center, and it would seem that the galaxies absorbed by the Milky Way in the “recent” past of the universe, and which have left traces in the form of stellar streams strung out by tidal forces, were stellar clusters or dwarf galaxies something like the Magellanic clouds.

Astronomers have managed to detect an exoplanet around a star in the Helmi Stream, which is a stellar stream likely the result of another galaxy absorbed by the Milky Way. The star and the planet are sufficiently old that they likely originated in their formerly independent galaxy, before it was absorbed by the Milky Way. And so it is that we can “see,” after a fashion, an extragalactic planet right here as part of the Milky Way. From this we can infer that exoplanets are not only to be found elsewhere in the Milky Way, but also in other galaxies, and indeed in galaxies of a very different construction than ours.

Almost a year ago, in Other Worlds, I discussed our increasing knowledge of extrasolar planets. At one time, all of this was sheer speculation. Now we have a growing body of scientific knowledge about extrasolar planets, what other solar systems are like, how plentiful they are, where we are likely to find them, and the like. This growing body of exoplanetary science has been inductively confirming the Copernican Principle, also called the Principle of Mediocrity, which holds that there are no privileged observers in the universe, which is equivalent to the statement that we are not unique. Now we know, and can demonstrate, that planetary systems are not unique to the Milky Way. From this stronger inductive position, we can with greater confidence extrapolate our existing knowledge to the furthest reaches of the universe.

The Copernican Principle tutors us in metaphysical modesty, but the growing evidence for the Copernican Principle, and the paucity of counter-examples, inspires us to metaphysical ambition (perhaps driven by metaphysical pride). Scientific knowledge is the expression of this metaphysical ambition as much or more than it is an expression of metaphysical modesty.

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Copernicus continues to shape not only how we see the universe, but also our understanding of our place within it.

Copernicus continues to shape not only how we see the universe, but also our understanding of our place within it.

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