Billions and Billions … of Planets
There are infinite worlds both like and unlike this world of ours. For the atoms being infinite in number, as was already proven, (…) there nowhere exists an obstacle to the infinite number of worlds. - Epicurus (341-270 B.C.)
Where is everybody? - Fermi’s Paradox, ca. 1950
In a universe of infinite possibilities, everything is not only possible, it’s probable. Of course, the universe isn’t really infinite. But in certain respects, it is big enough and contains so many of certain “things” that what is possible begins to get awfully close to what is probable.
So it is with planets, and perhaps life outside of our solar system.
But first, the news! NASA has announced its newest estimate for the number of planets in the Milky Way galaxy. Based on observations of a known system of extrasolar planets (those outside our own solar system), they determined there are at least 100 billion planets in our galaxy. At least. That means there is perhaps one planet for every star! That number doesn’t even take into account the existence of forever-alone rogue planets, wandering lifeless and free of warm parent stars.
Which brings us back to Fermi’s Paradox. Lower estimates say there are ~2 x 1011 stars in the Milky Way (200 billion). If we do some more extrapolation and guesstimation, we can say there are roughly 7 x 1022 stars in the known universe … 70 thousand million million million stars. If most galaxies are like the Milky Way, then that means each star would have one planet, on average. Now let’s say that a tiny fraction of those are at the right distance from their stars and composed of the right stuff and subject to a host of other perfect conditions, and they may be capable of supporting life. That’s still a ridiculously huge number.
So where is everyone?! It stands to reason, by odds and probability, that after 13.7 billion years of cosmic evolution we can’t be the sole special case in which that perfect storm of cosmic ingredients came together to produce life. That’s what bugged Fermi, and in a way, Epicurus before him.
NASA’s Kepler project continues to identify and track extrasolar planets, accompanied by occasional announcements of Earthiness. Yet what we have discovered is a fraction of a fraction of a fraction of the planets in but our galaxy. the odds of any of them harboring life is immensely small, while the chance that life exists somewhere remains likely … to people like me, anyway.
To say that Earth is unique in the universe would be to say that one grain of sand on Earth (~1018 of them, by the way) is somehow more special than all the others. Of course, if we asked the grain of sand, it might say “Yes, I am special!" because it is the best grain of sand it knows of. So it is with us, waiting alone in the "biggitude" of it all, to find out if we’re special.
It’s one of those classic questions that have tickled the human brain for ages. Let’s take something whose scale we have no way of reconciling, like the number of stars, and compare it to something else whose scale is completely neuron-warping, like the number of grains of sand on every beach.
Which is bigger?
This likely isn’t the first time you’ve heard this mind-bender. So I won’t be spoiling it by telling you that stars win … big-time. The universe is a very large place.
But we don’t have to go big to beat the stars. You’ll never guess how little of a certain common liquid it takes to beat both of those numbers … I’ll leave that to Robert.