Earlier this month, Bill Clinton was asked by Jimmy Kimmel if he saw any evidence of alien visitation when he was president. Clinton said no, but added he wouldn’t be surprised if we were visited some day.
“We know now we live in an ever-expanding universe,” he said. “We know that there are billions of stars and planets literally out there — and the universe is getting bigger. We know from our fancy telescopes that just in the last two years more than 20 planets have been identified outside our solar system that seem to be far enough away from their suns — and dense enough — that they might be able to support some form of life.
“So it makes it increasingly less likely,” Clinton concluded, “that we’re alone.”
It’s a common argument: The universe is so large, how can we possibly be unique?
But what if that thesis is only half right? Yes, there are vast numbers of Earth-like worlds out there. They’re also extraordinarily rare. And the odds of finding intelligent life are against us.
One reason Earth-like planets are rare is that there are just so many different ways to make a world. No two worlds are the same — even when they have similar dimensions and a similar location.
Venus and Earth started life with almost identical compositions, and are near neighbors in space. Despite this, it would be hard to find a less pleasant and less Earth-like place than the surface of Venus, a planet that has justifiably been called “our evil twin sister.”
The relatively small difference in solar heating may explain this, but other factors also play a role. In particular, Earth’s magnetic field, which is uniquely strong and long-lived among rocky worlds in the solar system, could be important.
Planets like Earth, which retain a strong field over billions of years, may be more conducive to life because a magnetic field acts as a shield against the solar wind. The solar wind is a million-mile-an-hour stream of energetic particles sent out by the sun that, if not deflected, can remove water from a planet’s atmosphere as collisions between the wind particles and water split the molecules into their constituent hydrogen and oxygen atoms, allowing the lightweight hydrogen to escape into space. This probably happened to Venus, which has a thick but completely dry atmosphere and no magnetic field.
Habitability may also be affected by properties that are not directly related to planet type. For example, location in the galaxy may matter (are there lots of nearby supernovas?), and the lifetime of a star is almost certainly a major consideration (will the planet be habitable for very long?).
A planet’s rate of rotation, the circularity of its orbit, the angle between a planet’s axis and its orbit, and even the locations of other planets in the system can affect habitability.
Imagine taking all the planets that satisfy one of the properties to sustain life — say, having the right amount of rock — and for the sake of argument, imagine that describes one in 10 planets.
If this argument is carried through 12 general properties a planet needs to be habitable, assuming that at each cut we keep one in 10 planets, we end up with only one planet in a trillion.
I should emphasize that the numbers I’ve used here are for illustration only. I do not know how many properties Earth has that are fine-tuned for life, and I do not know what the probability of each of these happening by chance is. So please don’t take my “one in a trillion” too seriously; the true frequency of habitable worlds could be much higher than this or it could be very much lower.
Nevertheless, unless there are surprisingly few planetary properties necessary for complex life, habitable worlds are going to be pretty rare.
The flip side of this argument is that there must be huge numbers of planets in the universe to ensure that, despite poor odds, worlds like ours with the right combination of characteristics will still occasionally appear.
Hundreds of billions of galaxies each containing hundreds of billions of stars implies more than 10 thousand billion billion (10,000,000,000,000,000,000,000) stars in the visible universe, and the number of planets is probably similar.
Numbers this large are notoriously difficult to visualize but, to get some idea, imagine sand so fine that you can barely see the individual grains. Now imagine having enough of these sand grains to fill a box a mile long by a mile wide by a mile high. That’ll be about the right number. And that’s just the visible universe. What our telescopes can’t see may be many, many times larger.
This unimaginably large quantity of planets allows highly habitable worlds like ours to be both very rare and, at the same time, very numerous.
One such world on average in a volume the size of the observable universe is pretty sparse but still allows at least a billion of them in the entire universe. Even if life-friendly planets are vanishingly rare, the universe is so enormous that they remain inevitable.
The question is: Could we ever find one?
Adapted from “Lucky Planet: Why Earth Is Exceptional — and What That Means for Life in the Universe” by David Waltham. Out now from Basic Books.