Long-time readers of Skeptophilia know I'm kind of obsessed with the idea of extraterrestrial life. I guess it's natural enough; I'm a biologist who's also an amateur astronomer, and grew up on Lost in Space and Star Trek and The Invaders, and later The X Files and Star Wars. (Although I'm aware this is kind of a chicken-and-egg situation, so what the ultimate origin of my obsession is, I'm not certain.)
Until fairly recently, there was no particularly good way to determine the likelihood of life on other worlds. Decades ago astronomer Frank Drake came up with the famous Drake Equation, which uses the statistical principle that if you know the probabilities of various independent occurrences, to find the probability of all of them happening, you multiply them together. Here's the Drake Equation:
[Image is in the Public Domain courtesy of NASA/JPL]
The problem, of course, is that the more uncertainty there is in the individual probabilities, the more uncertainty there is in the product. And there was no way known to get even a close value -- or, worse, to know if the value you had reflected reality or was just a wild guess.
What's cool for us alien enthusiasts, though, is that our research and techniques have improved to the point where we do have decent estimates for the values of some of these. Even better, every time one of them is revised, it's revised upward. Today I'd like to look at two of them -- f(p) and n(e) -- respectively, the fraction of stars that have planetary systems, and the fraction of those systems that have at least one planet in the habitable zone.
Given that we started out with a sample size of one (1) solar system, no one knew whether the coalescence of stellar debris into planets was likely, or simply a lucky fluke. Same for planets in the habitable zone; here we have only a single planet that is habitable for organisms like ourselves. Again, is that some kind of happy accident, or would most planetary systems have at least one potentially habitable planet?
Once we started to find exoplanets, though, they seemed to be everywhere we looked. The earliest ones were massive (probably Jupiter-like) planets, often in fast, close orbit, so they'd be pretty hostile places from our perspective. (Although, as I dealt with in a recent post, what we're finding out about the resilience of life may mean we'll have to revise our definition of what constitutes the "habitable zone.")
So the estimates for f(p) and n(e) crept upward, but still, it was hard to get reliable numbers. But just last week, two studies have suggested that f(p) -- the percentage of stars with multiple-planet systems -- may be very close to 100%.
In the first, we hear about a recently-developed technique to improve our ability to detect exoplanets even at great distances. Before this, most exoplanets were discovered using one of two methods -- looking for stellar wobble as a planet and its star circle their mutual center of gravity (which only works for nearby stars with massive planets capable of generating a detectable wobble), and luminosity dips as a planet occludes (passes in front of) its host star (which only works if the orbital plane is lined up in such a way that the planet passes in front of the star as seen from Earth). As you might imagine, those restrictions mean that we might well be missing most of the exoplanets out there.
Now, a new orbiting telescope developed by NASA -- called WFIRST (Wide Field Infrared Survey Telescope) -- has the capability to detect microlensing. Microlensing occurs because of the warping of the fabric of space-time by massive objects. As a planet rotates around its host star, that warp moves, creating a ripple -- and the light from any stars behind the planet gets deflected. An analogy is when you're looking down to the bottom of a clear pond and a ripple on the surface passes you; the image of the pebbles on the bottom appears to waver. That wavering of light from distant stars is what WFIRST is designed to detect.
The nice thing is that WFIRST isn't dependent on visible wobbles or planets with precisely-aligned orbital planes; it can see pretty much any planet out there with sufficient mass. And it can detect them from much farther away than previous telescopes -- the Kepler Space Telescope could detect planets up to around a thousand light years away, while WFIRST extends that reach by a factor of ten. It's also capable of scanning a great many more stars; the estimate is that the first sweep will look at two hundred million stars, which is a thousand times the number Kepler studied.
So chances are, we're going to see an exponential jump in the number of exoplanets we know of, and a corresponding uptick in the estimate for f(p).
The second study is much closer to home -- about as close as you can get without being in our own Solar System. Proxima Centauri is the nearest star to us other than the Sun, at 4.244 light years away. In 2016 we were all blown away by the announcement that not only did Proxima Centauri have a planet, it was (1) Earth-sized, and (2) in the habitable zone. (Anyone want to board the Jupiter 2?)
Now, astronomers have discovered a second planet around Proxima, at a distance about 1.5 times the orbit of the Earth, and a mass of about twelve times Earth's. This means it's probably something like Neptune, and very cold -- Proxima is a dim star, so the habitable zone is a lot closer to it than the Sun's is -- the estimate is that its average temperature is -200 C.
Even though it probably doesn't host life, it's exciting from the standpoint that Proxima's planetary system is looking more and more like ours. As astronomer Phil Plait put it, over at his fantastic blog Bad Astronomy, "I hope this new planet candidate turns out to be real. Having one planet orbiting the star is already pretty amazing, but having two? In my mind that makes it a solar system. And if two, why not more? How about moons orbiting the planets, or asteroids and comets around the star, too?"
The impression I'm getting is that f(p) (the fraction of stars with planetary systems) and n(e) (the fraction of stars with at least one planet in the habitable zone) are both extremely high. This bodes well for our search for life -- and as the techniques improve, my sense is that we'll find planets like ours pretty much everywhere we look. So life is looking more and more likely to be plentiful out there. Now, intelligent life that is sufficiently technological to communicate across interstellar space... that's another matter entirely.
But in my opinion, any time we can revise some part of the Drake Equation upward, it's a good thing.
This week's Skeptophilia book recommendation of the week is brand new -- only published three weeks ago. Neil Shubin, who became famous for his wonderful book on human evolution Your Inner Fish, has a fantastic new book out -- Some Assembly Required: Decoding Four Billion Years of Life, from Ancient Fossils to DNA.
Shubin's lucid prose makes for fascinating reading, as he takes you down the four-billion-year path from the first simple cells to the biodiversity of the modern Earth, wrapping in not only what we've discovered from the fossil record but the most recent innovations in DNA analysis that demonstrate our common ancestry with every other life form on the planet. It's a wonderful survey of our current state of knowledge of evolutionary science, and will engage both scientist and layperson alike. Get Shubin's latest -- and fasten your seatbelts for a wild ride through time.