Readers of this blog know that I’m a big fan of Quora, because it lets non-experts raise the kinds of speculative questions that don’t normally come up in formal scientific discussions. One frequent theme that comes up is the issue of what we would do if we found intelligent life on a planet around another star. A recent posting in particular caught my eye: “What would we do if we found an Earthlike planet with intelligent life that is 500 years behind us in technology and advancements?”
Well, that’s a fun thought experiment! It’s not one question, really, but a whole set of nested questions about how to find alien life, how to determine the presence of alien intelligence, how to determine the nature of that intelligence, and then how we would study it, or even try to make contact with it. There’s a big moral issue at the end, but a lot of juicy scientific ones along the way. And that got me thinking…
First, how will we find a truly Earthlike planet? Despite all the amazing recent discoveries of planets around other stars, astronomers have yet to find a true Earth twin—that is, an Earth-size planet orbiting a sunlike yellow star at an Earthlike distance. The closest they’ve come is a planet called Kepler 452b. It’s roughly five times the mass of Earth and probably belongs to the class of planets called super-Earths. Habitable, maybe; Earthlike, no. The same goes, even more so, for the recently discovered Earth-size planets around the star Trappist-1. These are extremely exciting worlds to study, and it is possible that some of them could support life. But the Trappist-1 planets orbit a dim red-dwarf star, which means that they are blasted with energetic radiation and are probably tidally locked, with one hemisphere always facing toward the star.
Closer analogs are surely out there, but they are hard to find. NASA’s Kepler space telescope spots planets by the way they blot out a tiny bit of their star’s light when they transit: that is, they pass between the star and us. But for a planet in a one-year orbit (like ours!), that means you see a shadow only once a year, and very briefly. To confirm that the planet is real—and not, say, a random flickering of the star—you want to observe at least three events. So you need to watch a lot of stars, and watch them all for several years. Even then, the only planets you’ll find are the ones that happen to be lined up exactly between us and their star.
Second, how would we know if a superficially Earthlike planet is truly habitable? The leading method right now is to look at light streaming through the planet’s atmosphere when it passes between us and the star. That’s an extremely difficult task even for giant planets. Right now, we don’t have the technology to do it for a true Earth twin—if we even had one to study. Another approach is to search directly for other Earths alongside their stars, and then analyze their light. To do that, you need both a very powerful telescope and a highly effective way to block out the glare of the star, since an Earthlike planet would be right alongside it in the sky, but on the order of a billion times fainter!
Corey S. Powell - Discover Magazine