How Will We Find Alien Life?
8:59 AM EST on January 11, 2023
What will first contact look like? The universe is almost certainly teeming with life—probability alone makes it a near-guarantee—but where and when and, more practically, how will we find it? I think about this from time to time, because I am a normal, sapient life form wondering why it seems so quiet out there.
It will not happen with little gray men with big heads stepping out of a flying saucer, we are pretty certain of that. It may very well happen when we drop a lander in the right place on the right satellite of Jupiter; given what we know of the rich, wet, Jovian moons, your average astrobiologist might even put money on it. Or maybe SETI's search will pay off and we'll spot a transmission from the stars.
Given the current state of technology, these three options, or some variation thereof—something comes here; we find something (relatively) nearby; we overhear something very far away—are our only options for finding life beyond Earth. In a matter of decades, however, we could have a fourth way to look.
"We are calling it the ‘Habitable Worlds Observer,’ or ‘Observatory’. We’re going to find those planets. We’re doing it.” That's John C. Mather, NASA’s Senior Project Scientist for the James Webb Space Telescope, which is a miracle and a tribute to human ingenuity, but simply doesn't have the power to look at exoplanets, or planets around other stars, and isn't designed to anyway. Mather was speaking with the boundless optimism of someone who both believes deeply in the mission and needs to publicly cheerlead to get the money to do it (science is politics). He was referring not to Webb, nor to the next space telescope—that'd be the Nancy Grace Roman Observatory, a 2.4-meter survey telescope scheduled to launch in 2027 that will look for dark energy and exoplanets—and probably not even to the one after that. But as soon as the 2040s or '50s, we could launch the Habitable Worlds Observer, the HWO, which for the moment exists only in the design and proposal stage, and atop NASA's wishlist to Congress.
Once every 10 years, the U.S.'s scientific agencies are polled on their priorities for the future, which not so subtly primes the pump to get them funded. High on NASA's list on the most recent decadal survey was the HWO, and it has an irresistible sales pitch: Point it at exoplanets and it'll tell us if there's life on them.
There's nothing rare about planets. We've got eight of 'em. (Don't start.) Given how planets form—from leftover bits of the swirling dust disks that coalesce to make a star—there's no reason other stars shouldn't have them. It's just that other stars are so very far away, and so bright that it's very hard to see if anything's orbiting them. So we've found a few workarounds. We can measure the wobble of a star to see if it's being wobbled by the gravity of its planets. We can see if a star system is giving off too little light or too much heat, because its planet is getting in the way, or emitting its own reflected heat. We can see if a star system is a little too dense, by noticing how its gravity bends light as it passes by. We can, on rare occasion if the planets are large enough and far enough from their star, spot the planets directly. There are several other methods.
The point is that once we figured out how to find them—the first was confirmed in 1992—we found exoplanets everywhere. We're finding them monthly. We know of more than 5,000 of them now, and those are just the easiest to find. In fact, it's a pretty well-accepted estimate that there are at least as many planets as stars in our galaxy, though they're not evenly distributed. And roughly one in five Sun-like stars, they think, has an Earth-like planet: the right size and the right distance from their star, in the so-called Goldilocks Zone. There may be 40 billion Earth-like planets in this galaxy alone. We've already found several dozen. Even some that might be better than Earth.
If you're looking for alien life, you'd first look on an Earth-like planet. This isn't a given; just playing the percentages. We don't know that life needs Earth-like parameters—liquid water, certain temperatures and pressures and radiation exposure levels—but we know life can arise under those parameters, because it did, at least once, here.
Enter the Habitable Worlds Observatory, which as you may have guessed, would observe these habitable worlds. Mark Clampin, director of NASA's astrophysics division, revealed some of the proposed design details of the HWO at an American Astronomical Society conference this week, and it's a beast of a telescope. It will need a mirror shaped orders of magnitude finer than even the Webb's. It will need to filter out light sources 100 times brighter than even the upcoming Roman telescope will be able to. It could weigh 15 times as much as the Webb. To accomplish all this, Clampin revealed, HWO would be, like Webb, stationed at a Lagrange point a million miles from Earth, but, uniquely, it would be designed for robotic servicing and upgrades. Think of it less as a telescope, then, and more as a platform for progressively better telescope technology to be mounted upon. HWO, with all that power, would then turn to known Earth-like exoplanets and, for lack of a better term, take a sniff.
Spectroscopy would be the HWO's secret weapon for sussing out signs of life. (To be clear, we already do spectroscopy without the HWO; we just need something as sensitive as the HWO to use it effectively on exoplanets.) This technique relies on the fact that light is absorbed by certain gases at specific wavelengths, and if we see some light that has passed through an exoplanet's atmosphere, we can tell by the missing wavelengths what gases it met along the way. And certain gases tend to be associated with life.
"Oxygen is still the first thing to go after," Victoria Meadows, head of the Virtual Planet Laboratory at the University of Washington, which is partnered with NASA, told Science. Oxygen and methane? Even better. On Earth, even among extremophiles, life tends to need or to use or to give off compounds made of some combination of oxygen, carbon, hydrogen, nitrogen, phosphorus, and sulfur—a relatively narrow list. Scientists are already hard at work trying to game out what we'd see in the spectroscopic signatures of an exoplanet that contains life.
This carries the obvious potential flaw in that it assumes a lot about how life arises and exists, namely that it needs to be in fundamental ways much like Earth life. There are reasons to think at least some of these assumptions are sound: life anywhere might be carbon-based because of how easily and how firmly carbon atoms bonds with other elements, especially other important-to-life ones ... but then, silicon does this too, under some conditions. (This debate is called carbon chauvinism and it's a fascinating one, but entirely theoretical until further notice.) It's possible we look at an exoplanet that is utterly teeming with life, and just ... don't notice, because its life doesn't resemble ours, and farts out entirely different gases. That's a real risk. But the highly scientific principle of "Do you have a better idea? Didn't think so" dictates that this is how we're going to go about the search for exobiology.
There's another latent disappointment in the HWO's search, on a more philosophical level. Let's say we find all the telltale signatures of life on some planet light years upon light years away. What then? We will know nothing about it—what it looks like, how diverse it is, how smart it has gotten, whether it also has invented religion and pro wrestling—and we will have no way of finding out. We cannot go there, and if it's not sentient and radio-communicative, we cannot talk to it, and even if we can, any exchange would take place on or beyond the scale of human lifetimes. Finding evidence of life this way would be the greatest scientific discovery humanity has ever made, and at the same time the biggest letdown I can conceive. What would it mean to make contact without contact? To be no longer alone, yet as lonely as ever? I still think it's worth looking.