These are exciting times for planet hunters, with Kepler and CoRoT in the hunt, three ongoing searches for rocky worlds around Centauri A and B, and the continuing WISE mission, which may identify planet-bearing red and brown dwarfs that we haven’t spotted yet, not to mention numerous radial-velocity, transit and microlensing projects. But stepping back to get the big picture is a bit sobering. Jean Schneider did that recently in a paper looking at the far future of direct imaging, wondering where we were headed after Kepler and CoRoT. Schneider (Paris Observatory) talks about a ‘conceptual or knowledge horizon,’ one we’ve discussed earlier, that limits us to detecting biomarkers and keeps us from going much further than that for centuries.
Seeing Alien Life Up Close
Why? A short article on Schneider’s work in Astrobiology Magazine condenses the paper’s argument. Suppose we do discover signs of life on a planet in the habitable zone of a nearby star. Huge space arrays could help follow up our investigations of such a discovery, but Schneider notes that to get a 100-pixel image of a planet with twice the Earth’s diameter some 16.3 light years away would require the elements of the array to be more than 43 miles apart. Having set up this interferometric system, we could snap images of things like rings, clouds, oceans or continents, and could monitor changes in cloud cover. But we would still be missing something we’d really like to know. Just what do the inhabitants of this place look like?
Learning the answer is well beyond our present capabilities. Suppose, Schneider muses, some truly unusual creatures exist in the closest known system to our own. From the article:
To begin imaging even giant organisms 30 feet long and wide on the closest putative exoplanet, Alpha Centauri AB b, some 4.37 light years away, the elements making up a telescope array would have to cover a distance roughly 400,000 miles wide, or almost the Sun’s radius. The area required to collect even one photon a year in light reflected off such a planet is some 60 miles wide. To determine if the lifeform is moving with a speed of even 2 feet per minute — and that the motion you’re seeing is not due to errors in observation — the area required to collect the needed photons would need to be some 1.8 million miles wide.
It was 23 centuries ago that Epicurus predicted the existence of other worlds. “Unfortunately,” says Schneider, “we are perhaps as far away from seeing aliens with our own eyes as Epicurus was from seeing the first worlds when… he predicted the existence of these planets.”
The Consolation of Philosophy (with Apologies to Boethius)
Epicurus probably never dreamed that humans would one day see a planet on which an alien creature lives. But he was certainly fired with the zeal of philosophy and perhaps it didn’t matter. A handy place to find a condensation of his views is in a letter written to the historian Herodotus, where he reveals himself to be a champion of the investigation of the natural world. Here he has been discussing atoms and their motion and the notion of eternity, which to him implies that everything we see on this planet offers a kind of template for what else exists:
Moreover, there is an infinite number of worlds, some like this world, others unlike it. For the atoms being infinite in number, as has just been proved, are borne ever further in their course. For the atoms out of which a world might arise, or by which a world might be formed, have not all been expended on one world or a finite number of worlds, whether like or unlike this one. Hence there will be nothing to hinder an infinity of worlds.
Later in the letter, the philosopher amplifies on the argument:
After the foregoing we have next to consider that the worlds and every finite aggregate which bears a strong resemblance to things we commonly see have arisen out of the infinite. For all these, whether small or great, have been separated off from special conglomerations of atoms; and all things are again dissolved, some faster, some slower, some through the action of one set of causes, others through the action of another.
And further, we must not suppose that the worlds have necessarily one and the same shape. For nobody can prove that in one sort of world there might not be contained, whereas in another sort of world there could not possibly be, the seeds out of which animals and plants arise and all the rest of the things we see.
Our tools give us the unquenchable desire to see these things up close, which is why we’re planning space-based coronagraphs that can block out the light of a central star to help us get direct images of small planets, while next-generation interferometers will help us study their reflected light and could show us bio-signatures. Ground-based observatories are coming online that, with adaptive optics, can make direct detections of distant planets.
Constraints to Overcome
These are wonderful prospects, but Schneider’s view is that it may well be millennia before we can find a way to image an alien being. All of which calls up the question of putting instrumentation directly into a distant solar system, a possibility he considers remote because of the dangers of movement at high speeds. As summarized in the article:
The only alternative would be to dispatch spacecraft out to the planet, but such a journey would be long and perilous. At speeds of 30 percent the speed of light, a 100-micron-thick interstellar grain roughly the width of a human hair would pack roughly as much kinetic energy as a 100-ton body traveling 60 miles per hour. No currently available technology could protect against such a threat without a spacecraft massing hundreds of tons, which in turn would be extraordinarily difficult to accelerate up to high speeds. One could instead travel more slowly and thus more safely, but at even 1 percent the speed of light (or about 1,860 miles per second) it would take millennia for the spacecraft to reach its target destination.
No one could argue that these considerations aren’t huge. But two thoughts come to mind. First of all, if the goal is to create the best possible images of a distant world’s surface, we do have the resources of the Sun’s gravitational lens to consider. We’ve often discussed the first step in exploiting that lens, a mission called FOCAL that would travel to 550 AU and beyond to take advantage of the huge magnifications that would be available there. If we do eventually master the technologies to send an interstellar probe to a star of interest, we’ll surely send a gravitational lens mission, a direct descendant of FOCAL, in the opposite direction to study the planet up close via the gravitational lens before we do anything else.
Dealing with interstellar dust and debris at high velocities is only one of the challenges of interstellar flight, nor do we have the technologies today to get to the one percent of the speed of light Schneider is discussing on the lower end. But ramping up mission velocities through new technologies supplemented by oncoming nanotech methods of construction could change the game, and Robert Forward’s dictum that while interstellar flight would always be difficult and expensive, it could no longer be considered impossible comes to mind once again.
Challenges are there to be overcome, and if we can find ways to overcome them within the realm of known physics, then it is possible that our civilization will one day find the energy sources and raw materials to produce the deep space infrastructure interstellar flight will demand. Forward’s huge lightsails powered by laser or microwave beam are one possibility, but who knows what a sufficiently mature nanotech-minded culture might produce within the next few centuries. No one knows when or if we’ll be seeing alien creatures up close, but philosophy is energized by finding out how far we can proceed by pushing theory and experiment.
The paper is Schneider et al., “The Far Future of Exoplanet Direct Characterization,” Astrobiology Vol. 10, Issue 1 (22 March, 2010). Also available in preprint form.