Beyond Darwin: The Future of Exoplanet Imaging

After we’ve found an Earth-like planet with a potential for life, what further things can we do to investigate it? A team led by Jean Schneider (Paris Observatory) asks this question in a new paper, speculating that there are things a technological society does that leave a sure trace. Given the right instruments (no small requirement), we might look, for example, for Carbon Fluoro Compounds (CFCs). Well known for their damaging effects on our ozone layer, CFCs absorb infrared light at characteristic wavelengths, making their signature a revealing one.

Spotting an Extraterrestrial ‘Techno-Signature’

Schneider calls markers like this ‘techno-signatures’ (as opposed to the more familiar ‘bio-signatures’). They’re spectral features that can’t be explained by complex organic chemistry. Find CFCs in the atmosphere of a distant world and you’ve got a snapshot of technological chemical synthesis at work. We might speculate as to whether the average civilization produces CFCs in abundance, or for that matter, whether such cultures would simply move through a period of CFC production before scouring them from their ecosystem.

That could leave us with a relatively tiny window of observing time, just as with radio waves, where we listen for an extraterrestrial signal knowing that our own culture is gradually going silent as it turns to cable and satellite. Schneider’s team also ponders the possibility of detecting artificially produced light on a planet, noting that earth’s total energy production is about 40 TW. This is roughly one millionth of the sunlight energy reflected by the whole planet, making artificial light at this power level an unlikely catch.

In fact, seeing alien city light would demand an aperture with a diameter of 1.5 kilometers. It’s a sobering perspective, and only one of many in this absorbing paper, which also touches on Luc Arnold’s speculations about detecting artificial constructions that might transit in front of a distant civilization’s star (we’ve discussed Arnold’s work before on Centauri Dreams). Clearly, we’re pushing into a technology area well beyond the proposed Terrestrial Planet Finder and Darwin missions, but as long as we’re doing so, why not push even farther?

Direct Imaging and Its Limits

Thus Schneider posits our finding a promising planet around a nearby star, like Centauri B. His assumption is that finding biomarkers on this world would trigger two types of projects, the first being an attempt to directly visualize living organisms. What would it take to pull in a direct image of an organism with a size of ten meters? Let me quote from the paper on the staggering numbers:

A spatial resolution of 1 meter would be required. Even on the putative closest exoplanet alpha Cen A/B b, the required baseline would be at 600 nm B = 600,000 km (almost the Sun radius). In reflected light the required collecting area to get 1 photon per year in reflected light is equivalent to a single aperture of B = 100 km. In addition, [if] this organism is moving with a speed of 1 cm s-1 it must be detected in less than 1000 sec. To get a detection in 20 minutes with a SNR of 5, the collecting area must then correspond to an aperture B = 3 million km.

And there you are: Snapping a photo of our ten-meter cousins on Alpha Centauri Bb is going to take a light-collecting area so vast that the project is rendered phantasmagorical. More realistic in these circumstances, I think, to turn to FOCAL, the telescope sent to the Sun’s gravitational focus in a mission design conceived by Claudio Maccone (and an idea originally developed by Jean Heidmann). For all the tough technology that one would require, it’s simplicity itself compared to an aperture of 3 million kilometers.

Beyond the Conceptual Horizon

Schneider’s take on the second project — getting a human or even robotic mission to a nearby star — is also sobering. For one thing, such a mission would need shielding from cosmic rays and interstellar dust. A water shell one meter thick could provide protection, but we still have the problem of accelerating up to 0.3 c or whatever cruising velocity we hope to use. Dust is worse still. Let me turn to the paper again for the grim numbers:

As for the threat by interstellar dust, a 100 interstellar grain at 0.3 the speed of light has the same kinetic energy than a 100 tons body at 100 km/hour. No presently available technology can protect against such a threat without a spacecraft having itself a mass of hundreds of tons, in turn extremely difficult to accelerate up to 0.3 c.

Schneider’s team, in discussing these matters, talks in terms of a ‘conceptual or knowledge horizon,’ one which limits us to making biomarker detections and then leaves us frustratingly unable to probe deeper until, in their view, many centuries of technological obstacles have been overcome. But as we wait for that horizon to shrink, what can we realistically hope to do? In addition to advanced spectroscopy, large, space-based interferometers could conceivably allow many of the following direct imaging possibilities:

Direct imaging of habitable moons of giant planets

Highly improved transit spectroscopy for transiting planets

Detecting planetary moons by astrometry (measuring the displacement of the planet’s position due to the gravitational pull of the moon)

Constraining planetary radius for transiting planets

Direct measurement of planetary radii

All of this leads up to what could be the ultimate step, at least in terms of forseeable technology. That would be the direct imaging of surface features like oceans and continents on a world light years away. And as the paper notes, this approach may also allow us to detect forests and savannahs there, investigating the equivalent of the ‘red edge’ of terrestrial vegetation at 725 nm.

We can hope the time frame for moving beyond these limits is shorter, and that we are not as far from seeing other worlds up close as Epicurus was some 2300 years ago when he first predicted that such places must exist. But in noting the Greek philosopher, the paper also reminds us that even as today’s technology would have been inconceivable to Epicurus, what may emerge in an indefinite future could help us overcome these obstacles in ways we cannot yet imagine.

Reference (and a Thought on Preprints)

The paper is Schneider et al., “The far future of exoplanet direct characterization,” accepted at Astrobiology and available as a preprint. And a note on the arXiv site: Now and then I hear people say that a preprint site like arXiv is all we need to consult. After all, the thinking goes, all new scientific papers appear there.

But nothing could be further from the truth. For one thing, many papers appear in print only, depending on the journal involved and its policies. But more significantly, a preprint may or may not be identical to the published version. I’ve seen many papers that have undergone substantial revision after the preprint first appeared. We look at preprints at Centauri Dreams to get word of what’s coming in the research, but what eventually appears in the journals should always be considered the gold standard.

Addendum: A note from Jean Schneider points out that the Darwin mission is no longer a part of ESA’s program. Indeed, the current Web site on the idea will soon be taken down. Our near-term future in space-based exoplanet detection beyond Kepler and CoRoT is looking more and more problematic.