Beyond Darwin: The Future of Exoplanet Imaging

by Paul Gilster on October 20, 2009

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.

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Paul Titze October 20, 2009 at 11:27

Paul Gilster wrote:

“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…”

Unfortunetly that’s true, many papers don’t make it on arXiv. One of my problems (as a “student”) is to get access to (expensive) print only articles that aren’t on arXiv by spending $0. If I study say 2 papers a day and some charge US$30 per paper, I’d be broke long time ago, local universities don’t always subscribe tophysics journals I’m interested in and besides I don’t have the time to goto the physics library at uni.

Others who have the same problem can get around this though. Most print journals (that I follow anyway) publish their electronic counterpart online (access for fee) however the titles of the papers appear online for free. Do a search on arXiv for the papers you want to read, no luck?, search the authors for others they have published on arXiv. Send them a polite Email asking them for the PDF, most are happy to send you a copy.

Cheers, Paul.

ethanol October 20, 2009 at 12:18

I’m not sure that we could take CFC’s alone to be an indicator of technological civilization. Life on earth doesn’t produce CFC’s but this isn’t due to any fundamental principle preventing their production. Indeed there are both chlorinated and fluorinated natural products here on earth. Additionally, plants have been known to remove toxic elements from the soil by converting them into gaseous compounds.

One minor typo: “As for the threat by interstellar dust, a 100 interstellar grain” Units?

Thomas October 20, 2009 at 13:09

A 3,000,000 km wide mirror? We got this!! Let’s do it!

More seriously though, a mission at the gravitational focus point is seeming more and more needed as time continues on. But I don’t see how a spacecraft can get there in a reasonable fashion. 600 AU away? It’s a stretch…

tacitus October 20, 2009 at 13:30

While it’s disappointing to read that biomarkers will be so tough to detect on exoplanets, I’m still stuck on the following question:

At what point does visiting an exoplanet become the only way to possible grab a better photograph of it?

This is the key question regarding the future exploration of exoplanets.

Building a space telescope with 1.5km aperture sounds ludicrously tough on the face of it, but it’s it any harder than completing a successful interstellar mission? Even if it proved to be slightly more difficult to build the telescope, that telescope would trounce any comparable (in cost/difficulty) interstellar mission when it came to economy of scale since one giant telescope would be able to image many thousands of exoplanets during it’s operational lifetime.

We’ve barely reached the 20th anniversary of the “Age of Exoplanetary Discovery” and we’ve already reach the point where new discoveries are so routine they’re being announced in batches. In another 20 years there is an excellent chance that we’ll be seeing the first direct images of exoplanets, and as the article so rightly says, who knows what technological breakthroughs will come our way to help us overcome the sheer scale of the problem of snapping clearer photos of our interstellar twins?

So I believe that unless some entirely unforeseen FTL technology comes along, our only practical means of interstellar exploration for the next two or three hundred years at least will continue to be looking down the end of a telescope — as colossal as they will need to become to satisfy our lust for exploration and discovery.

Administrator October 20, 2009 at 13:56

Paul Titze wrote:

Most print journals (that I follow anyway) publish their electronic counterpart online (access for fee) however the titles of the papers appear online for free. Do a search on arXiv for the papers you want to read, no luck?, search the authors for others they have published on arXiv. Send them a polite Email asking them for the PDF, most are happy to send you a copy.

Good point. Remember as well that many scientists maintain their own Web sites and sometimes post papers on them that would otherwise be unavailable except through an academic library or an online account.

ASJ October 20, 2009 at 15:06

There is the internal Solar gravitational focal point at 23.5 AU, where
neutrino detectors could be placed, to search for communications and/or leakage signals from suspected extraterrestrial sources.

Reference: “Gravitational Lensing Characteristics of the Transparent Sun”,
http://www.iop.org/EJ/abstract/0004-637X/685/2/1297

Andrew W October 20, 2009 at 18:51

I would have thought a thin shield of metal traveling thousands of km ahead of the interstellar probe would vapourise dust grains, with the resultant explosion dispersing the debris before the probe reached it. the only damage the shield would suffer is a small hole because with the speed the shield is moving at the plasma expansion would happen behind the shield.

Bounty October 20, 2009 at 19:22

While 0.3 c might not be reasonable, 0.05 c might be reasonable. We’d just have to wait a lifetime+ for data.

Pointless Geometry October 20, 2009 at 23:35

An aperture of 1.5km you say? Sounds like a cargo job for solar sails.

Adam October 21, 2009 at 8:12

Hi All

The collision risk issues mean the ships will need to be BIG, for shielding and active defense systems. A stand-off shield is a really good idea for so many reasons. These are tractable issues.

Eniac October 21, 2009 at 9:40

According to here: http://www-ssg.sr.unh.edu/ism/what1.html
interstellar dust grains are only a fraction of a micron. Since energy goes with the cube of size, that would reduce the impact from the 100 tons cited to 100 grams (at 100 km/h). Comparable with a well-pitched baseball. Still quite a bump, but definitely more managable, especially considering that the shield needs to be only at the front of the vehicle.

Eniac October 21, 2009 at 13:10

To put the problem of interstellar gas (99%) and dust (1%) in perspective: If you compressed the column of ISM material between here and Alpha Centauri into a liquid or solid layer of normal density, it would be less than a micron thick. Much less, I think.

Due to the rocket equation, any fusion powered interstellar ship carrying it’s own fuel will need to start out with a large fuel/payload ratio to reach relativistic velocity. Making the shielding of anything other than fuel would be a gigantic waste of valuable payload capacity.

Li6 is one of the best fuels, as it has competitive maximum exhaust velocity (although somewhat lower than D+He3), and is useful as a structural material, obviating the need for tanks. The ship could easily be made 99.99% fuel, initially, enough for a pretty good fraction of light speed, given reasonably efficient engines.

It appears, then, that the most likely configuration for a fusion ship would be a large rod or cone of solid Lithium, with Li6+Li6 fusion engines at the back, payload towards the front, and some more lithium ahead for shielding. After acceleration and most of the decelaration, when the fuel at the back is used up and velocity down to normal, the shield can be dismantled and burned for final deceleration and exploration.

philw1776 October 21, 2009 at 15:55

Add me to the list of those seriously bummed that neither ESA nor NASA has a substantive next gen exoplanet program funded in the works post COROT and Kepler. Exoplanet science has a low national and space ‘exploration’ priority.

Adam October 21, 2009 at 19:07

Short-sighted penny-pinching bureaucrats will be the death of all spaceflight unless we can make it pay!

Hank October 21, 2009 at 23:25

If we find a “habitable” planet, we should send them EM Transmissions.

After the requisite prime numbers… Bach… and whatever, I suggest we start an insterstellar IM conversation:

“hihi, 400k/human being/earth). a/s/l?” (age, species, location)

The four-year delay or whatever shouldn’t be too annoying. ^^

ljk October 22, 2009 at 10:22

philw1776 said on October 21, 2009 at 15:55:

“Add me to the list of those seriously bummed that neither ESA nor NASA has a substantive next gen exoplanet program funded in the works post COROT and Kepler. Exoplanet science has a low national and space ‘exploration’ priority.”

It should be fairly obvious that no national space agency on this planet, small or large, has any serious intention of conducting a real interstellar mission, not even robotic, any time soon. Even China recently played that game, mentioning their desire to send a probe to another star, but as they say, talk is cheap and easy.

With the uncertainty of NASA sending humans to even our closest celestial neighbors, Luna and Mars, at any point before 2050, it makes little sense to take seriously for now their mantra of those worlds being stepping stones to the stars. They will be some day, but I wonder if they will be the ones doing it?

My hopes for galactic exploration and colonization are lying more and more with the kinds of groups that have historically gone where no one has gone before: The societal “misfits” both religious and political. That should not be taken in a negative way, please note. These groups may have very legitimate reasons for not wanting to be under the control of Earth-based authorities. Besides, evolution has shown for billions of years that life naturally moves on when territories are occupied or used up.

And when corporations finally get a real toehold on space and truly appreciate how many resources there are up there and how much money is to be made, we’ll colonize at least this solar system in no time. That space infrastructure is what will do much for getting the aforementioned private groups heading for the stars. And nope, they won’t be noble astronauts on a dedicated mission sponsored by a government space agency.

spaceman October 22, 2009 at 20:40

It was said: “Our near-term future in space-based exoplanet detection beyond Kepler and CoRoT is looking more and more problematic.”

Indeed, I could not agree more. Right now and for at least the next ten years extrasolar planet science will be be both exciting and frustrating. Here’s why: we are closing in on the galactic planetary census e.g. especially with the recent HARPS results we are getting an idea about what fraction of stars have planets. As exciting as it is to be on the verge of pinning down Fp in the Drake equation, we don’t know much about this population of planets other than their mininum masses and orbits (which is, don’t get me wrong, a monumental scientific accomplishment). In other words, it is like knowing that a country has big people and small people, the small people outnumber the giants; however, we don’t know about the shapes of their faces, the color of their eyes, skin, let along their personalities. This gray area will only become more well-defined if and when TPF and Planet Imager-like missions (remember Planet Imager) are constructed and then launched and neither is likely to occur soon. Also, it will take next-gen microlensing searches, Kepler, and radial velocity searches even more powerful than HARPS to finish even the basic census. Though again, great accomplishments thus far!

nick October 22, 2009 at 21:11

Humans building large space structures to detect aliens emitting CFCs, which humans have already rendered obsolete after extremely brief use, strikes me as oddly backwards. It is the aliens, most likely hundreds of millions of years more advanced than us, who will have the astronomically sized structures, not ourselves in the near future. ETI should be as easy or easier to detect than planets.

Still, looking for CFCs does hint at a good strategy — look for artificial surfaces with very optically improbable properties, including chemicals that are highly unlikely or rare in natural dust clouds. For example, the surfaces of human satellites and many skyscraper windows have improbably very high concentrations of gold due to its good optical and thermal properties. Paint contains many artificial molecules. The result is that artificial surfaces have very artificial-looking optical properties and spectra. Given that ETI of the most probable ages (i.e. hundreds of millions of years older than us) could have already spread across galaxies and converted most of their visible surfaces into artificial surfaces, this strategy should work for other galaxies, and indeed given the statistics is more likely to work than looking at star systems within our own galaxy. And it can be done with normal-sized telescopes and standard spectroscopy.

spaceman October 26, 2009 at 0:10

This problem of collisions with interstellar dust is indeed a thorny one. I like Andrew W’s idea of having something fly ahead of the spacecraft to take the hits. I suspect that other solutions will arise out of sheer creativity. How much astrophysical data do we have on the amount of interstellar dust of certain sizes an ultra-fast spacescraft might encounter on its way to a distant extrasolar planetary system?

ljk October 26, 2009 at 9:16

The BIS Daedalus interstellar probe design of the 1970s had a “dust bug”
that flew ahead of the main vessel spewing dust to vaporize anything up
to a large asteroid in the ship’s flight path.

Now what would protect the dust bug is another story.

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