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Detecting Exoplanet Oceans

Is the discovery of oceans on planets orbiting distant stars within our reach? Finding such an ocean would be of immense interest from an astrobiological perspective because water on the surface is the traditional marker for a habitable zone. Astrobiology Magazine has just written up work by Nicholas Cowan (Northwestern University) and colleagues, who have been looking at the ways we might detect such oceans. The researchers are thinking ahead to a time when we have an actual image of a terrestrial world to look at, even if that image is little more than the ‘pale blue dot’ Voyager saw in its famous portrait of the Solar System. When we have identified that ‘dot,’ we can do a lot with it by studying the way its light varies as it orbits its star.

Let’s assume we deploy a starshade and use it in conjunction with the James Webb Space Telescope to block the light of the star and reveal the faint signature of the planet. A disk tens of meters wide with petal-like extensions, the starshade would be placed between the telescope and the star under observation, its shape designed to prevent the rings and refractions that would be created by a circularly shaped shade. One option under consideration is to place the starshade about 160,000 kilometers away from the telescope, which will orbit at the L2 Lagrangian point. Such a configuration could yield the image of a terrestrial world in the habitable zone, a planet whose variations in light can tell us something about what is on its surface.

Finding oceans then becomes a major first step in characterizing the planet. The Cowan paper presents the three methods that have so far been proposed for detecting alien oceans:

  • Changes in color

Variations in the color of the planet are useful because oceans are darker and have different colors than the surface features of continents. Watching the planet over time should reveal these changes.

  • Polarized light

Oceans polarize light, whose phase variations can flag the presence of water. The trick is that light also scatters off molecules in the atmosphere (‘Rayleigh scattering’), masking the effect, but rotational variation in polarization may allow us to infer the presence of an ocean.

  • Specular reflection

Oceans can throw bright reflections, especially when the planet is in its crescent phase, making the planet appear brighter than would otherwise be expected. Variations in reflectivity (albedo) as the planet circles its star can thus be markers for an ocean if properly interpreted.

Image: Glinting sunlight off Lake Erie. Can we use this kind of specular reflection to identify the oceans of an alien world? Credit: Image Science and Analysis Laboratory/NASA JSC.

Cowan and team focus largely on the specular reflection method, noting that all three techniques have been studied in terms of cloud cover and changes in albedo due to seasonal changes on the surface. But they also identify what they call the ‘latitude-albedo’ effect, which can play havoc with these observations by mimicking the glint of an ocean when none actually exists. The reason: A planet in the habitable zone with low axial tilt (obliquity) would tend to have highly reflective snow and ice in the regions least illuminated by sunlight. A false positive for ocean glint is thus produced.

In other words, the polar regions will make the apparent reflectivity of a planet with low axial tilt increase when the planet is seen in its crescent phase, an effect that will diminish in the gibbous phase. The latitude-albedo effect thus limits our ability to use ocean ‘glint’ as a marker for water on the surface, though the authors note there are some ways around the problem. It will be necessary to study the color variations of the planet during its own rotation and during the entirety of its orbit to develop an estimate for the planet’s obliquity. The rotational albedo map that can be generated out of this should allow better interpretation of the variations in light observed. The JWST/starshade combination may be powerful enough to monitor these tiny changes.

If you’re wondering how significant the ‘glint’ effect could be, consider that Tyler Robinson (University of Washington), modeling the Earth as it would appear to a distant observer back in 2010, was able to show that the Earth would be as much as 100 percent brighter at crescent phases when modeled with the glint effect than without it. Thus specular reflection can be a major player in characterizing an exoplanet, but only if we learn how to interpret it properly.

The Cowan team’s simulation worked with a planet whose obliquity is 23.5 degrees, the same as the Earth’s, calculating light curves as they would appear to a distant observer. Subtracting out the kind of reflection that would produce an ocean glint, they still found the false positive, phase variations that mimic the glint. Planets like the Earth have enough axial tilt that the methods above can correct for the latitude-albedo effect, but the authors note that zero-obliquity planets will be extremely hard to investigate. It’s worth noting that planets like these should be fairly common around red dwarf stars, where the planet has become tidally locked to the primary.

The paper is Cowan, Abbot and Voigt, “A False Positive For Ocean Glint on Exoplanets: the Latitude-Albedo Effect,” accepted at Astrophysical Journal Letters (preprint). Thanks to Antonio Tavani for the pointer to this paper.

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Comments on this entry are closed.

  • Alex Tolley July 16, 2012, 13:39

    We still seem to be chasing earth analogs in our search for life. The problem with looking for water (oceans of it) is that we are playing the “water as indicator for life” game, rather than looking for life directly, a mistake IMO.

    Supposing a planet had water, but very few open bodies of it. That planet would not register with the polarized light and specular reflection methods, but probably would with the color change method. Mars would also register with the color change method, although the colors wouldn’t match our expectations of life. And what about a planet in a “snowball earth” condition, frozen over, yet teeming with life below the ice sheets? Would a world with heavy cloud cover also be missed?

    I’m waiting for techniques that can detect some sort of bio-signature, although I don’t yet know what this might be, although I suspect it will not be a simple, one size fits all, signature.

    The fact that we are still thinking in terms of subterranean microbial life possibly in Mars, or in the oceans of Europa, suggests that a bio-signature may be difficult to obtain, and that there is no substitute for a local probe.
    If so, these probes will need to be small and relatively cheap, so that we can hope to send out many of them to likely stellar targets.

  • Interstellar Bill July 16, 2012, 14:36

    The exoplanet’s orbital inclination is also a major factor, determining the range of angles between our line of sight (LOS) and the direction of stellar illumination. At high inclination the illumination is mostly perpendicular to the LOS, while at low inclinations it varies nearly 180 deg, offering a full range of star-glint angles and thus much more data.

    Also, finding oceans is but a first step. Then how do we find out if there’s any land? Without land there’s no hope of life advancing beyond fishes. If 99% of water worlds turn out to have no land it would be quite SETI-discouraging.

  • ljk July 16, 2012, 16:45

    Interstellar Bill said on July 16, 2012 at 14:36:

    “Also, finding oceans is but a first step. Then how do we find out if there’s any land? Without land there’s no hope of life advancing beyond fishes. If 99% of water worlds turn out to have no land it would be quite SETI-discouraging.”

    How do you know there are not high societies of cetaceanoid ETI who think that worlds with lots of dry land are a disappointment in terms of finding other intelligent beings in the galaxy?

    After all, those inferior land creatures seem to do nothing but pollute, fight, and make noises that drown out the oceanic communications system the cetaceanoids set up ages ago.

    Sophisticated intelligent life can only best evolve in a vast liquid environment with all its mentally stimulating complexities. The cephalapodoid cultures are in wide agreement on this view too.

    “For instance, on the planet Earth, man had always assumed that he was more intelligent than dolphins because he had achieved so much – the wheel, New York, wars and so on – whilst all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man – for precisely the same reasons.”

    – Douglas Adams, The Hitch-Hiker’s Guide to the Galaxy, 1979

  • Ron S July 16, 2012, 16:50

    Alex: “The problem with looking for water (oceans of it) is that we are playing the “water as indicator for life” game, rather than looking for life directly, a mistake IMO.”

    You’re being unreasonably negative. The thing is astronomers are constrained to look for what it is possible to look for, not necessarily what we want to find. This is like complaining about all these hot Jovians we keep detecting rather than the small, rocky planets we want to find. The detection technology is what it is, so they do what they can with what they have and can build.

    Alex: “I’m waiting for techniques that can detect some sort of bio-signature, although I don’t yet know what this might be…”

    Indeed. You can be sure that astronomers would love to to have an effective bio-detector for exoplanets, but they can’t build such as thing at present. So why not look for water if that is within the reach of present technology. It will tell us something rather than nothing.

  • ljk July 16, 2012, 17:40

    Ron S said on July 16, 2012 at 16:50:

    “The thing is astronomers are constrained to look for what it is possible to look for, not necessarily what we want to find. This is like complaining about all these hot Jovians we keep detecting rather than the small, rocky planets we want to find. The detection technology is what it is, so they do what they can with what they have and can build.”

    The same thing can be said for the current state of SETI. If society and governments cared enough, we could have been doing a lot more than we have since 1960, including observatories on the lunar farside and very large projects in deep space.

    Instead we have a few projects that have to borrow time on telescopes and the one that was supposed to be dedicated to SETI, the Allen Telescope Array (ATA) is now spending its efforts looking for satellite debris in Earth orbit at the behest of its main benefactor, the USAF. Sadly symbolic of the whole state of affairs.

    The same could also be said of interstellar vessels. If people cared and appreciate the idea enough, we would have at least had an Orion or two on its way to Alpha Centauri, and the equivalent of Daedalus/Icarus would be under serious consideration if not already seeing development in space.

  • Dan Ibekwe July 16, 2012, 20:24

    Ijk’s waterworld ETI societies must be waiting for us to arrive and put them in touch with each other.

    Being a sophistcated intelligence on a planet without dry land has one drawback – you can’t light fires. Which makes smelting metals quite challenging.

    (And cetaceans did a lot of evolving on land before they went back into the water…cephalopods, maybe?).

  • bigdan201 July 16, 2012, 20:49

    Life may indeed exist on a wide variety of planets. But oceans have great potential, and are also relevant to our interests of eventual colonization.

    Would it be possible to detect an ocean by analyzing the spectral signature of an exoplanet?

    PS – My Firefox browser puts a red underline on “exoplanet”, assuming it’s a misspelling. That should change.

  • Eniac July 16, 2012, 21:11

    It is my understanding that you can distinguish intensity and color variations due to glint quite readily from those due to other types of albedo. The variations due to glint would be relatively abrupt (on and off), as the “glint spot” traverses coastlines. This should be a dead giveaway. In addition, glint can be recognized by its polarization. If we observe the planet long enough, we can eventually piece a map of the continents together from the observed coastline transitions.

  • Alex Tolley July 16, 2012, 23:57

    @ Ron S.
    I may be harsh, but perhaps not unreasonably so. The techniques being suggested are no doubt being funded with the hook that they are helping to find life bearing planets. Note, not worlds with oceans, but worlds that may support life. But we could get a shot at that just with spectroscopy to detect H2O, even though that will detect worlds without abundant surface water.

    Nasa has played that game ever since Viking. Look for water (ancient or recent), but don’t actually look for life. Even Curiosity is not directly looking for life, although the chemical analyses are a tangential approach.

    For all the criticism SETI takes, at least they are using techniques that, if successful, would be definitive for their search goal.

  • ljk July 17, 2012, 1:00

    Dan Ibekwe said on July 16, 2012 at 20:24:

    “Being a sophistcated intelligence on a planet without dry land has one drawback – you can’t light fires. Which makes smelting metals quite challenging.”

    How about those undersea hydrothermal vents and especially those erupting underwater volcanic vents?

    I can long remember the experts saying things like planets could not exist around double suns due to gravitational instabilities and Jupiter type worlds also formed and stayed far out from their stars. We now know both ideas have been invalidated. I can just imagine what we will learn next about what is and is not possible about alien life.

  • Keith Cooper July 17, 2012, 10:37

    A similar previous study by Doughty and Wolf in Astrobiology has looked at the possibility of detecting what would essentially be trees on exoplanets through albedo changes, although I don’t know how conclusive this method would be: http://online.liebertpub.com/doi/abs/10.1089/ast.2010.0495

    To quote from the abstract: “For multicellular photosynthetic organisms on Earth, competition for light and the need to transport water and nutrients has led to a tree-like body plan characterized by hierarchical branching networks. This design results in a distinct bidirectional reflectance distribution function (BRDF) that causes differing reflectance at different sun/view geometries. BRDF arises from the changing visibility of the shadows cast by objects, and the presence of tree-like structures is clearly distinguishable from flat ground with the same reflectance spectrum. We examined whether the BRDF could detect the existence of tree-like structures on an extrasolar planet by using changes in planetary albedo as a planet orbits its star.”

    I guess we know what we are looking for when searching for planets that resemble Earth, which is why we spend so much time looking for water. Finding planets that are habitable but in a way we don’t recognise would be much more difficult. And subterranean biospheres will be all but impossible to detect remotely.

    Titan’s an interesting case I think. Cassini observations have shown ethane and acetylene to be depleted at the surface and atmospheric models indicate that something is also causing hydrogen to vanish at the surface. This is interesting because Chris McKay and Heather Smith have proposed that methanogens on Titan could take in ethane and acetylene and react them with molecular hydrogen to produce energy, with carbon dioxide and methane as waste products. The trouble is, even by having a craft in the Saturnian system we can’t even say whether these observations really are pointing to the existence of life on Titan or not, so detecting such biosignatures at distances of light years on Titan-like worlds around other stars would be impossible, certainly with our present capabilities. What we could possibly do, and I suppose this is an analogy to ‘following the water’, is search for evidence of methane-rich worlds in ‘methane habitable zones’ around red dwarfs and possibly even detect methane hazes in the atmosphere. High altitude haze has been spotted in the atmospheres of some hot jupiters because the haze dampens the emission lines of atmospheric elements. It wouldn’t tell us there is life, but it would maybe tell us that there is a world like Titan there that potentially could be habitable to some form of life.

    Once we can directly image terrestrial exoplanets I think we’re going to be able to do a lot more in terms of characterising these worlds and searching for biosignatures.

  • NS July 17, 2012, 21:06

    “astronomers are constrained to look for what it is possible to look for”

    Yes. SETI especially has been compared to a drunk looking for his keys under a lamppost because that’s where the light is. Yet for SETI we don’t even know if there are any keys, and if there are we couldn’t find them in the dark anyway. So we are indeed constrained.

  • Ron S July 17, 2012, 21:50

    ljk: “The same thing can be said for the current state of SETI. If society and governments cared enough, we could have been doing a lot more…”

    The same could be said of anything, science or otherwise. But also keep in mind that even with the full redirection of global wealth not everything is possible in this or other areas you mention. When new science or even just new technology is required, throwing unlimited resources at a problem will often not get you where you want or when you want. Sometimes we just have to be a patient for the impossible to become possible, and affordable. Impatience is not a strategy.

  • Ron S July 17, 2012, 21:59

    Alex: “Nasa has played that game ever since Viking. Look for water (ancient or recent), but don’t actually look for life.”

    Yet you said in your original comment (quite correctly) we don’t know how to detect life (or at least not without great ambiguity). It seems for every experiment we think up there are non-biological possible to explain a positive result (aka false positive). This was true of Viking and remains true of other direct experiments. This isn’t likely to improve for an experiment conducted using astronomy from afar.

    Detecting water proves nothing definite, although it would be useful data. It is also a test that appears to be becoming tractable, unlike other astronomical experiments we can do that confidently target detection of life.

  • Alex Tolley July 18, 2012, 12:11

    @ Ron S
    Yet you said in your original comment (quite correctly) we don’t know how to detect life (or at least not without great ambiguity). It seems for every experiment we think up there are non-biological possible to explain a positive result (aka false positive).

    Obviously we can detect life with the appropriate experiments. The problem is building them for use on an automated spacecraft. A human on Mars could probably successfully run definitive tests.

    Remote detection is hard because we have only very limited tools and a very limited idea of what to look for. I think the latter can be improved, even if it must default to terrestrial-like life. My complaint is that we don’t seem (AFAICS) to be thinking much about this, compared to lower hanging scientific fruit.

  • ljk July 19, 2012, 8:54

    http://news.discovery.com/animals/dolphins-math-geniuses-120717.html

    Dolphins May Be Math Geniuses

    The brainy marine mammals could be far more skilled at math than was ever thought possible before.

    By Jennifer Viegas

    Tue Jul 17, 2012 07:00 PM ET

    THE GIST
    Complex, nonlinear math appears to explain a primary dolphin hunting technique.
    The math involves addition, subtraction, multiplication and ratio comparisons.

    It is possible that dolphins possess remarkable inborn math skills.

    Bottlenose dolphins swimming. Analysis of a dolphin hunting technique suggests the animals may be natural math geniuses. Click to enlarge this image.

    Dolphins may use complex nonlinear mathematics when hunting, according to a new study that suggests these brainy marine mammals could be far more skilled at math than was ever thought possible before.

    Inspiration for the new study, published in the latest Proceedings of the Royal Society A, came after lead author Tim Leighton watched an episode of the Discovery Channel’s “Blue Planet” series and saw dolphins blowing multiple tiny bubbles around prey as they hunted.

    “I immediately got hooked, because I knew that no man-made sonar would be able to operate in such bubble water,” explained Leighton, a professor of ultrasonics and underwater acoustics at the University of Southampton, where he is also an associate dean.

    NEWS: Dolphins – Second Smartest Animal?

    “These dolphins were either ‘blinding’ their most spectacular sensory apparatus when hunting — which would be odd, though they still have sight to reply on — or they have a sonar that can do what human sonar cannot…Perhaps they have something amazing,” he added.

    Leighton and colleagues Paul White and student Gim Hwa Chua set out to determine what the amazing ability might be. They started by modeling the types of echolocation pulses that dolphins emit. The researchers processed them using nonlinear mathematics instead of the standard way of processing sonar returns. The technique worked, and could explain how dolphins achieve hunting success with bubbles.

    WATCH VIDEO: Dolphins invent a new way to hunt fish. The math involved is complex. Essentially it relies upon sending out pulses that vary in amplitude. The first may have a value of 1 while the second is 1/3 that amplitude.

    “So, provided the dolphin remembers what the ratios of the two pulses were, and can multiply the second echo by that and add the echoes together, it can make the fish ‘visible’ to its sonar,” Leighton told Discovery News. “This is detection enhancement.”

    But that’s not all. There must be a second stage to the hunt.

    NEWS: Dolphins, Humans Share ‘Brainy’ Genes

    “Bubbles cause false alarms because they scatter strongly and a dolphin cannot afford to waste its energy chasing false alarms while the real fish escape,” Leighton explained.

    The second stage then involves subtracting the echoes from one another, ensuring the echo of the second pulse is first multiplied by three. The process, in short, therefore first entails making the fish visible to sonar by addition. The fish is then made invisible by subtraction to confirm it is a true target.

    In order to confirm that dolphins use such nonlinear mathematical processing, some questions must still be answered. For example, for this technique to work, dolphins would have to use a frequency when they enter bubbly water that is sufficiently low, permitting them to hear frequencies that are twice as high in pitch.

    “Until measurements are taken of wild dolphin sonar as they hunt in bubbly water, these questions will remain unanswered,” Leighton said. “What we have shown is that it is not impossible to distinguish targets in bubbly water using the same sort of pulses that dolphins use.”

    If replicated, the sonar model may prove to be a huge benefit to humans. It might be able to detect covert circuitry, such as bugging devices hidden in walls, stones or foliage. It could also dramatically improve detection of sea mines.

    “Currently, the navy uses dolphins or divers feeling with their hands in such difficult conditions as near shore bubbly water, for example in the Gulf,” he said.

    In terms of dolphin math skills, prior studies conducted by the Dolphin Research Cetner in Florida have already determined that dolphins grasp various numerical concepts, such as recognizing and representing numerical values on an ordinal scale. Marine biologist Laela Sayigh of the Woods Hole Oceanographic Institution said, “In the wild, it would be very useful (for dolphins) to keep track of which areas were richer food sources.”

    While dolphins are among the animal kingdom’s most intelligent animals, they are not likely the only math champs.

    Parrots, chimpanzees and even pigeons have been shown to have an advanced understanding of numerical concepts. The studies together indicate that math ability is inborn in many species, with number sense, mathematical skills and verbal ability perhaps being separate talents in humans that we later learn to combine.

  • Ron S July 19, 2012, 9:39

    Alex: “Remote detection is hard because we have only very limited tools and a very limited idea of what to look for. I think the latter can be improved, even if it must default to terrestrial-like life. My complaint is that we don’t seem (AFAICS) to be thinking much about this, compared to lower hanging scientific fruit.”

    Who is “we”? It most certainly is being thought about, and actual experiments have been done. Consider that at least two interplanetary spacecraft have recently turned their instruments on Earth from some distance to see what can be seen. Using a target known to host terrestrial-like life (heh!) they look at what the instruments show, in particular whether the spectral signatures they get can reasonably and unambiguously distinguish a life-bearing planet from others known (or suspected) not to host life. Getting these signatures and quantifying the detection parameters is critical information needed to decide what instruments capabilities would be needed to investigate exoplanet targets.

  • Alex Tolley July 19, 2012, 14:57

    Ron S. This seems the best link I can find on life bio signatures.
    Exoplanet Characterization and the Search for Life

    I don’t see much that isn’t very basic and targeted at terrestrial-life-like worlds. Perhaps you have a link or two that looks into the question in a much deeper way.

    Yes, finding a terrestrial world with oceans, land life and an oxygen atmosphere would be a slam dunk, but if we found one in a million observed worlds, what does that have to say about life?

    In some respects, finding life in/on apparently dead worlds like Mars and Europa would be more interesting, especially if the biology is very different and offers insights into the space of possibilities, rather than a replay of terrestrial evolution.

  • Ron S July 19, 2012, 16:26

    Hmm…

    I did a search on arXiv and got 12 hits for biosignature in astro-ph. You may have to reconstruct this search URL, but let’s see if it comes through here:
    http://xxx.lanl.gov/multi?archive=astro-ph&search_year=all+years&field_1=abs&query_1=biosignature&%2Ffind=Find&file=new+abstracts&year=%2712&month=07&subj_astro-ph=-%3E+astro-ph+subject+classes&subj_cond-mat=-%3E+cond-mat+subject+classes&subj_physics=-%3E+physics+subject+classes

    That paper you mention (which appears to be from 2010) says that there are no single-pixel investigations of Earth from spacecraft. This is wrong, now. Unfortunately I couldn’t find a paper in my quick look, but I am sure that even Paul has a post up on this subject from 2011, I think.

    As to other “catalogs” of potential exoplanet biosignature discovery, there are lots, of what reliability I cannot say. Here’s one that on a quick look seems not bad — just remember this is a bit of a rambling list of biosignatures, for terran life and other, conceptual forms of life:
    http://www.biocab.org/Astrobiology.html

    Another catalog is the “Astrobiology Primer” which is somewhere on arXiv. The paper id I have is 0610926, but sorry no link. It looks quite good.

    Unfortunately for the present discussion I almost never record these things. If something interests me I may read it, maybe say something about it, then delete it.