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Two Takes on Extraterrestrial Life

“With exoplanets we are entering new territory,” says René Heller (Leibniz Institute for Astrophysics, Potsdam), talking about recent studies looking at axial tilt as a parameter for habitability on a planet. Heller is getting at the fact that while we’ve studied the axis of a planet’s spin relative to the plane of its orbit rather thoroughly here in our own Solar System, we are a long way from being able to discern the axial tilt of exoplanets, much less make definitive statements about its effect on habitability. Right now we can say something about the size, mass and orbital period of many distant planets (and in a few cases, some of the components of their atmosphere) and that’s about where our knowledge stops.

Heller imagines the Earth with an axial tilt something akin to that of Uranus, whose equator and ring system run almost perpendicular to the plane of its orbit. Introduce such high obliquity to the Earth and the north pole would point at the Sun for a quarter of the year and in the opposite direction for another three months. As the year progresses, one pole is scorched while the other freezes, and then the situation reverses. As Heller told Astrobiology Magazine in a recent story, “The hemispheres are cyclically sterilized, either by too strong irradiation or by freezing.”

In the same article, George Williams (University of Adelaide) speculates that an axial tilt of as little as 40 degrees would provide a tough environment for complex animals because of the exceedingly hot summers and cold winters that would prevail over much of the globe. He points to evidence for high obliquity in the early Earth, arguing that it may have been a change in axial tilt to something closer to today’s 23.5 degree obliquity that prompted the explosion of lifeforms in the Cambrian era some 540 million years ago. Glacier studies may be giving us some evidence for high obliquity throughout much of Earth’s early history, though what would have altered our planet’s tilt roughly 550 to 600 million years ago to make all this happen remains a mystery.

Plants on the Early Earth

Exoplanets raise so many questions about our own planet’s past, and the obliquity issue is but one factor in all this. We can also home in on the question of plant life, examined in a recent Scientific American blog entry with the assertive title Thanks to Plants, We Will Never Find a Planet Like Earth. The story looks at an open question — how was the surface of the early Earth shaped, particularly by the waters that weathered bare rock to create the soil plants needed to thrive? Martin Gibling (Dalhousie University, Nova Scotia) and Neil Davies (University of Ghent) have analyzed sediment deposition back hundreds of millions of years.

The evolution of vascular plants with complex root systems around 450 million years ago was a major factor in drawing carbon dioxide out of the atmosphere, but Gibling and Davies think these plants were also critical in breaking down rock to form the minerals and mud that helped river banks to form. I went to their paper in Nature Geoscience to look at their views on the interplay between organisms and their physical environments (the growing discipline is called biogeomorphology). The researchers identify a series of changes in Palaeozoic fluvial systems forced by plant evolution, including the introduction of stable muddy floodplains well suited for vegetation and soil development.

The authors see plants acting as ‘geomorphic engineers,’ creating a framework for the complexity of further life:

…the development of meandering rivers with strengthened banks through the Late Silurian and Devonian promoted stable muddy floodplains partially protected by levees and highly suitable for vegetation growth and soil development, including carbonate-rich soils… Early Pennsylvanian suites of narrow fixed channels on muddy plains and of island-braided styles in sand-bed rivers imply an increased length of channel margins and riparian corridors with their varied plant communities and subsurface water prisms, which are of crucial importance for many animal species. The Devonian to Pennsylvanian development of avulsive channel systems generated abandoned channels suitable for organic occupation, especially during dry periods. Towards the sea, concomitant expansion of muddy coastal plains and deltas would have also had many biological consequences. Feedback loops are likely to have been complex.

And so on. Just how valuable was this interplay between plants and the physical environment? This entire issue of Nature Geoscience is devoted to the question, noting how many of Earth’s environmental features were brought about by the evolution of life. The effect of plants goes beyond geochemistry and extends deeply into the landscape, controlling the stability of river banks. Before plant life was ubiquitous, water flowed in broad sheets with few clearly defined channels and floodplains. The editors of the journal think this has implications for exoplanet life:

Even if there are a number of planets that could support tectonics, running water and the chemical cycles that are essential for life as we know it, it seems unlikely that any of them would look like Earth. Even if evolution follows a predictable path, filling all available niches in a reproducible and consistent way, the niches on any Earth analogue could be different if the composition of its surface and atmosphere are not identical to those of Earth. And if evolution is random, the differences would be expected to be even larger. Either way, a glimpse of the surface of an exoplanet — if we ever get one — may give us a whole new perspective on biogeochemical cycling and geomorphology.

Let’s hope that we are indeed able to get that glimpse, and a glimpse at many of the exoplanets being discovered today, particularly those in their star’s habitable zone. It’s hard to argue with the belief that evolution will take a wild variety of courses depending on initial conditions ranging from planetary atmospheres to, as we saw above, axial tilt. What we have ahead of us stretching far into the forseeable future is the chance to begin cataloguing exoplanet ecosystems that we’ll learn more about from spectroscopy and eventual imaging as we tune up our tools. We’ve already learned how wildly one planetary system can differ from another. We can only imagine the extensive range life will take, assuming it’s out there, as it fills each unique niche.

The paper is Gibling and Davies, “Palaeozoic landscapes shaped by plant evolution,” Nature Geoscience 5, 99–105 (2012), published online 01 February 2012. Thanks to my friend Antonio Tavani for the heads-up on this paper and the related story from Scientific American.


Comments on this entry are closed.

  • Mark Wakely February 12, 2012, 15:24

    One thing the article fails to mention is the distribution of land masses on any tilted alien planet in the goldilocks zone. Assuming adequate mobility with the ability to migrate fast enough to remain in an acceptable climate, a land-based species still needs someplace to migrate to in order to escape extreme weather changes. If a planet doesn’t have at least one sizable land mass that is always in a habitable zone, it seems unlike that those species would evolve in the first place. (One “supercontinent” in one hemisphere would be an example.) Widely separated continents would be another problem unless the species also had the ability to transverse oceans, either on the oceans or above them (flight). Earth, with its many past and present “land bridges” (some of ice) between continents allowed for species migrations, many of which continue today as seasons change. The same might not be true of tilted alien planets with “poor” or inadequate land mass distribution, where weather extremes like in the illustration above would make it difficult if not impossible for life to evolve and adapt.

  • Eniac February 12, 2012, 19:06


    The 60 civilizations in my database endured a total of 25,234 years, so L = 420.6 years. For more modern and technological societies, L became shorter, with the 28 civilizations since the fall of Rome averaging only 304.5 years.

    Estimating L in the Drake equation this way makes no sense whatsoever. The Drake equation has no factor taking into account more than one civilization per planet, and the author counts 60 on one planet alone.

    Civilization as used by the author is a fleeting political construct. What we are really interested in is how long will there be beings around that qualify as technological.

    The correct interpretation of L is the length of time until technology disappears from a planet, never to arise again. In my view, this is not going to happen before the planet is made uninhabitable by its swelling sun, so L should be the average lifetime of a planet. Or longer, if the “civilization” gets their act together and move somewhere else.

    The Drake equation assumes that L is the lifetime of civilization after it has gone through all the other steps, and that after L, the whole process must start anew. This is obviously not true, since the author counts

  • Eniac February 12, 2012, 19:11

    Oops, disregard that last incomplete paragraph, please.

  • amphiox February 14, 2012, 1:36

    Earth, billions of years of life, only a handful of tool using species, ever (even counting the crows from New Caledonia), 100 years of electronics, and still no signs if civilization detectable by alien astronomers.

    That COULD mean that tool-using species are rare in the universe.

    It could ALSO mean that EVERY SINGLE HABITABLE PLANET OLDER THAN 4 BILLION YEARS will have produced at least one such species.

    Until we actually find another example of a habitable planet, and figure out if that habitable planet does or does not have a tool-using species, there is no way to distinguish which or the above two scenarios is more correct, or to which of the above two poles reality most closely resembles.

    No amount of additional knowledge obtained from studying earth alone will increase our certainty on this question, even if we somehow manage to learn EVERYTHING about earth’s life.

    You can’t coherently calculate probabilities with a known denominator of only one.

  • amphiox February 14, 2012, 1:38

    Estimating L in the Drake equation this way makes no sense whatsoever.

    Absolutely agreed!

    It is in fact better to express L not in years, but in a proportion (just like all the other Drake variables), as in the proportion of a planet’s habitable lifespan during which a technological species exists and possesses technology to a level that is detectable. This would then account for the possibility that multiple technological species could arise, and go extinct, at different time points, or for a single species to experience rises and falls over time in technological levels (wherein they might spend several centuries being detectable, bomb themselves back to the stone age, recover over the next several thousand years, and be detectable again).

  • amphiox February 14, 2012, 1:42

    I never found the planetary obliquity arguments very convincing at all. So what if there are big seasonal swings in climate? Complex lifeforms like animals can migrate. Complex lifeforms like plants can have life cycles with protected dormant stages (or even find means of migrating too.)

    Even if you postulate that complex life can’t evolve in the first place in such a situation, the one thing we know for certain is that obliquity will change, so all you need is one short period of low obliquity that allows for complex life to evolve, and after it has established itself, it should be immune to future swings in obliquity.

    Imagine if earth suddenly swings into an Uranus-like obliquity today. Assume the process takes several tens of thousand years. It might not be pleasant, but it is easy, EASY, to envision that humanity would survive, and indeed human civilization would survive. And so would most lineages of plants and animals.

  • Quiet Professional February 17, 2012, 2:41

    I would posit one civilization per galaxy at any given point in time.

  • what March 4, 2012, 14:39

    “The hemispheres are cyclically sterilized, either by too strong irradiation or by freezing.”

    What is sterilization by freezing?

  • ljk May 22, 2012, 13:08


    Biophoton Communication: Can Cells Talk Using Light?

    A growing body of evidence suggests that the molecular machinery of life emits and absorb photons. Now one biologist has evidence that this light is a new form of cellular communication.

    kfc 05/22/2012

    One of the more curious backwaters of biology is the study of biophotons: optical or ultraviolet photons emitted by living cells in a way that is distinct from conventional bioluminescence.

    Nobody is quite sure how cells produce biophotons but the latest thinking is that various molecular processes can emit photons and that these are transported to the cell surface by energy carying excitons. A similar process carries the energy from photons across giant protein matrices during photosynthesis.

    Whatever the mechanism, a growing number of biologists are convinced that when you switch off the lights, cells are bathed in the pale fireworks of a biophoton display.

    This is not a bright phenomena. Biophotons are usually produced at the rate of dozens per second per square centimetre of cell culture.

    That’s not many. And it’s why the notion that biophoton activity is actually a form of cellular communication is somewhat controversial.

    Today, Sergey Mayburov at the Lebedev Institute of Physics in Moscow adds some extra evidence to the debate.

    Mayburov has spent many hours in the dark watching fish eggs and recording the patterns of biophotons that these cells emit.

    The question he aims to answer is whether the stream of photons has any discernible structure that would qualify it as a form of communication.

    The answer is that is does, he says. Biophoton streams consist of short quasiperiodic bursts, which he says are remarkably similar to those used to send binary data over a noisy channel. That might help explain how cells can detect such low levels of radiation in a noisy environment.

    If he’s right, then this could help to explain a number of interesting phenomenon that some biologists attribute to biophoton communication.

    In several experiments, biophotons from a growing plant seem to increase the rate of cell division in other plants by 30 per cent. That’s a growth rate that is significantly higher than is possible with ordinary light that is several orders of magnitude more intense.

    Other experiments have shown that the biophotons from growing eggs can encourage the growth of other eggs of a similar age. However, the biophotons from mature eggs can hinder and disrupt the growth of younger eggs at a different stage of development. In some cases, biophotons from older eggs seem to stop the growth of immature eggs entirely.

    Mayburov’s work won’t end the controversy; not by any means. There are still many outstanding questions. One important problem is to better understand the cellular mechanisms at work–how the molecular machinery inside cells produces photons and how it might be influenced by them. Another is to understand the kind of evolutionary pressures that are at work here–how has this ability come about?

    Clearly, there’s more work to be done here.

    Ref: http://arxiv.org/abs/1205.4134: Photonic Communications and Information Encoding in Biological Systems