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Life, Unbounded

Life, Unbounded


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Exomoons Can Spoof Exoplanet Biosignatures

The views expressed are those of the author and are not necessarily those of Scientific American.


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Astronomers hope that one day soon we’ll obtain a spectrum of light that might tell us whether or not an Earth-sized exoplanet harbors life. This spectrum could be of starlight filtered through the planetary atmosphere, or of reflected and emitted radiation. In either case it would probe the chemical composition of an alien world.

The detection of an imbalance, or disequilibrium, of atmospheric components has long been considered a smoking gun for a planetary biosphere. For example, a rich mix of oxygen and methane in a warm environment is not a stable state. Methane oxidizes relatively speedily to form water and carbon dioxide. So detecting the presence of both oxygen and methane would suggest an active replenishment mechanism. Life (as we know it) represents an excellent candidate for supplying these ingredients.

So far so good. Find that Earth-sized planet, gear up our best technology, and sniff for a spectral biosignature.

Except there may be a hitch. It’s possible for nature to throw us a curve ball. In a new paper in the Proceedings of the National Academy of Sciences, Rein, Fujii, and Spiegel explore a disquieting possibility. They ask what would happen if this juicy looking exoplanet also happens to have a substantial moon that itself harbors an atmosphere, yet both objects are devoid of life.

If the Earth-sized exoplanet has an oxygen rich atmosphere (but nothing that would react with that gas), and the exomoon has a Titan-like methane rich atmosphere, we might be duped into thinking that we’re seeing a single, out of equilibrium, biologically driven environment – an inhabited planet. That’s a problem, and it arises because, like it or not, our data is going to be comparatively low-fidelity.

It’s going to be extremely difficult to distinguish between a planetary spectrum originating from one object or two closely spaced objects, but this is as good as our near term (or even long term) technology is going to give us. Oxygen and methane represent just one example of such confusion, other chemical pairings would be similarly affected.

What’s the solution? The authors suggest two simple ways forward. One is that we might be lucky enough to identify a true ‘Earth-twin’ within some 30 light years distance – close enough to give us a shot at beating this kind of false-positive with careful measurements. The other is to just forget about looking at Earth-twins. Other potentially habitable planets, such as those around low-mass stars, or so-called super-Earths, could allow us to test for this kind of confusion by making the spectral measurements easier.

Of course, we don’t know how common such ‘spoof’ systems might be, but when we’re talking about determining whether or not we’re alone in the cosmos it would pay to be very, very certain about what we’re seeing.

Caleb A. Scharf About the Author: Caleb Scharf is the director of Columbia University's multidisciplinary Astrobiology Center. He has worked in the fields of observational cosmology, X-ray astronomy, and more recently exoplanetary science. His books include Gravity's Engines (2012) and The Copernicus Complex (2014) (both from Scientific American / Farrar, Straus and Giroux.) Follow on Twitter @caleb_scharf.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. countryend 1:39 pm 04/30/2014

    Then, perhaps, are there fossils or life expressions elsewhere? Is it possible? Isn’t the emergence and maintenance of life a process of radical contingency? That is, is a unique and unrepeatable past totally necessary? Or does life emerge through space like mushrooms when some conditions are present? So, how many conditions are necessary: three, four, trillions, infinite? Only one, water or any sort of God? Is God the word that means infinite conditions, absolute necessity? Anyway, how did the life that emerge in a given conditions resist when switching to a different moment? How does life resist time itself, the effects of entropy? But, is it possible for human beings to recognize a simpler life than their own brain only? On the other hand, beyond likeness, is it possible to recognize a complex life than their brain, is this the extra-terrestrial life that some people are searching unsuccessfully? However, is there an origin of life or would it be as finding a cut in the material history of the universe, an infinite void that human language patches now? Along these lines, there is a peculiar book, a short preview in goo.gl/rfVqw6 Just another suggestion, far away from dogmas or axioms.

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  2. 2. SJCrum 5:10 pm 05/1/2014

    There was one telling item in the writing for this subject, and one that reveals a lot about the science. And, that was the statement about a mixture of oxygen and methane being an indicator of a possible planet.
    In order to understand the science involved with that, our sun, for example, gets its burning in a way that is the same for all stars. And, that is that stars have huge amounts of methane gas in the “atmosphere” surrounding them, and another item that I will mention later.
    First, though, I will describe why the real science of this is so easy to know, while trying to normally figure out how the sun burns is nearly impossible in just observation-type analysis.
    Somewhere near 2005, there was a picture on the web that was put there by an MIT professor, and which he used as a challenge to his physics class for them to try to describe what was shown in the picture.
    The image was a photograph looking down at a newly poured slab of concrete, and with the top surface of the concrete still wet. And, in the center was a reflected image of the sun. The main thing was that the sun was greatly dimmed by grayish concrete, and that left a bright blue ring around the exterior of the sun, and a huge amount of burnt-yellow cloud-like gas all around the outer area.
    At the end of the term none of the students were ever able to describe what was shown, and the professor finally described it to them as a rainbow. In truth, though, it wasn’t that at all. But, it certainly reveals an awfully lot about the burning of stars.
    The real science is that all of the burnt-yellow gas was methane gas, and this color can easily be seen in Wikipedia images of common gas colors in chemistry.
    As for the blue ring, I will describe that later because it doesn’t have anything to do with the burning gas.
    The science with stars is that it is totally impossible for methane gas to burn without having oxygen present. And, another item is that if there was enough oxygen present at any time to cause all of the methane to explode in one huge explosion, the gas would go up instantly. So, the real science is that there is a method for stars that allows only a small, controlled burning to occur. And, that is exactly what exists with stars.
    The point is that when stars are made, it is an enormously hot formation that involves layers of liquid hot atoms being applied to cores of stars much the same as someone winding yarn around a center point to finally make a ball of yarn. Stars are made in the same exact rapid spinning formation in all directions of spin so the end result is a ball of rock-solid material. But, all of the atom material is at a temperature where dirt and rock would liquefy. And, no, accretion isn’t even close to the real science involved. And, that is a somewhat group hug of dust in clouds of dust.
    As for the star burning though, the surface of stars is a rock-like shell of calcium oxide, that when hot enough, it releases the oxygen that is locked into the solid oxide. Also, as time goes on, more and more oxygen is released as the extremely hot burning continues. And, even though the methane gas is totally there, it cannot all burn until all of the needed oxygen is provided.
    So, this is how stars work. And, this totally impossible science for evolution is exactly how the God of Creation made stars so they would burn extremely long times, and totally controlled amounts.
    As for the oxygen and the methane gas in the described situations described, unfortunately, those are a situation where a star from our galaxy has collided with a star from the Constellation of Andromeda. And, where the end result is that one of the two is still burning, and the second is just a hot core of the second star. And, all of the methane and oxygen gas is simply all coming from the atmosphere and hit surface shell that releases more oxygen.
    By the way also, all of the leaping jets shooting outward from our sun, are all caused by asteroids that are pulled into the sun, and when they hit the surface, larger amounts of oxygen are released, and this then causes far more methane to burn. So, that is a real science item also, with all of it related to how stars actually burn.

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