Biosignatures of Super-Earths



We have as yet not found another Earth. However, our discoveries have led us to super-earths; these super-earths have been found to orbit M-sized dwarf stars. Our technological limitations may be viewed as the obvious reason for the super-earth discoveries. While we continue to search for the sky for any signs of a biosignature, super-earths are a good case-study for what may be possible to discern from our Earthly perspective. Despite the discoveries, we “really” cannot see the super-earth very well. Our best telescopes can discern a planet but not a spectral composition of the atmosphere. Computations have been performed to discern what biosignatures are possible for our discoveries and we may be a few years away from “seeing” a spectrum of a super-earth. So, what can we expect? Does name “super-earth” imply that we may see another Earth; our twin, perhaps. Despite the name, we most probably will not see as much similarity as some sci-fi will have us believe.

Firstly, we orbit a G-type dwarf star, not a M-type dwarf star. The differences are an “apple-and-lemon” contrast of characteristics. G-type stars are much warmer and larger than their M counterparts. The size of the star will (for the most part) dictate the type of solar radiation coming-upon the planet. We perceive radiation of G-type stars as “yellow” light on our planet; while M-type stars radiate primarily infrared light. Our perception of infrared light “would” be darker and “possibly hot to the touch.” If one were to wear “night-vision” glasses, then objects and individuals would be readily discerned. “We don’t see heat, but we certainly feel it.”

So, if we could not readily discern infrared objects, then an “infrared spectrum” of the two different planets would allow us to “possibly” see biosignatures. The figure below is illustrative of infrared spectra of bio-signatures under different kinds of “suns.”

Fig 1 (below) Representative Biosignatures in the Infrared Spectrum of different stars for twin-planet Earth.

Legend of colors:

Star Surface Temperature

Black 6000 K (the Sun)Red 3300 K (Gliese 388) Star known to eject flares.

Green 3800 K

Blue 2800 K

Magenta 2500 K

Each species shows variations in peak intensity and whether the species is discerned. Each species is known for our planet.

Figure adapted from:

H. Rauer et. al. Astronomy & Astrophysics (2011) Potential Biosignatures in super-Earth atmospheres.

Paper available from Physics E-print server.

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2 thoughts on “Biosignatures of Super-Earths

  1. Torbjörn Larsson, OM

    Not central here, but why wouldn’t life see objects in IR on M star planets? Snakes do it through their pit organs, and the archaea photosynthesis is AFAIK based on the opsins that we have adapted to our main atmosphere & star visibility window.

    Reply
  2. jaksichja

    To a first approximation the IR light impinging upon the surface of the planet would be absorbed at an initial wavelength and emit at a lower wavelength. Given that fact, humanity may not possess the ability to see anything on the planet if the initial wavelength was not within a person’s (optical) sensitivity range.

    Your question is very good—I obviously did not perform any calculations to satisfy the numbers. And–I am arguing to a (rough) first approximation.

    I can appreciate your comment but it is occasionally very hard to find the middle ground of argumentation when attempting to do public outreach in science.

    Reply

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