Indexing of exoplanets in search for potential habitability: application to Mars-like worlds

Abstract

Study of exoplanets is one of the main goals of present research in planetary sciences and astrobiology. Analysis of huge planetary data from space missions such as CoRoT and Kepler is directed ultimately at finding a planet similar to Earth—the Earth’s twin, and answering the question of potential exo-habitability. The Earth Similarity Index (ESI) is a first step in this quest, ranging from 1 (Earth) to 0 (totally dissimilar to Earth). It was defined for the four physical parameters of a planet: radius, density, escape velocity and surface temperature. The ESI is further sub-divided into interior ESI (geometrical mean of radius and density) and surface ESI (geometrical mean of escape velocity and surface temperature). The challenge here is to determine which exoplanet parameter(s) is important in finding this similarity; how exactly the individual parameters entering the interior ESI and surface ESI are contributing to the global ESI. Since the surface temperature entering surface ESI is a non-observable quantity, it is difficult to determine its value. Using the known data for the Solar System objects, we established the calibration relation between surface and equilibrium temperatures to devise an effective way to estimate the value of the surface temperature of exoplanets.

ESI is a first step in determining potential exo-habitability that may not be very similar to a terrestrial life. A new approach, called Mars Similarity Index (MSI), is introduced to identify planets that may be habitable to the extreme forms of life. MSI is defined in the range between 1 (present Mars) and 0 (dissimilar to present Mars) and uses the same physical parameters as ESI. We are interested in Mars-like planets to search for planets that may host the extreme life forms, such as the ones living in extreme environments on Earth; for example, methane on Mars may be a product of the methane-specific extremophile life form metabolism.

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Notes

  1. 1.

    http://phl.upr.edu/projects/habitable-exoplanets-catalogue.

  2. 2.

    http://phl.upr.edu/projects/habitable-exoplanets-catalogue/data/database.

  3. 3.

    Bray-Curtis distance becomes Manhattan when it is applied to relative counts, as opposed to the absolute abundances (Greenacre and Primicerio 2013).

  4. 4.

    Maintained by the PHL, http://phl.upr.edu/projects/habitable-exoplanets-catalogue/data/database.

  5. 5.

    http://phl.upr.edu/library/notes/surfacetemperatureofplanets.

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Acknowledgements

We would like to thank Jayant Murthy (Indian Institute of Astrophysics, Bangalore) and Yuri Shchekinov (Lebedev Institute, Moscow) for the fruitful discussions. This research has made use of the Extrasolar Planets Encyclopaedia at http://www.exoplanet.eu, Exoplanets Data Explorer at http://exoplanets.org, the Habitable Zone Gallery at http://www.hzgallery.org/, the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program at http://exoplanetarchive.ipac.caltech.edu/ and NASA Exoplanet Archive at http://exoplanetarchive.ipac.caltech.edu and NASA Astrophysics Data System Abstract Service.

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Correspondence to Madhu Kashyap Jagadeesh.

Appendix: Calculation of Mars ESI as an example

Appendix: Calculation of Mars ESI as an example

\(\mathrm{ESI}_{x}\) calculations for Mars are performed using Eq. (10), with weight exponents from Table 1, by using the following values for the input parameters,

$$\begin{aligned} &R=0.53 \times 6371~\mbox{km} = 3376.63~\mbox{km},\\ & \rho=0.71 \times 5.51~\mbox{g}/\mbox{cm}^{3} = 3.9121~\mbox{g}/\mbox{cm}^{3},\\ &V_{e}= 0.45 \times 11.19~\mbox{km}/\mbox{s} = 5.0355~\mbox{km}/\mbox{s} , \\ &T_{s}= 240~\mbox{K} . \end{aligned}$$

The ESI for each parameter are, accordingly,

$$\begin{aligned} & \mathrm{ESI}_{R}= \bigl(1- \vert 3376.63~\mbox{km}-6371~\mbox{km} \vert / \vert 3376.63~\mbox{km} \\ &\phantom{\mathrm{ESI}_{R}=\,\,}{} +6371~\mbox{km} \vert \bigr)^{0.57} = 0.8124 ,\\ & \mathrm{ESI}_{\rho} = \bigl(1-|3.9121~\mbox{g}/\mbox{cm}^{3} - 5.51~\mbox{g}/\mbox{cm}^{3}| /|3.9121~\mbox{g}/\mbox{cm}^{3}\\ &\phantom{\mathrm{ESI}_{R}=\,\,}{} +5.51~\mbox{g}/\mbox{cm}^{3}| \bigr)^{1.07} = 0.8218 ,\\ & \mathrm{ESI}_{v_{e}} = (1-|5.0355~\mbox{km}/\mbox{s}-11.19~\mbox{km}/\mbox{s}| /|5.0355~\mbox{km}/\mbox{s}\\ &\phantom{\mathrm{ESI}_{R}=\,\,}{} +11.19~\mbox{km}/\mbox{s}| )^{0.7} = 0.7162 ,\\ & \mathrm{ESI}_{T_{s}} = (1-|240~\mbox{K}-288~\mbox{K}| /|240~\mbox{K}+288~\mbox{K}| )^{5.58}\\ &\phantom{\mathrm{ESI}_{R}}{} =0.5875 . \end{aligned}$$

Interior ESI from Eq. (14) is:

$$\mathrm{ESI}_{I} = \sqrt{0.8124\times0.8218} \approx 0.8171. $$

Surface ESI from Eq. (15) is:

$$\mathrm{ESI}_{S} = \sqrt{0.7162 \times 0.5875} \approx 0.6487. $$

And the global ESI for Mars (Eq. (16)) is:

$$\mathrm{ESI} = \sqrt{0.8171 \times 0.6487} \approx 0.728. $$

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Kashyap Jagadeesh, M., Gudennavar, S.B., Doshi, U. et al. Indexing of exoplanets in search for potential habitability: application to Mars-like worlds. Astrophys Space Sci 362, 146 (2017). https://doi.org/10.1007/s10509-017-3131-y

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Keywords

  • Earth-like planets
  • Habitability
  • Astrobiology