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Exchange of material between solar systems by random stellar encounters

  • Robert Zubrin (a1)


It is shown that a mechanism involving only random motion of the sun with respect to the surrounding star field can account for the ~1 per 25 Myr characteristic frequency of large cometary impacts on Earth. In the proposed mechanism, the sun travels through the Oort Cloud of an encounter star, most typically a Type M dwarf, while the dwarf flies through the Oort cloud of our Sun. As a result, Oort Cloud objects from our Solar System are precipitated in large numbers to impact planets in the dwarf star system, while the dwarf's Oort Cloud objects are destabilized to impact planets in our Solar System. It is shown that it is this exchange of Oort cloud object between stellar systems, rather than the precipitation of Oort Cloud objects within a stellar system, that can account for the apparent periodicity of mass extinctions. Because the sun is more massive than ~90% of stars, its Oort cloud extends further, resulting in it delivering about a factor of three more bombardments on other solar systems than our Solar System receives. About 60% of the bombardments on our Solar System are found to be delivered by Type M dwarfs, about 20% by type K dwarfs, with the remaining 20% being delivered by stars of type G or larger. Foreign star Oort cloud objects can be captured by our Sun at typical ranges of 10 AU, resulting in a cometary approach to perihelion within a few years. It is found that assuming an effective Oort Cloud radius of 40 000 AU for a star of solar mass, increasing in size with the square root of the mass, accounts for the observed characteristic frequency of mass extinction events on Earth, given the local stellar number density of 0.003 stars per cubic light year. The frequency of mass extinction events in other solar systems would increase or decrease in linear proportion to the local stellar number density. It is shown that this exchange of materials between solar systems during close stellar encounters could be an important mechanism for spreading life throughout the galaxy. Implications for the evolution of life on Earth and in other solar systems are discussed.


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Author for correspondence: Robert Zubrin, E-mail:


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Alvarez, W (1997) T. Rex and the Crater of Doom. Princeton, NJ: Princeton University Press.
Alvarez, L, Alvarez, W, Asaro, F and Michel, H (1980) Extraterrestrial cause for the cretaceous-tertiary extinction. Science 208, 10951108.
Brasser, R, Duncan, MJ and Levison, HF (2008) Embedded star clusters and the formation of the Oort cloud. Icarus 184, 5982.
Drake, N (2017) This may be the oldest known sign of life on Earth, National Geographic, 1 March, 2017, Available at (Accessed 12 March 2017).
Feng, F and Bailer-Jones, CAL (2014) Exploring the role of the Sun's motion in terrestrial comet impacts. Monthly Notices of the Royal Astronomical Society 442, 36533673.
Feng, F and Bailer-Jones, CAL (2015) Finding the imprints of stellar encounters in long-period comets. Monthly Notices of the Royal Astronomical Society 454, 32673276.
Fouchard, M, Froeschle, C, Rickman, H and Valsecchi, G (2008 a) Methods for the study of the dynamics of the oort cloud comets I: modelling the stellar perturbations. In Benest, D, Froelich, C and Lega, E (eds), Topics in Gravitational Dynamics: Solar, Extra Solar, and Galactic Systems. Berlin: Springer, pp. 257272, Chapter 10.
Fouchard, M, Frieschle, C, Breiter, S, Ratajczak, R, Valsecchi, G and Rickman, H (2008 b) Methods for the study of the dynamics of the Oort cloud comets II: modelling the galactic tide. In Benest, D, Froelich, C and Lega, E (eds), Topics in Gravitational Dynamics: Solar, Extra Solar, and Galactic Systems. Berlin: Springer, pp. 273295, Chapter 10.
Fouchard, M, Froeschle, C, Rickman, H and Valsecchi, G (2011) The key role of massive stars in oort cloud comet dynamics. Icarus 214, 334347.
Heisler, H, Tremaine, S and Alcock, C (1987) The frequency and intensity of comet showers from the Oort cloud. Icarus 70, 269288.
Hornek, G, Klaus, D and Mancinelli, R (2010) Space mircobiology. Microbiology and Molecular Biology Reviews 74, 121156.
Hughes, AM, Duchêne, G and Matthews, BC (2018) Debris disks: structure, composition, and variability. Annual Review of Astronomy and Astrophysics 56, 541591.
Levison, H, Duncan, M, Brasser, R and Kaufmann, D (2010) Capture of the Sun's Oort cloud from stars in its birth cluster. Science 329, 187190.
Mamajek, E, Barenfeld, S, Ivanov, V and Kniazev, A (2015) The closest known flyby of a star to the solar system. The Astrophysical Journal Letters 800, L17.
Melosh, J (1988) The rocky road to panspermia. Nature 332, 687688.
Muller, R (1988) Nemesis the Death Star: The Story of A Scientific Revolution. New York: Weidenfeld and Nicolson.
Randall, L (2015) Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe. New York: Ecco.
Randall, L and Reece, M (2014) Dark matter as a trigger for periodic comet impacts. Phys. Rev Letters 112, 161301.
Raup, D (1999) The Nemesis Affair: A Story of the Death of Dinosaurs and the Ways of Science. New York: W.W. Norton.
Raup, D and Sepkowski, J (1984) Periodicity of extinctions in the geologic past. Proceedings of the National Academy of Science 81, 801805.
Stern, A (1987) Extra-solar Oort cloud encounters and planetary impact rates. Icarus 69, 185188.
Weisman, PR (1997) Long period comets and the Oort cloud. Annals of the New York Academy of Sciences 82, 6795.
Zubrin, R (2001) Interstellar panspermia reconsidered. Journal of the British Interplanetary Society 54, 262269, Avaialable at, (Accessed 26 February 2017).
Zubrin, R (2017) Interstellar communication using microbial data storage: implications for SETI. Journal of the British Interplanetary Society 70, 163174.
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International Journal of Astrobiology
  • ISSN: 1473-5504
  • EISSN: 1475-3006
  • URL: /core/journals/international-journal-of-astrobiology
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