Inhabiting the Galaxy

Earth has a nearly unimaginable variety of life–to creatures living at the bottom of the ocean to birds in the sky, Earth is teeming with life. But where, outside of Earth, are there conditions friendly to life?
This is often thought of in terms of the ‘stellar habitable zone’–the area around a star that would not be too hot, or too cold, and in combination with the planet’s natural atmosphere, would create a world hospitable to liquid water and complex (multicellular) life. This is sometimes called the ‘Goldilocks Zone’ because it is ‘just right’.  This concept, however, can be extended out to the idea of a galactic habitable zone; where in a galaxy might such planets be found? Suitable terrestrial (rocky) planets need metals too form; too little metals, and we won’t have anything suitable. Near too many supernovae (the death of massive stars), and the radiation from them would harm the chances of life developing or surviving. The Goldilocks Zone for galaxies would be somewhere with a high enough metallicity to form terrestrial planets, but sufficiently far away from supernovae, especially regular ones. It should be noted that metallicity is a measurement of metals compared to non-metals; for astrophysicists, anything heavier than hydrogen is a ‘metal’. It should also be noted that the effects of radiation on life are not well understood.
This is galaxy NGC 1300, a barred spiral galaxy. The Milky Way is also a barred spiral galaxy. Image credit: Hubble Heritage Team, ESA, NASA
This is galaxy NGC 1300, a barred spiral galaxy. The Milky Way is also a barred spiral galaxy. Image credit: Hubble Heritage Team, ESA, NASA.
This brings us to today’s article, “The Quest for Cradles of Life: Using the Fundamental Metallicity Relationship to Hunt for the Most Habitable Type of Galaxy“. Published in 2015 in Astrophysical Journal Letters and written by Pratika Dayal1 Charles Cockell, Ken Rice  and Anupam Mazumdar, this takes the above galactic habitable zone idea and transforms it into a model based in mathematical equations. Galaxies do indeed come in various types: spiral galaxies with curving ‘arms’ full of stars and a bulge in their middle, elliptical galaxies that resemble circles or ovals, and irregular galaxies of no particular shape or structure are a few examples.
Three main criteria for life in a galaxy become obvious: (1) the number of stars in a galaxy, (2) the likeliness of those stars to host a suitable planet, which is correlated with metallicity, and (3) the amount of planets affected by supernovae. They made the assumption that stars are spread out evenly throughout a galaxy, in the name of simplicity. The equations were further derived by a previously discovered ‘fundamental metalliticy relationship’ that links three things important to the equations (stellar mass (related to number of stars), rate of star formation, and metallicity). The result were equations that gave the probability of suitable planets with relation to the Milky Way (that’s our galaxy). Terrestrial planets (Mercury, Venus, Earth and Mars being our solar system’s examples) and gas giants (i.e. Jupiter, Saturn, Uranus, Neptune) were done separately. This is not to suggest that the gas giants themselves are habitable, but their moons may be suitable for life.
Galaxy NGC 1132, the focus of this image, is an example of an elliptical galaxy. Image credit: NASA/ESA/STScI/M., NASA/CXC/Penn State/G. Garmire
Galaxy NGC 1132, the focus of this image, is an example of an elliptical galaxy. Image credit: NASA/ESA/STScI/M., NASA/CXC/Penn State/G. Garmire
Generally it was found that the habitability of a galaxy increases as number of stars increase (as there are more stars that could host planets) and lower star formation rates (which is linked to supernovae). Low mass galaxies, if they have any star formation at all, were 100 to 10 million times less friendly to life than the Milky Way. Massive galaxies (at least two times the mass of the Milky Way) with under a tenth of the Milky Way’s star formation rate yielded the highest probability. Using a study linking stellar mass to the shape of the galaxy, it was found that giant elliptical galaxies would be the best bet for life (ten to ten thousand times more suitable planets than the Milky Way would likely host; for gas giants, this increased to up to a million times more life-friendly than the Milky Way).
Of course, this model is not perfect–it made simplifying assumptions, and as mentioned, the effects of radiation on life is not well known (some point out the radiation could create mutations that would help evolution and is thus plausibly beneficial to life). It is, however, a starting point for future studies.

 

Bibliography
Bennett, Jeffrey, Megan Donahue, Nicholas Schneider, and Mark Voit.  The Cosmic Perspective: the Solar System. 7th ed. Boston: Addison-Wesley, 2014. Print.
Astronomy textbook used for reference on the galactic habitable zone.
Boen, Brooke, ed. NGC 1132: A Mysterious Elliptical GalaxyNational Aeronautics and Space Administration, 7 May 2008. Web.
The source of the second picture in today’s post.
Dayal, Pratika, et. al. “The Quest for Life: Using the Fundamental Metallicity Relationship to Hunt for the Most Habitable Type of Galazy“. Atsrophysial Journal Letters. 810.1 (2015) (5pp).
Today’s featured article.
Nemiroff, Robert and Jerry Bonnell. Barred Spiral Galaxy NGC 1300. Astronomy Picture of the Day. 9 Jan. 2016. Web.
The source of the first picture in this post.
The Milky Way GalaxyNASA Goddard Spaceflight Center. 3 Feb. 2016. Web.
A page aimed at the non-scientist about the Milky Way galaxy and why we suspect it is a barred spiral. There is also a button on the top right to show a more advanced version for those more familiar.
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