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12 - How Many Inhabited Planets?

from Part III - Planetary Systems and Life

Published online by Cambridge University Press:  24 September 2020

Wallace Arthur
Affiliation:
National University of Ireland, Galway
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Summary

Here, I ask the question: how many planets are there with life in the Milky Way? This question has many variants. We can ask about simple microbial life. Alternatively, we may ask about more complex life – for example multicellular animals and plants. Then again, we may ask specifically about intelligent life. An approach that can be used for all of these variants is the one pioneered by the American astronomer Frank Drake. In this chapter, I use the Drake equation to estimate the number of microbial worlds and the number of worlds with animals. (In a later chapter I use the same approach to estimate the number of worlds with intelligent life.) When Drake first devised his equation, we were hard put to come up with meaningful numerical values for any of its parameters. Now we have reasonably good values for at least some of them. Hence our estimates are better than before. However, there are still wide errors, so I investigate the effects of these errors on our estimates. Bearing them in mind, I only attempt estimates to the nearest order of magnitude. These estimates are: 1 billion planets with microbial life; and 10 million planets with animal life.

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Chapter
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The Biological Universe
Life in the Milky Way and Beyond
, pp. 186 - 202
Publisher: Cambridge University Press
Print publication year: 2020

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  • How Many Inhabited Planets?
  • Wallace Arthur, National University of Ireland, Galway
  • Book: The Biological Universe
  • Online publication: 24 September 2020
  • Chapter DOI: https://doi.org/10.1017/9781108873154.016
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  • How Many Inhabited Planets?
  • Wallace Arthur, National University of Ireland, Galway
  • Book: The Biological Universe
  • Online publication: 24 September 2020
  • Chapter DOI: https://doi.org/10.1017/9781108873154.016
Available formats
×

Send book to Google Drive

To send content items to your account, please confirm that you agree to abide by our usage policies. If this is the first time you use this feature, you will be asked to authorise Cambridge Core to connect with your account. Find out more about sending content to Google Drive.

  • How Many Inhabited Planets?
  • Wallace Arthur, National University of Ireland, Galway
  • Book: The Biological Universe
  • Online publication: 24 September 2020
  • Chapter DOI: https://doi.org/10.1017/9781108873154.016
Available formats
×