Wednesday, December 28, 2016

How many Earth-size planets are orbiting Sun-like stars?

Size and number of Kepler Planet Candidates
Size and number of Kepler Planet Candidates
In the search for planets that may be habitable outside of our own solar system, one of the elements that should be considered is planet size.  Planet size is important for various reasons. There is evidence that suggests that plate tectonic activity on a global level is a necessary factor for supporting life on a planet similar to Earth.  Planets must be of a particular size in order to allow for global plate tectonics.[001] Larger planets (between the range of Mars and Earth) may be necessary to maintain an atmosphere with the right composition to support life.[002] Based on the limited examples available within our own solar system, life may require specific conditions that are associated with Earth's size.

So, how many Earth-size planets exist?  Well, to start with, small terrestrial planets drastically outnumber larger Earth-size and Jupiter-size planets.  That being the case, recent observations suggest that Earth-size planets are fairly common around Sun-like stars.[003] There are so many such planets, the 2013 study Prevalence of Earth-size planets orbiting Sun-like stars states that (given certain circumstances),
...the nearest such planet is expected to orbit a star that is less than 12 light-years from Earth and can be seen by the unaided eye.[003]
That's not only a lot of planets, 12 light-years is a distance that seems at least somewhat reachable with technology that is currently being investigated.  Recently, such a planet seems to have been discovered around the closest star to our own, Proxima Centauri.[004]  Technically, Proxima Centauri cannot be seen with the unaided eye on its own, but rather as part triple star system that appears as one the dot in the Southern Hemisphere sky called Alpha Centauri, but close enough (literally).[005]

Kepler's Small Habitable Zone Planets
Kepler's Small Habitable Zone Planets
Overall based on Kepler space observatory results, it is calculated that about 22% of all Sun-like stars have an Earth-size planet within its Habitable Zone.  Prevalence of Earth-size planets orbiting Sun-like stars seems well informed in its conclusion (2013),
Future instrumentation to image and take spectra of these Earths need only observe a few dozen nearby stars to detect a sample of Earth-size planets residing in the Habitable Zones of their host stars.[003]
The number of Sun-like stars in the Milky Way Galaxy is said to be about 10%.*  The number total stars is a matter of debate, but it often stated as 100 billion stars.[006]  10% of that is 10 billion stars.  Therefore, 22% of 10 billion is 2.2 billion stars.  With that determined, how common are planets that are so similar to Earth that events naturally occur in the right sequence to spark and nurture life?  How likely is that life to evolve to develop the human-level expression of intelligence and curiosity?  What is the likelihood of any of species developing in the same timeframe as us?  Are other species close enough to us to communicate with us?  Should we really trying to reach out to these others?  Some of these questions will be addressed in further articles.

*10% is stated by multiple tertiary sources for "sun-like" stars, and 7.5% is stated for "g-type" stars, but I could not verify these percentages from any original sources.

Primary reference:
E.A. Petigura, A.W. Howard, G.W. Marcya, Proceedings of the National Academy of Sciences of the United States of America, 110 no. 48, (Nov., 2013), 19273-19278, 10.1073/pnas.1319909110,  Prevalence of Earth-size planets orbiting Sun-like stars

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