Planetary formation patterns and the probability of alien civilisations existing in the universe

Artist's impressions of a potential future space colony designed by NASA scientists in the 1970s. This shows a painting with astronauts working on a space colony that contains land and oceans.

Image credit: Don Davis/NASA Ames Research Center/Public domain.

First published on 24th November 2015. Last updated 11 August 2018 by Dr Helen Klus

Our understanding of planets has increased dramatically in the last decade, particularly since the launch of NASA's Kepler satellite in 2009. This has resulted in the detection of thousands of extra-solar planets, and led to a better understanding of how planets form[1].

In a recent article in MNRAS (which can be read for free here), physicists Peter Behroozi and Molly Peeples from NASA's Space Telescope Science Institute look at what we can conclude from the most up-to-date theories of planetary formation[2a].

Behroozi and Peeples show that the Earth may have formed before 92% of all the habitable planets that will ever form in the observable universe.

The fact that a civilised species arose so early means there's a 92% chance that another civilised species will arise somewhere in the observable universe before planets stop forming altogether.

1. Planetary formation patterns

Behroozi and Peeples determined the number of planets formed per galaxy, per year - the planetary formation rate – by using the most recent theories regarding the formation rate of stars[3]. They then adjusted the equations to account for the fact that stars can have multiple planets, and the fact that planets need elements heavier than hydrogen and helium to form. Giant planets, like Jupiter and Saturn, are particularly sensitive to this[4].

Behroozi and Peeples show that there should be about 100 billion billion potentially habitable planets currently within the observable universe[2b]. These follow the formation rate of stars, decreasing over time. Giant planets, on the other hand, need larger amounts of heavy elements to form. These are only formed in massive stars, and so giant planets are typically billions of years younger than habitable planets, and are more likely to form over time. Low-mass galaxies might not contain any giant planets at all.

In the Milky Way, there should be about a billion habitable planets and about 10 billion giant planets. The Earth is predicted to have formed before about 80% of other habitable planets currently present in both the Galaxy and the observable universe.

In order to put this into perspective, Behroozi and Peeples worked out how many planets are predicted to form before there's no more gas left to form new stars. They concluded that the Earth formed before about 61% of all planets that will ever form in the Milky Way, taking into account its future collision with the Andromeda galaxy. The Earth is thought to have formed before 92% of all habitable planets that will ever from in the observable universe. Most of these planets will form hundreds of billions of years from now. At this point, the universe will have expanded so much that their galaxies will no longer be visible from the Milky Way.

Hubble deep field, showing many different galaxies.

"Somewhere, something incredible is waiting to be known." - Sharon Begley.
Hubble image of galaxies over 12 billion light-years away. Image credit: ESA/NASA/Public domain.

2. The probability of alien civilisations

These results can be fed into the Drake equation, which can be used to calculate the number of intelligent, communicative civilisations in the universe[5]. The Drake equation depends on a number of highly uncertain factors - such as the length of time civilisations release detectable signals into space - and so the results are highly uncertain. Behroozi and Peeples' results lead to the prediction that there are currently between one and a million billion civilisations in the observable universe, and between one and 10,000 in the Milky Way[2c].

A different approach can be found by considering the fact that if only one civilisation were ever to form in the observable universe, then you would expect it to form somewhere in the middle of the distribution. The fact that a civilisation developed on one of the first 8% of planets to form means it's quite likely that another civilisation will form at some point.

Behroozi and Peeples use Bayes' rule to show that there is at least a 92% chance that we're not the only civilisation the observable universe will ever contain. They then use Poisson statistics to determine the probability of more civilisations forming if ours was the second, third, forth, or nth civilisation to form.

They determined that if evidence of just one other civilisation were found in the Milky Way, then Earth could be the ten billionth planet with a civilisation in the observable universe.

3. References

  1. Lissauer, J. J., Dawson, R. I., and Tremaine, S., 2014, 'Advances in exoplanet science from Kepler', Nature, 513, pp.336-344.

  2. (a, b, c) Behroozi, P. and Peeples, M. S., 2015, 'On the history and future of cosmic planet formation', Monthly Notices of the Royal Astronomical Society, 454, pp.1811-1817.

  3. Muñoz, J. A. and Peeples, M. S., 2015, 'A framework for empirical galaxy phenomenology: the scatter in galaxy ages and stellar metallicities', Monthly Notices of the Royal Astronomical Society, 448, pp.1430-1445.

  4. Levison, H. F., Kretke, K. A., and Duncan, M. J., 2015, 'Growing the gas-giant planets by the gradual accumulation of pebbles', Nature, 524, pp.322-324.

  5. SETI Institute, 'The Drake Equation', last accessed 01-06-17.

  6. Dyson, F. J., 1960, 'Search for artificial stellar sources of infrared radiation', Science, 131, pp.1667-1668.

  7. (a, b) Harp, G. R., et al, 2015, 'Radio SETI Observations of the Anomalous Star KIC 8462852', arXiv preprint arXiv:1511.01606.

  8. Boyajian, T. S., et al, 2015, 'Planet Hunters X. KIC 8462852 - Where's the Flux?', arXiv preprint arXiv:1509.03622.

  9. Bad Astronomy by Phil Plait, 'Did Astronomers Find Evidence of an Alien Civilization? (Probably Not. But Still Cool.)', last accessed 01-06-17.

  10. Becker, W. and Pavlov, G. G., 2002, 'The Milky Way-pulsars and isolated neutron stars', arXiv preprint astro-ph/0208356.

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