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 on 5th June 2017 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.

Fewer habitable planets are formed over time, and so most of these planets will form in the distant future, 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. Potential evidence of alien civilisations

There's currently no evidence of life anywhere else in the universe, however it's possible that we could find evidence of alien civilisations if they exist. This will probably only happen if they choose to communicate with us, or if they're advanced enough to change the environment around their stars.

Diagram of a Dyson sphere around the Sun that extends to Earth.

Diagram of a Dyson sphere. Image credit: Bibi Saint-Pol/Public domain.

It's been suggested that advanced life forms may form Dyson spheres, or Ringworlds around their stars in order to harness more of the star's energy and increase the liveable land mass[6]. A Dyson sphere with a radius at 1 AU (the distance between the Earth and Sun), for example, would have a surface area of about 550 million times the surface area of the Earth*.

Painting of a space colony from the outside, showing layers of land.

Artist's impression of a potential future space colony designed by NASA scientists in the 1970s. Image credit: Rick Guidice/NASA Ames Research Center/Public domain.

Painting of a space colony from the inside, showing people on land near a lake.

Artist's impression of a potential future space colony designed by NASA scientists in the 1970s.
Image credit: Don Davis/NASA Ames Research Center/Public domain.

If Dyson spheres are possible, and an alien civilisation is able to build them, then they may be detected using the same method the Kepler spacecraft uses to look for planets. A fully formed Dyson sphere would block all of the visible light from the star, and would only be detected from its heat or through gravitational affects, but a ring system, or partially constructed Dyson sphere would cause the light of the star to flicker as it orbits, periodically blocking the light. This would cause a dip in brightness, and the shape of the dip can be used to determine the shape of the object that made it.

It has recently been suggested that some sort of alien megastructure might be responsible for the strange light curves of the star KIC 8462852[7a]. KIC 8462852 is an F-type main sequence star, which makes it very similar to the Sun, and is about 1400 light-years away. This means that light from KIC 8462852 takes 1400 years to reach us, and so we are seeing what it was like 1400 years ago.

Evidence of KIC 8462852's unusual fluctuations in brightness were first discovered by amateur astronomers working as part of the Planet Hunters project, they then collaborated with professional astronomers, and their results are available here[8]. While Dyson spheres are not discussed in the paper, the lead author, American astronomer Tabetha Boyajian, has since discussed the possibility with other scientists[9], and last month, scientists at SETI spent two weeks looking for evidence of artificial radio signals coming from KIC 8462852, although they did not find any[7b].

While it's possible that there's an alien megastructure around KIC 8462852, it seems much more likely that the unusual fluctuations will be due to a new type of natural phenomenon. A similar thing happened when pulsars were first discovered in the 1960s[10]. Either way, KIC 8462852 is a highly unusual system that's bound to tell us something new about the universe, and we are just beginning to scratch the surface of what we can learn from the Kepler data.

You can join the search for exoplanets at Planet Hunters, where after following a simple tutorial, you'll be shown actual light curves compiled by Kepler so you can look for ‘dips'. If evidence of alien civilisations is out there, this may be the best way to find it.

Surface area of a sphere = 4π × Radius2.

Radius of Earth = 6371 km.
Surface area of Earth = 4π × 63712 = 510 million km2

Radius of Dyson sphere of 1 AU = 149,597,871 km.
Surface area of Dyson sphere of 1 AU = 4π × 149,597,871 km2 = 281 million billion km2.

Surface area of Dyson sphere of 1 AU/Surface area of Earth
281 million billion km2/510 million km2
= 551 million.

4. 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.

Back to top

The Star Garden is a science news and science education website run by Dr Helen Klus.

How we came to know the cosmos covers the history of physics focusing on space and time, light and matter, and the mind. It explains the simple discoveries we made in prehistoric times, and how we built on them, little by little, until the conclusions of modern theories seem inevitable. This is shown in a timeline of the universe.

The Star Garden covers the basics for KS3, KS4, and KS5 science revision including SATs, GCSE science, and A-level physics.