3 minute read

I recently read one of the first papers on the search for exoplanet technosignatures published in Nature in 1959 by Cocconi and Morrison [1]. You can see my notes from reading this paper and an accompanying Jupyter notebook I created to understand the maths within it.

Next on my list of historical papers is another Nature paper by Bracewell published in 1960 [2]. This paper gives a different perspective on the search for communications from an extraterrestrial civilisation. Bracewell challenges the strategy proposed by Cocconi and Morrison [1] of searching nearby stars for radio signals, with the view that if there were another civilisation that close to us then advanced societies would have to be extremely abundant, which is unlikely. Instead, Bracewell proposes that we should be looking for radio signals from probes sent by an advanced society to thousands of their nearest stars (including the Sun). Here are my notes from reading this paper.

I am tracking all of the papers I’ve read and want to read on this topic in this roundup of the technosignatures literature.

Alien artifacts in our solar system

Bracewell proposes that societies advanced enough to find us are likely already in communication with one another and will work together to find additional civilisations, such as ours. The nearest society to us in this galactic network is therefore going to be the one to send a probe to our solar system.

If there is already a probe listening to transmissions from Earth, communication on this will already be well on its way to reach the society it originated from. Bracewell argues that we can expect a follow-up mission to join the probe in due course and that we should be listening for messages from the probe, and possibly an initial transmission of our own signals back to us as a test.

Societies within our reach

Bracewell plots the number of stars that could potentially support life with distance from Earth (with an estimated total of 5% of the stars in the galaxy at (\(10^{10}\) stars)), based on mass distribution in the galaxy, as well as the total number of communities in the galaxy with more advanced technology than our own that there would need to be to find one at that distance.

By taking an estimate for the number of advanced civilisations in the galaxy we can use the plot to find the estimated distance to the nearest one and the number of likely stars within this distance to give the frequency of advanced civilisations at that distance. By multiplying the frequency with the time it takes an advanced civilisation to evolve (\(5 \times 10^9\) years if it is the same as Earth) we get the average lifetime of a civilisation.

If there is a technologically advanced civilisation within 10 light years of Earth, according to Bracewell’s estimates, technologically advanced civilisations would have to be extremely abundant (\(10^{10}\)) in the galaxy.

Survival times of advanced societies

An abundance of advanced civilisations implies that they are long-lived. Bracewell argues that this also allows them plenty of time to communicate and form a network. If advanced civilisations are rare then this implies they are short-lived and are less likely to find one another.


Bracewell argues that advanced civilisations are unlikely to be within 10 light years of Earth and that we should look further afield. He also proposes that technologically superior societies are likely to send out probes to the nearest 1000 or so stars to find others and that we should look for one of these artifacts in our own solar system.

All calculations in the paper assume that a technologically advanced civilisation is an inevitable outcome of evolution, which may not be the case. It also has a higher estimate for the number of stars that could support life than current estimates.

This paper was the first to propose the existence of autonomous alien probes lurking in our solar system, connected to an interstellar communication network, and these theoretical artifacts have been named Bracewell probes. Several searches have been conducted for them but at limited frequencies and nothing has been detected so far.


  1. 1. Cocconi G, Morrison P: Searching for Interstellar Communications. Nature 1959, 184:844–846.
  2. 2. Bracewell RN: Communications from Superior Galactic Communities. Nature 1960, 186:670–671.