“Alone in The Universe”: John Gribbin’s Argument for Why Most Star Systems in the Milky Way are Uninhabitable

By Cory Davis

This is a breakdown of John Gribbin’s argument as made in his book Alone in the Universe: Why Our Planet is Unique (2011) for why most star systems in the Milky Way Galaxy are uninhabitable for technological life.

Alone in the Universe -- Why Our Planet Is Unique.jpg
Alone in the UniverseWhy Our Planet is Unique (2011) by John Gribbin

On December 19, 2020, I wrote a review of this book, giving it 4/5 stars. I thought it was a fun thought experiment which made me wonder and ponder our existence in the universe. If you are interested in checking it out, please find it here. There were four reasons that I did not give it a 5/5: (1) there was no summary of the argument, (2) there was no clear outline about why it is important, (3) the conclusion was extreme, and (4) the title of the book is not honest.

On December 30, 2020, I summarized the author’s argument consicely into 12 premises and a conclusion for why we are alone in the Milky Way Galaxy as a technological civilization. If you are interested to check it out, please find it here. His argument first reduces the the amount of stars that could harbor life from 100% of the Milky Way to just 10%, based on metallicity and location. On January 1, 2021, I broke down this argument for why most of the Milky Way area is uninhabitable for technological civilizations here. But this still accommodates 10% of the stars in the Milky Way. That leaves between 10 – 40 billion stars to consider as potential systems to harbour life. However, the author shreds this down from 10% to 0.06% in his discussion about habitable star systems and types, the topic of this post.

In this post, I will break down John Gribbin’s argument for why most star systems in the Milky Way are uninhabitable for technological civilizations. His argument is as follows:

Most stars are not the right type to accommodate complex life like ours.

Star Types (Not to Scale)

75% of the stars in our neighbourhood are red-dwarf stars with about 10% the mass of our sun.

  • Their habitable zones are very narrow and extremely close to their star, much closer than mercury is to the sun. They are around 5 million kilometres to their star, whereas mercury never gets closer than 46 million km to the sun. Because of this, these planets would be tidally locked, where one side of the planet always faces the sun, and the other side faces away. This means that one side the atmosphere would freeze off, and the other side would get scorched. You are also much closer to the star and therefore more vulnerable to solar radiation.
  • A note that the author does not make here is that some argue that tidally locked planets would have a zone of habitability between the frozen and scorched sides where temperatures would be just right. The oceans may not completely freeze or burn off because of circulation. This may be true, and create a habitable area under water. However, air moves in response to differences in pressure (related to temperature). Hot air rises and cold air (more dense) moves in. If you have an extremely cold side, and an extremely hot side, such as with a tidally locked planet, this would create winds more violent than we can imagine on earth. The chance of a “green belt” (as they call it) forming on land plummets. There may be a possibility of some lichen-like life on land here, but I am skeptical of complex life thriving on land. Please correct me if I am off-base.
  • Red Dwarves are also much more active than the sun. They blast their solar systems with radiation that would strip away atmospheres and blast life with dangerous radiation. You are also more vulnerable to things like solar flares and coronal mass ejections.

Large stars also have more difficult habitable zones. They have much shorter lives and are way hotter. Therefore, intelligent life most likely couldn’t evolve there either. The range of habitable stars would be only K-type, F-type and G-type.

Most stars inhabit multiple star systems.

Artist’s impression of the double-star system GG Tauri

Most stars orbit at least one other star which threatens stable planetary orbits, circular planetary orbits, stable habitable zones, and stable climates. Therefore, most multiple star systems cannot accommodate the evolution of technological life.

Only 20% of stars are single. Even in single-star systems, there is a lot that could go wrong for complex life. However, binary, or triple star systems are more dangerous for the planets who reside there. Very stable orbits can only occur in a narrow range of conditions in them. But even where they do occur, it is unlikely that they are circular, causing the planet to dip in and out of the habitable zone.

Fluctuating heat caused by the binary system would also cause problems (different temperatures of each star). Just a 4.5-degree (Celsius) shift on earth today could threaten civilization. The habitable zone would also vary greatly. The two stars would be getting hotter at different rates as they mature, which would impact the stability of planetary climates.

Most stars do not show refractory element patterns conducive for rocky planet formation

The Sun, with Bird Silhouette

Stars that have depleted refractory elements at their surface tend to have rocky planets, stars who have not tend to have gaseous giants in their early solar system. Only 10% of stars exhibit refractory element patterns.

  • When the sun was about 2 million years old it received a blast of iron-60 and aluminium-26 from a supernova very close to us at the time.
  • Our sun had an unusually high metallicity – which is a puzzle easily solved by the above statement.
  • However today, sun has less heavier elements at its surface than the interior – vaporizing refractory elements like calcium and aluminium at high temperatures – these elements are common in rocky planets like the earth. So, the sun has been depleted of refractory elements which in turn have been used to form rocky planets.
  • The behaviour of our sun and the formation of rocky planets is related. That depleting these “refractory” elements at its surface is related to the formation of rocky planets. Early on these elements that would have been present in the sun’s atmosphere went into the formation of rocky planets.
  • Depletion of refractories seems to be a signature that there are rocky planets like ours.
  • Stars that don’t exhibit this behaviour tend to have gaseous planets in the inner solar system to the detriment of any possible rocky planets that formed there.
  • Refractory elements are not volatile and include things like silicate that make up rocks – and form asteroids, rocky planets, and moons. Other elements are volatile and form gaseous planets.
  • Only about 10% of stars exhibit refractory element patterns that fit into this category


In a previous post, I broke down John Gribbin’s argument for why most of the Milky Way Galaxy is uninhabitable. He argued that 90% of stars in the Milky Way are not habitable for technological life due to their location with respect to metallicity content. That leaves only 10% of stars to consider for the discussion today.

  • 75% of these stars are red dwarves, or M-Stars. This lowers the percentage of stars to consider from 10% to 2.5%.
  • Only K-Type, F-Type, and G-Type stars should be considered to accommodate technological life. This reduces the number of stars from 2.5% to 2%
  • Multiple star systems are dangerous. Only 30% of the 2% are left – 0.6%. He notes that only 20% of stars are single. So here he includes some multiple star systems as candidates by incorporating 30% for consideration.
  • Only 10% of stars exhibit patterns of depleting refractory elements at their surface to be used for rocky planet formation. That leaves only 0.06% of stars left.

Even if it we were to ignore all other premises (here), you could still argue that 0.06% is rare. That is 0.06% of 400 billion. That leaves roughly 240 million stars out there, in the galactic habitable zone, with the right kind of star, in the right kind of star system, and exhibits behaviour that depletes refractory elements at their surface to form rocky planets. Many of these stars may be younger or older than our sun, in various stages of their life cycles.

So, it is worth noting that stars live in and die, leaving a relatively narrow period of time that they can accommodate technological life:

  • It took almost 4 billion years to produce complex life on earth
  • The sun is heating up giving us a limited amount of time before the oceans are evaporated
  • Time is important, and the stage in the star’s life cycle is indicative of the probability that complex life exists within its solar system

These are just a few premises for why John Gribbin argues that we are alone in the Milky Way Galaxy. Other interesting points to drive that 0.06% down even further (he argues) are: (1) that evolution is not goal-oriented, but rather adapts to environmental changes, (2) our solar system is uniquely accommodating, (3) our planet is uniquely accommodating, (4) the explosion of complex life was a rare event that does not guarantee lineages who could result in technological civilizations, (5) mass extinctions play a critical role in evolution, (6) there are several looming existential threats to humanity, and (7) human level intelligence is rare, even in the context of complex life.

If you are interested in having those arguments broken down further, as argued in the book, please let me know in the comment section below.


I think the author makes a compelling argument, that provokes imagination and big thinking across vast expanses of time and space. However, I am not necessarily convinced by the argument, as the conclusion at the end of the book is extreme. If we take his argument at face value, I think that one conclusion we could make is that technological life in our Milky Way Galaxy is rare, and the the likelihood of two existing at the same point in time and close enough in space to recognize each other is vanishingly small. With that said, not all arguments were totally convincing, and there are still many unknowns.

As always, I found this topic very interesting. But I am more interested to hear what your thoughts and opinions are. If you have any thoughts, comments, feedback or ideas, please share them in the comment section below. I am sure they will create an interesting dialogue. If you enjoyed this post, please give it a like and subscribe, or follow me on twitter @interestpeaks. I always appreciate it when you do.

Thank you so much for reading this post. I look forward to my next book review on “This Is Marketing” by Seth Godin. If this book interests you, stay tuned for future posts.

2 thoughts on ““Alone in The Universe”: John Gribbin’s Argument for Why Most Star Systems in the Milky Way are Uninhabitable

    • Hey Richie! Thank you for your comment 😀 I think there is some interest! I sure thought his book was interesting. However, his conclusions I am unsure about. I think it is possible that we are alone in the universe, but don’t think we have enough data to make that conclusion. On the flip side, I think it was an interesting thought experiment to make that argument. It helped me see the universe and our world through a different lens. Certainly a worthwhile endeavor. If you are interested, I recommend checking out The Earie Silence, Rare Earth, and Aliens (by Jim Alkalili). Aliens was great as it is a book of essays that brings a variety of perspectives. Take care and thank you for checking this blog out!


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