It is a common trope of science fiction that a space ship or shuttle crashes on a nearby planet, and for the human occupants to step outside and comfortably breath the planet's atmosphere.

My question is:

  • Given 400 billion planets in the Milky Way galaxy in accordance with this article
  • Given Dr Drake's initial assessment of Drake's equation of 50 million planets within the Milky Way galaxy that have evolved life in this article
  • Given no change to our genetic makeup nor special breathing apparatus

What is the percentage chance that any random planet that our shuttle crashes on within the Milky Way galaxy has a breathable atmosphere?

Breathable = tolerable. Does not need to be too comfortable, just not immediately fatal within say an hour. Temperature ranges within survivable human limits.

Bonus: If you could substantiate a figure even loosely based on your assumptions that would help immensely. Estimates based on current data from exoplanet discoveries get extra bonus points.

  • 2
    $\begingroup$ Aren't you just asking for the Drake equation with fewer terms? If you're going to use the values for the Drake equation in the link, just use the same estimated values for the same terms in the link. $\endgroup$
    – DKNguyen
    Commented Jun 16, 2021 at 5:07
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    $\begingroup$ From our own solar system, oxygen does not accumulate in the atmosphere by geological processes. A carbon dioxide atmosphere is more likely on rocky planets. The only reason Earth has the amount oxygen in its atmosphere that it does is because of microbial life, a magnetic field to protect the atmosphere from losses & a gravity strength to give a sufficiently high escape velocity to hold oxygen & nitrogen. $\endgroup$
    – user81881
    Commented Jun 16, 2021 at 5:15
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    $\begingroup$ I'd say the odds are Puts on sunglasses Astronomical. $\endgroup$
    – Jafego
    Commented Jun 16, 2021 at 5:31
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    $\begingroup$ @Jafego - yes but astronomically large, or astronomically small? $\endgroup$
    – flox
    Commented Jun 16, 2021 at 7:15
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    $\begingroup$ @flox As per Fred's comment free oxygen is likely too reactive to just float around untouched without something producing it to offset its removal from the atmosphere. $\endgroup$
    – DKNguyen
    Commented Jun 17, 2021 at 1:07

5 Answers 5


Havng a breathable atmosphere and temperatures with liquid water, etc., are requirements for a planet to be habitable for humans.

Most discussions of planetary habitabilty are about planets where any of the many possible types of carbon based, liquid water using lifeforms could live. Places where humans could live unproteted, breathing the atmosphere should be a small subset of planets which are habitable for carbon based, liquid water using lifeforms in general.

For example, the biosphere of Earth, inhabited by various living organisms, extends several miles high in the atmosphere and several miles deep under the oceans, as well as several miles deep inside rocks. Unprotected humans are only able to survive on the land surface of Earth, and some parts of the land surface of Earth are too hot, too cold, too dry, etc. for unprotected humans to survive here.

So there are many lifeforms, even on Earth, that can survive in conditons where unprotected humans would die within hours, minutes, or seconds. Thus alien planets where some life forms can live and flourish, but where humans without protective clothing, vehicles, and buildings would swiftly die, should be several times as common as planets where unprotected humans can survive for long periods.

As far as I know. the only scientific discussion of habitabiity for humans is Habitable Planets for Man, Stephen H. Dole, 1964, 2007. Here is a link to a pdf of the 1964 edition:


After discussing many, many factors which affect the habitability of planets in earlier chapters, Dole discusses the probability of habitable planets in chapter four, pages 82 to 105.

On page 103 Dole concludes that there should be about six hundred million habitable for humans planets in our galaxy. That is a vast number, but there should be about 100,000,000,000 to 400,000,000,000 stars in the galaxy. Dividing the number of stars by 600,000,000 should mean that there is one star with a habitable planet out of every 166.6666 to 666.6666 stars, or that the probability that a particular star has a habitable planet is between 0.0015 and 0.006.

Dole calculates that the average distance between a star with a human habitable planet and the nearest star with a human habitable planet is about 24 light years.

That does not seem very promishing for space travelers who have to make an emergency landing on the nearest planet.

Of course there have been many advances in astronomy and astrobiology since Dole wrote in 1964, 57 years ago. It is possible that the 2007 edition was revised with more up to date information and gave a higher or lower estimate of the number of human habitable planets, but I haven't seen that edition.

So possibly you might want to write a story where some superadvanced society has terraformed many planets and so every star in the galaxy has at least one planet habitable for humans, where they could breathe the air after crash landing.


Your question can be rephrased to "What are the chances that a planet has photosynthetic life forms?", because oxygen won't last over geological times in a non life hosting planet.

We can calculate it by using a modified version of Drake equation

$N=R_* \cdot f_p \cdot n_e \cdot f_l \cdot f_i$


$R_∗$ = the average rate of star formation in our galaxy

$f_p$ = the fraction of those stars that have planets

$n_e$ = the average number of planets that can potentially support life per star that has planets

$f_l$ = the fraction of planets that could support life that actually develop life at some point

$f_i$ = the fraction of planets with life that actually go on to develop intelligent life (civilizations)

here I am assuming that intelligent life is based on an oxygen rich atmosphere, because oxygen based biochemistry gives out more energy to the lifeforms using it.

If we use the same 'educated guesses' used by Drake and his colleagues in 1961

$R_∗$ = 1 yr−1 (1 star formed per year, on the average over the life of the galaxy; this was regarded as conservative)

$f_p$ = 0.2 to 0.5 (one fifth to one half of all stars formed will have planets)

$n_e$ = 1 to 5 (stars with planets will have between 1 and 5 planets capable of developing life)

$f_l$ = 1 (100% of these planets will develop life)

$f_i$ = 1 (100% of which will develop intelligent life)

We get the number of planets with an oxygen rich atmosphere in a galaxy.

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    $\begingroup$ Not quite... just having photosynthetic lifeforms producing oxygen doesn't mean that any given planet at a particular point in its history will have enough oxygen, or a sufficiently low concentration of carbon dioxide, for example. $\endgroup$ Commented Jun 16, 2021 at 22:07

The real question should be: how likely is it that a space ship will crash on a habitable planet?

Given that humans are mostly interested in habitable planets, it is save to assume that spaceships will either be very far away from any planet, or near a habitable one.

So if a ship crashes on a planet, the likelyhood that it is habitable will be close to 1.


Here is Carl Sagan on the matter.


The Abundance of Life-Bearing Planets

(This originally appeared in The Bioastronomy News, vol. 7, no. 4, 1995.)

By Carl Sagan

Editor's note: This is Carl Sagan's response to "A Critique of the Search for Extraterrestrial Intelligence" by Ernst Mayr, which appeared in The Bioastronomy News, vol. 7, no. 3, 1995.

Once you have found another planet of Earth-like mass, however, it of course does not follow that it is an Earth- like world. Consider Venus...

The best current estimates of the number and spacing of Earth-mass planets in newly forming planetary systems (as George Wetherill reported at the first international conference on circumstellar habitable zones [Doyle, 1995]) combined with the best current estimates of the long-term stability of oceans on a variety of planets (as James Kasting reported at that same meeting [Doyle, 1995]) suggest one to two blue worlds around every Sun-like star...

Nevertheless, the bulk of the current evidence suggests a vast number of planets distributed through the Milky Way with abundant liquid water stable over lifetimes of billions of years. Some will be suitable for life--our kind of carbon and water life--for billions of years less than Earth, some for billions of years more. And, of course, the Milky Way is one of an enormous number, perhaps a hundred billion, other galaxies.

Yet, shortly thereafter--Mayr adopts the number 3.8 billion years ago--some early organisms arose (according to the fossil evidence). Presumably the origin of life had to have occupied some time before that. As soon as conditions were favorable, life began amazingly fast on our planet. I have used this fact (Sagan, 1974) to argue that the origin of life must be a highly probable circumstance; as soon as conditions permit, up it pops!

In the article I excerpted that from there was not an estimate on how many sunlike stars there are. So from a different and more recent source:

From https://www.technologyreview.com/2020/11/06/1011784/half-milky-way-sun-like-stars-home-earth-like-planets-kepler-gaia-habitable-life/

And what better way to look for Earth 2.0 than to search around stars similar to the sun? A new analysis of exoplanet data collected by NASA’s Kepler space telescope, which operated from 2009 to 2018, has come up with some new predictions for how many stars in the Milky Way galaxy that are comparable to the sun in temperature and age are likely to be orbited by a rocky, potentially habitable planet like Earth. When applied to current estimates of 4.1 billion sun-like stars in the galaxy, their model suggests there are at minimum 300 million with at least one habitable planet.

The model’s average, however, posits that one in two sun-like stars could have a habitable planet, causing that figure to swell to over 2 billion. Even less conservative predictions suggest it could be over 3.6 billion.

For reference, the Milky Way has 100 billion stars. Dang! That is just one galaxy. In the Universe as a whole there are estimated to be a billion trillion stars. This is our actual universe.

Now numbers. We are using the Sagans liberal "up it pops!" guess for chance of life forming on an Earthlike planet, his rationale being it did not take very long on Earth for life to start after things cooled off. That means 100%. We will pair his liberal guess with the conservative 300 million stars in the Milky Way which are sunlike (0.3% of total stars) and assume each has at least 1 planet suitable for Earthlike life at any given time.

If I understand Sagan right he thought that each of those earthlike planets (as regards liquid water) was likely to have developed life. That is 300 million planets in our galaxy alone and 0.003 x 1 billion trillion (3 million trillion) in the universe.

I am assuming life is necessary for a breathable atmosphere because something like photosynthesis has to kick out the oxygen we crave. Abiotic photodissociation will not make enough in any environment that we can stroll around in. But not all planets with life will have a breatheable atmosphere. Earth had lots of life for a billion years but we would not have been able to breathe - that went on up until photosynthesis evolved and the Great Oxygenation Event happened. Mars might have been breathable once but no more.

Now it is speculation. Assuming each Earthlike planet has life, of the 3 million trillion there are what is the chance of an atmosphere we can breathe. 10%? 0.01% I feel like it gets a little silly, throwing around orders of magnitude with numbers like this. What is the practical difference between 10% and 0.01% of a million trillion?

Suffice it to say - there are a lot of planets with breathable atmospheres in the universe.


A few percent

According to the most recent paper I found on abiotic oxygen, desert planets accumulate O2 because XUV splits the scant water available, while on ocean worlds plate tectonics is shut down and after a long delay hydrogen is lost. Their model is speculative, and most of the runs don't yield oxygen rich enough to breathe. Nonetheless, it is some significant fraction of the overall habitable-zone worlds. (The paper's first greenhouse scenario doesn't apply here - it's too hot)


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