Where are the aliens?

Enrico Fermi (Italian physicist 1901 - 1954) was at lunch with colleagues in 1950 discussing aliens when he asked "Where are they?". Leo Szilárd (Hungarian physicist 1898-1964) (jokingly) told Fermi ""they are already among us - but they call themselves Hungarians".

Fermi asked the question (it has since become known as the Fermi paradox) because if you assume our Sun is a typical star then there should be many intelligent races in the universe. After all there are some 200 - 400 billion stars, say 300 billion (3 x 10^11) stars, just in our galaxy the Milky Way. A significant number of these should have planets and/or moons suitable for life even if we limit this to mean life as we understand it. A proxy for identifying such planets is to find planets which could have water. Assuming:

• Single stars (to avoid complicated orbits in binary or multiple star systems). It is possible for a suitable stable planetary orbit to exist in a binary system, but since we have little data about planetary orbits in binary star systems assume such a binary system is rare compared to single star systems.
• Say 35% stars are single [SolStation.com: Stars and Habitable Planets]
• Not too close to the centre of the Milky Way (to avoid intense hard radiation from the black hole there and other sources)
• Not too far out from the centre of the Milky Way to be too poor in metals for rocky planets to form, or indeed life to exist (metals are "seeded" from earlier supernovae, and the density of supernovae events is assumed to be too low too far out)
• Say 10% of stars lie in this ring allowing life
• Of types F, G (i.e. sun-like) or K on the main sequence [SolStation.com: Stars and Habitable Planets]. The mnemonic to remember the main sequence stars OBAFGKMN is: Oh Be A Fine Girl Kiss Me Now, with stars decreasing in mass from O to N. Types O - A are probably too massive to have planets with intelligent life, as the more massive a star the hotter and quicker it burns and so we assume they have too little time for intelligence to evolve. On the other hand, although stars of types M - N will last very long times, because they have such low masses they will not be very hot and the habitable zone around these stars will be close to the star. This will probably result in any planet in the habitable zone being in tidal lock with the star (as Mercury is with the Sun), which would result in temperature extremes inimical to life.
• Of main sequence stars in the Sun's neighbourhood [Wiki: Stellar classification]:
• 3% are type F
• 7.7% are type G
• 12.5% are type K
• 76% are type M
• Thus types F - K make up 23.2% of main sequence stars in a suitable sample of space in the ring allowing life around our galaxy, and further assuming 90% stars are main sequence stars [ChView: The Stars of the Milky Way].

Then 0.35 x 0.1 x 0.232 x 0.9 = 7 x 10^-3 of all stars in the Milky Way are possible candidates, i.e. 2.1 x 10^9 stars could potentially have planets with water. This is a reasonably firm estimate, and agrees in order of magnitude with another way of estimating the possibility of stars with planets capable of having water (but similar assumptions): that 30 systems in the 5000 (i.e. 6 x 10^-3) within 100 light years of our Sun [SolStation.com: HR 8501] should be targeted by SETI as candidates for having intelligent life. This estimate needs to factor in that 10% of stars lie in the galactic ring allowing life, giving a lower estimate of (6 x 10^-3) x 0.1 x (3 x 10^11) = 1.8 x 10^8 candidate stars in our galaxy.

However, further analysis requires more creative assumptions. Many extra solar system planets have now been detected, and extrapolating somewhat we assume that for our target F - K type single stars the chance of having planets is close to 100%. Further, the chance of having one planet in the habitable zone is perhaps 50% (the Sun has one - Earth, but nearly had three), therefore an estimate for the number of planets in the right place around the right star in our galaxy is about a billion (~ 1 x 10^9). But beyond that we do not know how important the fine details of a suitable system are, e.g.

• What is the chance of a rocky planet being in the habitable zone rather than a gas giant?
• Do we need a planet like Jupiter to clean up asteroids?
• Do we need the Moon: for large ocean tides, helping to heat the interior of the Earth, or rotational or axial tilt stability, etc.?
• Are plate tectonics essential?
• etc., etc.

But even with a suitable planet, we need to understand the likelihood of intelligence emerging amongst the creatures of that planet, and then of a technologically advanced civilization appearing and not destroying itself too soon. The Drake equation is an attempt to quantify these possibilities, although originally it was presented more in the form of an agenda to focus thought [Wiki: Drake equation]. It has been said that intelligence is unlikely, however other species on Earth show intelligence, so I rather think it is eventually going to appear if multi-cellular life does form. But even given intelligent life, the chance of a technologically advanced civilization appearing is another unknown. There are too many unknowns here to give any sensible estimate, but given the number of possible star systems, the chances of a technologically advanced civilization appearing have to be remote for there to be just us, e.g. if it were only a 1 in 10 million chance for any planet in the habitable zone around single system F - K type stars nurturing a technologically advanced civilization, we would expect ~ 100 such civilizations appearing sometime in our galaxy alone. Once a technologically advanced civilization arises it is assumed it will take a very short time (< 50 million years) for that civilization, even using technology known to us, to colonize or explore the galaxy. In which case "where are they?".

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