Where to Find Alien Life

One of the reasons life survives on Earth is that we have a big brother protecting us from sterilizing events (asteroid bombardment).   Jupiter has been a powerful sink for asteroids that would have killed us all.  To find alien life in another system we should focus on those systems which have a large planet well outside the habitable zone as seen here. Exoplanets.org has a list of known exoplanets, and it is sortable by the semi-major axis.  Click on that column to sort the exoplanets by their distance from the primary.

Jupiter is about 5 astronomical units (AU) from our G-type star.  Here are the first 4 likely candidates.

590 light years away near Pavo is HD 190984 , an F8 star with a big planet at 6 AU.

40 ly away is 55 Cancer, a G8 star with a big one at 5.9 AU.

At 85 ly in Libra, HD 134987 is a G8 star with a planet 0.8 Jupiter masses at 5.8 AU.

49ly away mu-Ara is a G3 star with a planet twice the mass of Jupiter at 5.3 AU.

Statistically speaking these are outstanding targets for hosting alien life.  A more detailed analysis may preclude one or more of these due to orbital eccentricity, other planets, or binary star systems.  But if spontaneous generation of life is common then at least one of these systems hosts alien life.

Too cold for life

Speculation about life on Titan or other cold worlds is misguided because the chemical reaction rates are simply too slow. All chemical reactions have a rate.  That rate has an exponential dependence on temperature as described by the Arrhenius equation.

rate = A exp (-Ea/kT)

Ea is the activation energy, k is Boltzmann's constant, T is the temperature, A is the pre-exponential factor

Chemical activation energies are best measured in electron-volts or eV.  Typical values are between 1 and 3 eV, usually closer to 2.0.  The ground temperature on Titan is around 94 oK or -179 oC.

Thus a chemical reaction on Titan would be roughly 73 orders of magnitude slower than the same reaction on Earth (298 oK).  So a reaction which takes a microsecond on Earth would require 10^60 years on Titan. (That's 1000000000000000000000000000000000000000000000000000000000000 years).

Pre-exponential factors for chemical reactions have a certain value which represents steric  effects.  They cannot be 60 orders of magnitude larger because molecular geometry is not that pliable.

Traditional chemical reaction based life is impossible on Titan or any other location where the temperature is so far below 298 oK.

Where are the aliens?

"Peering far beyond our solar system, NASA researchers have detected the basic chemistry for life in a second hot gas planet, advancing astronomers toward the goal of being able to characterize planets where life could exist. The planet is not habitable but it has the same chemistry that, if found around a rocky planet in the future, could indicate the presence of life."

If organic molecules are so prevalent where are all the intelligent space-faring aliens ?  The lack of evidence for aliens is strong evidence that we are alone, the random evolution of intelligent life is so rare that we are the only ones in the entire galaxy.  Note that this is not the same as saying "the random evolution of life is rare".  There probably are bacteria in most solar systems.

Has Optical SETI discovered aliens?

Optical SETI offers a much higher chance of finding a real signal because of the simple fact that there are no known natural sources of nanosecond pulses (or shorter).  Any 10 nanosecond pulse of light can only be from intelligent life (or of course a shocking new natural phenomenon). Recently a researcher in Australia has detected a pulse.  Unfortunately the details of the signal are not available.  Thanks to Sentient Developments for pointing out this article.

The details of optical SETI are a lot more complex than radio based SETI.  Radio SETI is fairly clear, just listen for a signal which shouldn't be there.   But optical SETI requires some understanding of light pulses.  For example, every day our own sun puts out flares, which would look like pulses from a few light years away.  Supernovae, gamma-ray bursters, novae, and variable stars all put out more light at some point than normal.  So what makes an optical pulse natural versus synthetic?  The answer is time.

There are no natural phenomena which generate nanosecond light pulses**.   A nanosecond pulse means that a burst of photons is detected, and all the photons arrive within a few nanoseconds of each other, then no more arrive until the next burst.   In general this means the light was generated with a laser.  Lasers are a little easier to understand, all the photons are the same, same color (or wavelength), and same phase.  There are continuous lasers which have no pulses, they emit a steady stream of photons.

From a few light years away it is plausible to measure the photon color and determine that a light source is a laser, but unlikely.  For example, amateur astronmers know that a planetary nebula emits OIII lines at 501nm.  All these photons are the same color but they are absolutely not from a laser.  With extremely precise spectrometry it might be possible to determine that a certain set of photons came from a laser.  But there would be a lot of arguing.

However, if the laser is pulsed, and the pulse width is less than 100 nanoseconds, then this signal came from a laser built by an intelligent species.  It's not even necessary to measure the color of the photons.  If they are nanosecond pulses then we are not alone.  All that is needed is that the pulse be bright compared with the background.  For example, point an 8" telescope at a magnitude 6 star.  The telescope collects some number of photons/second, let's say it's a million.  So on average about once every microsecond a photon hits the detector.  Now suddenly a pulse of 5 photons arrives, all within 10 nanoseconds.  Even if that pulse is not repeated it stands out like a beacon, 5 random photons never arrive on top of each other.  A 5 photon pulse means we are not alone.  This works until the average photon rate is around a 100 million per second, so really bright stars will drown out possible optical signals.

With a photomultiplier tube, boxcar integrator, and a decent oscilliscope we amateur astronomers could build our own optical SETI equipment and discover intelligent life.


**  There is of course always the chance that a shocking new natural phenomenon will be discovered which emits nanosecond pulses.  If you discover that you might have to settle for a Nobel instead of aliens.