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We are living in an exciting time for the human race; since 1995, astronomers have been hunting planets using spectrometers and other devices to sniff out alien worlds.  So far, we’ve mostly found planets that look like Jupiter and orbit the Sun like Mercury.  But as the instruments available to us become more sophisticated, we can zero in on much smaller planets orbiting much farther distances from its parent star, with the ultimate goal being to find a planet like Earth that could support life.  Alarmists point out that the Earth is running out of resources and that we need to search for another planet to sustain us, but there are other reasons to search for life beyond our world.  The philosophical, spiritual and scientific ramifications of finding alien life would be as profound for humanity as the first voyage across an ocean might have been for humans centuries ago.  Our galaxy is like an enormous ocean, each solar system a island, and every world habitable with life an oasis.  We have only explored a fraction of our galaxy from the telescopes on Earth or in orbit just above us.  Our search for life outside of Earth is just beginning, and even if we don’t find anything out there, by taking that first step into the abyss, we open our species to the possibility of so much more to discover along the way.

If we’re looking for a planet that support life, we should first take note of our planet.  What are some telltale signs that there is life on Earth?  For one thing, Earth contains a tremendous amount of liquid water that girdles 70% of the planet’s surface.  Liquid water is essential for life and helps to moderate daytime and nighttime temperatures on the planet.  Most importantly, perhaps, is the fact that when water freezes into ice, it’s less dense than in its liquid form.  This means that instead of freezing solid and sinking to the bottom of the ocean, ice floats to the top.  If the Earth’s climate ever became much more chilly, (which it has in the past), it would take much longer to thaw out the oceans if they ever froze over.  Moons like Enceladus and Europa might be caked over with ice, but underneath that frigid layer is an ocean of liquid water, kept warm by the friction from Jupiter’s gravity.  Water is unique among the elements in that its solid form is less dense than its liquid, only elements like bismuth can achieve this under standard temperature and pressure.  But unlike bismuth, water has other unique properties.  It has a high surface tension that allows bugs to walk across the surface of bodies of water or rocks to be skipped across it.  Water is also polar, meaning that it can dissolve a wide arrange of molecules; this is important for cells because without it, organisms wouldn’t be able to the proper salt balance.

What would our planet look like to a hypothetical alien observer? Certainly we humans leave an indelible imprint on the surface of our planet.  At night, one can see the light from our cities radiating out into space.  Not just the visible light from streetlights and headlights, but radio signals, too.  Our cities radiate infrared light, too. Tar, asphalt, cement and concrete tend to absorb heat very well during the day; and reflect that heat back during the night.  This occurs so much so that we can measure the heat island effect from our cities.  Airports also tend to have a large carbon footprint; higher than average temperatures are found in neighborhoods surrounding airports, reportedly because of the amount of fuel being burned to get planes airborne, And, of course, our modern society is dependent upon the burning of fossil fuels in every sector of the world economy.  The combined power of every country’s carbon dioxide emissions to absorb infrared heat is slowly broiling the planet.  Human development has radically altered the flow of energy on our planet.  According to a recent study, the amount of photosynthesis going on at any given time attributable to human agriculture may only represent 3% of all the plant productivity on Earth, but that doesn’t adequately sum up our influence over the environment.  Because not only does our agriculture account for 3% plant productivity, but our economy that harvests, processes, and delivers that food also requires a tremendous amount of energy to maintain.  So much so that it is estimate that human industry accounts for 30% of the plant productivity on the planet.  When the authors of that study accounted for the cost in productivity that comes with degraded ecosystems, we humans actually take up about 37-40% of all plant productivity on the planet. And this can clearly be seen from outer space.  Plants absorb red and yellow light and tend to reflect green light at highly specific wavelengths so satellites can pick up on the presence of vegetation based on observations.

But there are other ways that our presence on Earth could be inferred.  Our atmosphere shows many signs of chemical disequilibrium.  The ozone layer in our atmosphere is a clear signal that there is life on Earth; only photosynthesis could be responsible for that level of oxygen in the atmosphere.  The presence of methane in our atmosphere is another sign of life; methane is incredibly unstable and tends to get converted into carbon dioxide in the presence of oxygen.  So in order to be detectable in our atmosphere at any appreciable levels, there needs to some process to replenish it. There is a geological process called serpentinization that can create methane when boiling water comes in contact with organic minerals in rocks, but bacteria called methanogens are more likely to be the cause.  Nitrogen can be another indicator of life.  Nitrogen naturally tends to bond with itself to form nitrogen gas; it makes up about 70% percent of our atmosphere.  These bonds are so strong that it takes a lot of energy to tear them apart.  Usually, nothing short of a bolt of lightning in the atmosphere would suffice in tearing apart the atoms in a molecule of nitrogen.  When scientists detect nitrogen in any other form, it’s a good sign that life has been here.  Microbes recycle nitrogen into several different forms before returning it to the air as nitrogen gas.  Human industry also tends to emit nitrous oxides; these chemicals can be found as accidental by-products in car exhaust and coal-fired power plants.  But most nitrogen is deliberately created to sustain our agriculture; and most of that supply ends up in our lakes and rivers as a pollutant.

The search for alien life seems to straddle the line between fiction and reality, but advancements in our technology pushes us further and further into a realm that was once fantasy.  There is much to explore in the next few years and as our technology catches up to our imagination, the discoveries are unimaginable.  Happy hunting.