What to look for when searching for life on other planets

How did we get to the surface of this planet? Are we the product of a cosmic chemical catastrophe, or something brought us here? Will we be here forever?

For centuries, human thought has been interested in resolving these questions, understanding the universe, the origins of life on Earth, and whether there is more life in the cosmos. Hundreds of telescopes have been built around the world (and more sent into space!) to be able to observe more detail in the universe. In order to understand what the ideal conditions are for advanced life, we must first study the conditions on our planet and then look for similar patterns in our solar system. Little by little, today’s technology allows us to discover more details that were previously inaccessible.

Our planet is about 7,930 miles in diameter; it is a rocky world with oceans, clouds, an atmosphere composed of gases such as nitrogen, carbon dioxide, oxygen, methane, and a surface composition of heavy elements, such as silicon and the iron. The temperature of the Earth allows the existence of liquid water on its surface. Inside contains a core: the very center of our planet is a hard sphere of iron and nickel. Above the core there is a lighter layer, called the mantle, made up of silicon and oxygen, and then an even lighter layer, called the crust, containing granite and basalt. Finally, we find the outermost layer of cold and rigid rock, called the lithosphere.

The Earth has a natural satellite, the moon, which protects us from the impacts of small rocks like asteroids and comets. The internal heat of our planet, coming mainly from the radioactive decay of heavy elements, fuels geological activity. The Earth also produces a magnetic field that protects us from dangerous and highly energetic charged particles that emanate from the Sun. The interaction of these particles with the Earth’s magnetic field creates the magnificent auroras that periodically grace the winter skies of earthlings north and south.

After the formation of the Earth about 4.5 billion years ago, life appeared very quickly, about 3.85 billion years ago, essentially just after the planet had cooled sufficiently. In other words, when life could arise, it did. The oldest fossils show that bacteria-like organisms were present on Earth 3.5 billion years ago. Life evolves over time through natural selection and mutations. Cyanobacteria began releasing oxygen around 3.5 billion years ago, and all of this oxygen accumulated to become a major component of the Earth’s atmosphere around 2 billion years ago. 540 to 500 million years ago, a dramatic diversification of life occurred – an event scientists call the “Cambrian Explosion”. And about 225 to 65 million years ago, dinosaurs and small mammals populated the surface.

Studies suggest that the earliest life on Earth may have resembled present-day bacteria found near deep ocean volcanic vents and geothermal hot springs. We recently discovered that some microbes can survive for years in space; in fact, dozens of microbial species may have accompanied the Curiosity rover to Mars, where it landed in August 2012. Perhaps we have already sent the seeds of new life there.

The necessities of life as we currently understand them are nutrients, energy and liquid water. Are they easily found elsewhere, or only on Earth? When comparing the different celestial bodies in our solar system, there is an important point to consider: planets close to the sun are too hot for liquid water, and planets far from the sun are too cold for liquid water. The ideal planet for advanced life should exist in the “habitable zone” – neither too far nor too close to the sun. Yet other energy sources can keep liquid water under an ice cap, even on a freezing moon on a distant planet.

Research on our solar system has found a few candidates for hosting life: Saturn’s moon Enceladus, Saturn’s moon Titan, Mars and Jupiter’s moon Europa. Can we find life, or even ancient fossils, like those found on our planet?

We don’t know all the answers to our questions, but if human nature has made us explorers, and we keep that instinct alive, we can expect some fascinating planet-hopping in the near future. NASA is sending the Europa Clipper later this decade, the first spacecraft in a fleet for full exploration of Europa. We can only hope that this moment in human history can happen in our lifetime, when life elsewhere in the solar system is finally discovered.

lhuiyolitzin Villicana Pedraza, PhD., is a molecular spectroscopy specialist and member of the astronomy and physics faculty at Doña Ana Community College and New Mexico State University.

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