If there's one thing the Kepler mission has taught us, it's that planets are everywhere. Lately, it seems like if we're not discovering new exoplanets, we're finding evidence that there are hundreds of billions of them out there (many of them probably even Earthlike), and we just haven't found them yet.
On one hand, being up to our necks in planets is incredibly exciting business. It's entirely possible, for example, that we've already discovered Earth 2.0 and just don't know it yet. At the same time, this overabundance of exoplanets has raised some unsettling questions, and at the root of many of those questions is one big, glaring conundum: where the hell is the alien life?
In today's online edition of New Scientist, Nick Lane takes an in-depth look at how the Kepler team's discoveries are bringing this age-old paradox back into focus. It's a lengthy read, and you'll have to register with New Scientist to read it (registration is free), but it's a well-presented analysis of exobiological ideas, and definitely worth making time for.
Of course, the universe is a very big place. Even Frank Drake's famously optimistic "equation" for life's probability suggests that we will be lucky to stumble across intelligent aliens: they may be out there, but we'll never know it. That answer satisfies no one, however.
There are deeper explanations. Perhaps alien civilisations appear and disappear in a galactic blink of an eye, destroying themselves long before they become capable of colonising new planets. Or maybe life very rarely gets started even when conditions are perfect.
If we cannot answer these kinds of questions by looking out, might it be possible to get some clues by looking in? Life arose only once on Earth, and if a sample of one were all we had to go on, no grand conclusions could be drawn. But there is more than that. Looking at a vital ingredient for life - energy - suggests that simple life is common throughout the universe, but it does not inevitably evolve into more complex forms such as animals. I might be wrong, but if I'm right, the immense delay between life first appearing on Earth and the emergence of complex life points to another, very different explanation for why we have yet to discover aliens.
Living things consume an extraordinary amount of energy, just to go on living. The food we eat gets turned into the fuel that powers all living cells, called ATP. This fuel is continually recycled: over the course of a day, humans each churn through 70 to 100 kilograms of the stuff. This huge quantity of fuel is made by enzymes, biological catalysts fine-tuned over aeons to extract every last joule of usable energy from reactions.
The enzymes that powered the first life cannot have been as efficient, and the first cells must have needed a lot more energy to grow and divide - probably thousands or millions of times as much energy as modern cells. The same must be true throughout the universe.
This phenomenal energy requirement is often left out of considerations of life's origin. What could the primordial energy source have been here on Earth? Old ideas of lightning or ultraviolet radiation just don't pass muster. Aside from the fact that no living cells obtain their energy this way, there is nothing to focus the energy in one place. The first life could not go looking for energy, so it must have arisen where energy was plentiful.
Today, most life ultimately gets its energy from the sun, but photosynthesis is complex and probably didn't power the first life. So what did? Reconstructing the history of life by comparing the genomes of simple cells is fraught with problems. Nevertheless, such studies all point in the same direction. The earliest cells seem to have gained their energy and carbon from the gases hydrogen and carbon dioxide. The reaction of H2 with CO2 produces organic molecules directly, and releases energy. That is important, because it is not enough to form simple molecules: it takes buckets of energy to join them up into the long chains that are the building blocks of life.
A second clue to how the first life got its energy comes from the energy-harvesting mechanism found in all known life forms. This mechanism was so unexpected that there were two decades of heated altercations after it was proposed by British biochemist Peter Mitchell in 1961.
Read more at New Scientist. For more on the implications of an uncannilly quiet galaxy, see our recent feature: Does a galaxy filled with habitable planets mean humanity is doomed?