One question rises above all others when it comes to our place in a vast and ancient Universe, 'Are we alone?' With a billion habitable locations in the Milky Way galaxy, and more than ten billion years for biological experiments to play out, a search for intelligent life beyond Earth is well-motivated. Unfortunately, the single example of life on Earth gives no clear indication of whether intelligence is an inevitable or an extremely rare consequence of biological evolution. The search for...

Life on Earth is amazing and multifaceted. Ultimately all of life has descended from one common ancestor and has been guided by evolution by natural selection. On the one hand, the evolution of modern-day diversity and ecosystems may have been contingent on the initial chemical building blocks of life and the historical events that have characterized our planet over geologic time. On the other hand, there are numerous aspects of life pointing to regular and deterministic processes that shape...

One of the most fascinating developments in the last two decades is humankind's discovery of alien worlds orbiting stars near our Sun. Since the first such discovery in 1995 there has been a truly exponential growth in the detection of these new planets. Scientists have been puzzled and surprised by the diversity and extravagance of these new extra-solar systems. For example, we now know the most common type of planet is actually missing from our own Solar System. Recently, the space-based...

When Renaissance scholars figured out that the planets are, like Earth, orbiting the Sun, an immediate assumption was that they are inhabited worlds. In the last 50 years, spacecraft have determined that life on the surfaces of planets and moons in the Solar System is rare – if it exists at all. However, there are places where a search for life in the Solar System may still be fruitful. Although the current surface of Mars is a hostile environment, early Mars may have been much more clement...

Life is particularly fascinating in its ability to create complex and ever-changing forms out of simple building blocks. How does such complexity arise, and what are the conditions that allow never-ending evolution of new and more intricate forms of life? We now know that one of the main processes that allows this is that life consists of modules that interact with and feed back on one another. In the history of life on Earth, new levels of complexity have often arisen out of new types of...

It is generally accepted that planets or their satellites are required for life to originate and evolve. Thus, in order to understand the possible distribution of life in the Universe it is important to study planet formation and evolution. These processes are recorded in the chemistry and mineralogy of asteroids and comets, and the geology of ancient planetary surfaces in our Solar System. Evidence can also be seen in the many examples of ongoing planet formation in nearby regions of our...

Throughout history, our definition of 'life' reflects our assumptions about how the Universe works – and why we ask the question. The ways different human cultures, ancient and current, have talked about life provide some sense of how we have defined life, and illustrate the aspects of life that fascinate us. Many cultures used life as an analog to explain the movement of winds and currents, or the motions of the planets. Today we use those mechanical systems as analogs for life. Ultimately,...