What is Astrobiology?
published during a waning crescent moon.

Illustration of 55 Cancri e. Credit: Ron Miller/NASAblueshift

The recent observational detection of 55 Cancri e’s atmospheric composition was an incredible feat illustrating the precision technology now being used to probe the characteristics of exoplanets. These tools are a critical component of our search for biomarkers on habitable exoplanets, and could play a significant role in discovering life beyond Earth.

55 Cancri e is the so-called “Diamond Planet.” The planet is about six to seven times more massive than Earth and appears to be predominately made of carbon—which is how it got its name. Diamonds, after all, are made from carbon compressed to high pressure in the deep interior of Earth. On 55 Cancri e, we might have a situation where most of the mantle—maybe thousands of kilometers thick—would be crystalline carbon. The surface would more likely be dark amorphous carbon, and the deep interior might harbor heavier metals that comprise a more complex mixture than pure carbon. The pressures in the core are so high that diamond may flow on Cancri e as a liquid!

Such a planet is fun for speculation. Imagine volcanoes venting high-pressure liquid carbon from the deep interior that immediately cools and explosively crystallizes in the atmosphere, raining diamonds back down to the surface. Perhaps the carbon atmosphere glows green and yellow and interacts with its nearby star’s strong plasma wind to produce atmospheric fireworks thousands of times brighter than the aurora on Earth. This setting may seem fit for a science fiction story, but it is realistic for such an exotic planet!

But apart from speculation, just the fact of 55 Cancri e’s existence is stunning. Carbon is the backbone of Earth-based life and is an essential ingredient for life (as we know it). The other two key ingredients are liquid water and available energy. The existence of this planet means that carbon is ubiquitous in planetary systems.

We’ve already found many so–called “water worlds” that are also typically larger than the Earth. And we find water almost everywhere we look in our solar system: underground on Mars, subsurface oceans on Europa, Ganymede, Callisto – the moons of Jupiter, Enceladus – a small moon around Saturn, and perhaps other places as well. We have also found vast amounts of water in the interstellar medium where we observe stars and planets forming.

And of course, energy is readily available in various forms on and inside planets. Tidal dissipation may be the heat source that keeps the subsurface oceans of Jupiter’s moons liquid, and chemical energy is available for surface and subsurface life on Earth and probably inside other terrestrial planets as well. Furthermore, thermal (or geothermal) energy left over from a planet’s formation, in addition to the decay of radioactive isotopes, also provides important energy sources for life.

What’s important to note here is that our universe provides all three of the essential ingredients for life in unimaginable abundance. This recognition is one of the prime motivations behind the new science of astrobiology.

What is astrobiology?

Astronomy is generally considered the oldest of the sciences. It initially stemmed from people’s interest in the sky and the odd events that occasionally occurred there, such as comets. Astronomy was also an essential part of early survival, providing guidance on when to plant and harvest crops, when migratory animals were on the move, and how to navigate the seas. Like all sciences, it grew throughout the centuries as our understanding of the universe expanded.

The science of physics concerning matter, motion and energy turned out to be astronomy’s ideal cousin since many of the things observed in the sky were understandable through physics. The study of “astrophysics” became the first major historical specialization in the sciences to encompass the arena where these two sciences overlap. But overall, the science of astronomy kept its focus on distant things, whereas physics focused on understanding the fundamental nature of matter and energy.

With the advent of spectroscopy and the periodic table, the science of chemistry came to the forefront of 19th-century scientific investigations. After that, the microscope led the way to the newest of the traditional “hard” sciences—biology. The trend of specialization continued rapidly in the 20th century for many other sub-disciplines. For example, geology and chemistry became geochemistry, biology and physics became biophysics—and so on. The boundaries between new specializations have become rich domains of new discoveries, and this trend will probably continue into the foreseeable future. And this is a good thing. Specialization allows us to plumb the depths of how the stuff of the universe works.

However, in spite of how well the specialization of science has served us over the centuries, it has put blinders on our big-picture view of the universe. The universe is complex and patently not specialized. Nowhere in nature do we see physics separated from chemistry separated from biology. That’s not the way nature behaves. But nevertheless, that is the approach most of our educational system follows in teaching science. Unintentionally, this has had the extremely unfortunate consequence of encouraging “in the box” thinking when dealing with complex problems.

Enter the new(est) science of astrobiology. The name itself is interesting. The first half is from the oldest science (astronomy) while the last is from one of the modern sciences—biology. But more interestingly, this new science is bucking the trend of specialization – in fact, it does the opposite. It is perhaps the most interdisciplinary of all of the sciences and covers a larger domain than that of any other.

Such a name as “astrobiology” is almost too grand for a meaningful definition. If you Google a definition, you’ll find a variety of similar meanings, my favorite being “The interdisciplinary study of the origin, evolution, and distribution of life in the universe.”

The field of astrobiology combines the study of the formation of stars and planets, the origin and history of life on Earth, the importance and diversity of extremophiles, the habitability of the numerous exoplanets that we are finding around other stars (and between them as well!), and just about anything else that you can imagine that deals with life or the universe. It is a science that is simply too broad for us to get our minds around it all!

In some ways, humanity has now come full circle. Science began as an attempt to understand how the stars influenced people. The new science of astrobiology is an attempt to understand the full story of life in the universe.