Astrobiology is a relatively recent area of study that investigates the origin and evolution of life in the universe. It combines perspectives from many disciplines – biology, astronomy, chemistry, geology, and others. As we learn more about the omnipresence of water in our own Solar system, and about the existence of planets around other stars, there is growing interest in astrobiology. But what is it like working in this interdisciplinary scientific field? To learn more, we spoke with Jonti Horner, an astronomer and astrobiologist who works at the University of Southern Queensland’s Centre for Astrophysics.
Why did you decide to focus your work on astrobiology?
That’s a great question. When I started work as a researcher during my PhD, I was looking at the origins and evolution of one particular group of unstable Solar system small bodies known collectively as “The Centaurs”. I’d always been interested in comets, meteors, and asteroids ever since I was a kid – the Centaurs fascinated me. They are the parent population of our short-period comets. I wanted to know more about where they came from, how they moved, and how likely it was that a given Centaur would one day become a short-period comet.
Tied into all that was the idea that these objects could pose a threat to Earth – and in fact, we spend about a third of every year on Earth running through debris left behind when a giant Centaur was trapped in the inner Solar system. That Centaur gradually fell apart at some point in the last hundred thousand years. What’s left behind of that event is known as the “Taurid stream” – a broad swathe of debris, full of dust, small asteroids, and a couple of comets (most famously Comet Encke). Every year, when Earth passes through the Taurid stream, we see spectacular fireballs. Some of the bigger objects in the stream could pack a real wallop if they hit Earth. In fact, people think that a large Taurid, maybe 100 meters across, hit Earth back in June 1908, causing the largest impact on Earth in the last thousand years – the Tunguska event.
So, in my earliest work, I was already touching on astrobiology, although I wouldn’t at the time have realized that was what I was doing. I was running simulations based on the 23 Centaurs we knew at the time, working out how likely it was that they would eventually be flung onto Earth-crossing orbits. I found that, on average, a given Centaur has maybe a one-in-three chance of being launched into the inner Solar system by the giant planet Jupiter – this is how the short-period comet population remains populated. Without fresh comets being created in that manner, the number of short-period comets would fall away to nothing in just a few hundred thousand years. The comets would disappear because they would fall apart, collide with the planets, or be flung out of the Solar system, never to return.
Following on from that work, when I started my second postdoctoral research position, at the Open University, I started working with Professor Barrie Jones, who became a very dear friend and mentor to me. There is a prevailing myth in astronomy – the idea that the Earth is shielded from impacts by the giant planet Jupiter. According to this myth, if Jupiter were not there, Earth would be pummeled so frequently by impacts that life could not survive. Both Barrie and I had a feeling that this wasn’t quite the whole story and that this really seemed like an oversimplification.
There’s a great example of why we thought that in the historical record – an object called Comet Lexell. Comet Lexell flew very close to the Earth in 1770. It was the closest cometary approach to our planet in recorded history. It was spectacular – as bright as the brightest stars, and visible with the unaided eye even from the center of big cities. When astronomers studied its orbit, they realized it went around the Sun with a period of about six years. Being so bright, people immediately wondered why no one had seen the comet before. The answer was that, just three years earlier, in 1767, Jupiter’s gravity had flung the comet into the inner Solar system for the very first time, capturing it from an orbit that brought it nowhere near our planet, and flinging it into harm’s way.
So, why don’t we see Comet Lexell anymore? Well, its orbital period was pretty much half that of Jupiter’s, so twelve years after that first encounter, it came close to the giant planet again, and Jupiter flung it outward, most likely throwing it so fast that it will never return.
What this all demonstrates is that Jupiter can indeed be our friend – it can take things that threaten Earth and throw them away, never to return. But it can also be our foe – it can take things that come nowhere near the Earth, and throw them at our planet.
That was where my first true astrobiology research project began. I wanted to figure out whether Jupiter was really more of a friend, or actually more of a foe. Does it truly shield Earth from more impacts than it causes? That work occupied me for a few years, working closely with Barrie. We found that, as with most things in science, it’s complicated. If Jupiter wasn’t there, the Earth would be hit less frequently that it is in our Solar system. So Jupiter actually causes a net increase in the frequency with which objects hit Earth. But, if Jupiter were instead the mass of Saturn, the Earth would be hit far more frequently.
It’s all a bit weird, but to make it even weirder, there is an argument that we need at least some impacts to happen on Earth to encourage life to diversify and thrive. Without the impacts on Earth of icy objects flung inward by Jupiter in the Solar system’s youth, our planet might well have been too dry for life. And equally, it is likely that Jupiter helped direct the asteroid that killed the dinosaurs our way – and if that event hadn’t happened, we might not be here having this conversation.
So, I’d say I’ve always been interested in astrobiology, but I never made a hard conscious decision that “I will become an astrobiologist”. It was more a case of my following interesting research questions. What if this? What would happen if…? Where do these objects come from? And those questions brought me firmly into astrobiology.
It’s been helped, too, by the fact I’ve gotten involved in the search for planets around other stars – and one of the things I’ve spent a lot of time thinking about is how we will choose the best places to look for life beyond the Solar system. What is there we can do, with our current knowledge and tools, to lay the groundwork for that process? We’ll find lots of potential places to look for life before we’re able to survey them. And when we can look for evidence of life upon them, the observations will be the most challenging ever attempted. This all means we will need to focus on the few best targets. I’m really interested in finding out how we can ensure we pick the very best targets. What is there that will make one planet more tempting as a target for our search than another?
What do you think some of the most exciting areas are in the field of astrobiology?
There’s so much going on right now. It is a really interesting time. In the Solar system, we’re realizing that liquid water is everywhere – that sub-surface oceans are abundant in the outer Solar system – and so there are suddenly a wide variety of places we could look for life in the coming decades, here in our own backyard. I still think Mars is the best first place to look, though. The discovery of lakes of permanent liquid water in Mars’s south polar cap last year was really exciting in that light. I suspect people all around the world are now trying to work out how we could send a mission to the south pole, drill through the ice, and see whether we can search for life in those deeply buried lakes!
Beyond the Solar system, I’m really excited about the growing wealth of knowledge we have about planets around other stars. We haven’t found “Earth 2.0” yet. And despite what people would have you think, I suspect it will be some time before we do. But I’m really excited about the journey to that discovery. We’re learning more and more about how planets form and how diverse planetary systems are. In turn, we are learning ever more about our own place in the cosmos.
We’re still some way from finding out just how typical or atypical the Solar system is, but what we have learned is that planets are ubiquitous – they’re everywhere. We’ve also learned that planetary systems are far more diverse than we could possibly have imagined. In the coming years, we’re going to find thousands of new planets. We will gradually be getting closer and closer to finding planets that are really like Earth. That’s the first step along the road to finding out whether any of those planets could (or do) host life. I’m really excited about that journey – from finding alien worlds, to characterizing them, and then beginning to search for evidence of life upon them.
What would you suggest to someone who wants to specialize in astrobiology?
It depends where someone is in their career or studies. If they are at high school, I advise them to think carefully about what they want to study at university. There are many routes into astrobiology – it is a truly multidisciplinary science. Astrobiology conferences are amazing because they’re so diverse in terms of the research topics and backgrounds of the people there. You have biologists talking to geologists and chemists working with astronomers. All of the sciences have something to say and something to contribute to astrobiological research.
Obviously, my own work is very much in the “astro” part of astrobiology. But I know people who studied earth sciences and now research the possibilities for plate tectonics on alien worlds. I know others who studied biology and now work looking into the extreme conditions in which bacteria can survive and thrive. It’s such a broad field. Whatever your particular scientific interests are, it’s certain that you could get involved in astrobiology.
For people looking at starting research in the field, I’d just encourage them to keep asking questions, and trying to find out the answers – that’s the core of research, after all. I’ve found the best and most fun questions often fall through the cracks between disciplines. There’s always more to learn. The other thing is to read widely. One of the problems we often run up against in science is how siloed things can become. People in one field don’t talk to people in another. And I’m not talking about astronomers and biologists not talking, here. It can be one sub-field of a discipline not talking to another – stellar astronomers not talking to people who work on solar system objects, for example. It isn’t deliberate. It’s just the result of the fact that people go to conferences that are specialized in their field and they read papers in their field – that’s their daily bread and butter – and so they lose touch with other fields. That’s sad, because there’s so much fun work to be done where different fields intersect. This is particularly true in astrobiology!
If you want to understand the likelihood of life elsewhere, and where to look for that life, you need to understand everything from how galaxies work through to how bacteria cope with high levels of radiation. That’s more than one person can do. You need to collaborate, and collaborate outside your own field. That can be quite daunting, but it’s also really good fun. There’s always so much more to learn!