The search for planets beyond Earth's solar system has revealed countless surprises, including the existence of thousands of strange and unexpected worlds unimaginable only a few decades ago.
With the help of powerful ground- and space-based observatories, astronomers have confirmed the existence of more than 3,000 exoplanets, and counting. Many of these new worlds have been found in unexpected places, such as in orbit around a pulsar — a type of star that forms following a massive supernova explosion — or roaming the universe, untethered to stars and therefore devoid of an energy source. Also, scientists have found "Goldilocks" planets, which orbit at a distance from their stars where liquid water can exist on the planetary surface. Researchers have even found evidence of water — the basis for life as we know it on Earth — on some moons of Jupiter, Saturn and Neptune, and beneath Pluto's icy surface.
A new book, "Exoplanets: Diamond Worlds, Super Earths, Pulsar Planets, and the New Search for Life Beyond Our Solar System" (Smithsonian Books, 2017), released March 14, narrates the history of exoplanet research and illustrates the many different types of planets that have been discovered to date. [The Strangest Alien Planets (Gallery)]
Space.com spoke with "Exoplanets" authors Michael Summers, a planetary scientist at George Mason University in Virginia and a co-investigator on NASA's New Horizons mission to Pluto, and James Trefil, a physics professor at George Mason University, about finding exoplanets in the universe and the implications these worlds have for the existence of life and intelligence beyond Earth.
Space.com: Your book discusses "the curse of the single example" — the natural assumption that if you have only one example of something, such as life on Earth, then everything else you find will resemble the one thing you know about. Do you think life as we know it exists beyond Earth, or in a different, non-carbon-based form?
Michael Summers: Well, we really don't know. Given that we are seeing so many planets that are Earth-like, and that so many of them have water, as well as the kind of raw materials and usable energy that we've learned is needed for life on Earth, the possibility of finding life that is carbon-based like us seems to be really huge.
However, just because these requirements are met, doesn't mean that we automatically get life. There could be life-forms that are different than us and have different requirements, such as methane instead of water. Given that we have found such a diverse range of planets, it stands to reason that you could have complex things arise that are much, much different than life on Earth. For example, you could have complex things on a superconducting planet that might be made not of molecules, but of electric and magnetic fields. I expect we'll find a lot more complexity in the universe, including life that could be vastly smarter than humans, but might not meet all of our current criteria for life. So we'll have to come up with new criteria for what constitutes life.
James Trefil: The most common form of chemical life that isn't like us in science fiction is life based on silicon, which has similar chemical properties to carbon. That's the best bet for chemically based life. What nonchemical life might be like is anyone's guess.
Space.com: You mention in the book that if another form of life was found in our universe, it would likely be much more advanced than humans on Earth. Why is that?
Summers: Our galaxy is approximately 13 billion years old, and our sun has only been around for about 4.6 billion years, roughly a third of that time. So there are potentially Earth-like planets that are billions of years older than our solar system, which suggests there is the possibility that life has been out there for a much longer time than we have been on Earth and has had plenty of time to develop advanced, superior intelligences. There is also the extreme possibility that our civilization on Earth is it — we're the only life in the universe. But in my opinion, that would be even more astonishing than finding life everywhere, when there are just so many Earth-like habitable planets in the universe. [How Do You Spot an Alien Planet from Earth? (Infographic)]
Space.com: In your book, you also discuss the Fermi paradox — that even though astronomical calculations suggest life should exist beyond Earth, we have yet to find evidence for the existence of extraterrestrials in our galaxy. So, where is everybody?
Summers: That's a very poignant question because we are only recently beginning to see places in the universe where life could exist. And now that we are able to see other planets, if we don't find life pretty soon, that's going to tell us that the existence of life in the galaxy may not be very common at all. But the reality is that we have only been looking for exoplanets and life elsewhere for a short amount of time, and only in a tiny fraction of the universe. So I think the answer to the Fermi paradox is that we just haven't looked far enough and long enough. We'll find life elsewhere. We just have to do a little more work.
Trefil: Good question. Either we're way off the mark on the abundance of life or the Great Filter is in front of us, and every advanced life-form that discovered science wiped itself out — scary!
Space.com: You're referring to the Great Filter theory, which suggests that the reason we may not find life on other worlds is that some hurdle causes most evolving life to go extinct — either a stage of evolution humans have already passed, explaining why there aren't others out there, or one that still lies ahead. Can you elaborate on that?
Summers: In my opinion, anything that could act as a filter and destroy life almost always ends up opening new opportunities for the development of other life. For example, the asteroid that hit Earth 64 million years ago wiped out the dinosaurs, but from the perspective of the evolution of complex life, that event was not entirely negative, since it allowed for the evolution of mammals, including humans. It's also possible that civilizations never get much more advanced than ourselves because of the tendency to self-destruct. We keep developing new ways of destroying our own civilization, such as nuclear war, which could essentially be a Great Filter in our future. If you think about the far future, another filter would be the sun becoming a red giant, which would make our solar system uninhabitable.
I think that what [the Great Filter theory] tells us is that there is an urgency for us to move our civilization out in to the universe — establish colonies on Mars and elsewhere so that if civilizations on Earth were harmed by an asteroid impact, we would still have our children someplace else. The more places we are able to spread our civilization, the more likely we will be able to survive for thousands, millions or even billions of years.
Trefil: If you think about the chain of events leading to technological civilization, there are lots of places where there could be a bottleneck — the Great Filters. If we're really alone, then one of those must be a lot more restrictive than we think. As discussed above, if the Great Filter is in front of us, the enemy will not be the universe, but our own dark [human] nature.
Space.com: One of the things your book did was re-evaluate the Drake equation, which evaluates the likelihood of communication with intelligent life. The updated numbers you proposed imply that the existence of life is probably much more widespread than the equation originally suggests. Can you elaborate on the relationship between the Drake equation and exoplanet research?
Summers: Our exoplanet research has given us some of the factors in the Drake equation that we previously didn't know. For instance, we now know that the average star has planets and, if our solar system is typical, there is at least one or maybe several planets [in orbit around other stars] that have liquid water and are habitable. The next important step in understanding the factors of the Drake equation is to figure out how rapidly life originates. Some [life] may have very simple evolutions from bacteria to superintelligent beings, and some may be fraught with innumerable wars and battles like the Earth has seen. But every life-form is going to have its own story and develop in its own unique way, and the Drake equation oversimplifies that. As a guide in our research, however, the Drake equation has told us that locations for life do exist and are much more numerous than we expected 30 years ago.
Trefil: The Drake equation has served a useful purpose in focusing debate, but as we point out, it leaves out a lot of the exoplanet universe — subsurface oceans and rogue planets, for example. [The Father of SETI: Q&A with Astronomer Frank Drake]
Space: What do think the future holds for the discovery of exoplanets?
Summers: I think that we're going to see discoveries one after another, just like we've seen in the past two years — new types of planets and more habitable planets like [those in] the TRAPPIST-1 system. As a planetary scientist, I consider that job security. But in a way, it's overwhelming. I think we're going to have to develop better tools to study the diversity of planets, perhaps artificial intelligence to search the databases for us and to look for patterns that we can't see with our own eyes. I expect to find lots and lots of planets, signatures of life, and lots of things that we can't even imagine today.
What we need to do, and what we've got planned to do with the James Webb [Space] Telescope, is examine the atmospheres of other planets for signs of life — look at them for biomarkers, or the kind of chemical byproducts that life as we know it gives off, like methane, oxygen, carbon dioxide and water. If we see a combination of methane, oxygen and ozone in the atmospheres of these planets, then that would be an indication of life that is perhaps carbon-based like us.
Tefil: Just do the math — there are so many planets out there that almost any weird situation you can imagine is bound to show up sooner or later.
Space.com: What is the greatest challenge scientists face in the search for exoplanets?
Summers: There are two [challenges]: finding the signatures for life — what we call biomarkers — and imaging planets around their stars. Both of those fields are developing very rapidly.
Trefil: We're always limited by our instruments, of course, and the great distances involved will make the search for evidence of life very difficult.
Space.com: In your opinion, what is the biggest surprise to date surrounding exoplanet finds?
Summer: There have been so many, but I guess that in itself is the biggest surprise — that there are just so many diverse planets, everywhere we look. Also, I think the number of rogue worlds — the planets that are floating around between stars — comes as a surprise, because there are possibly more of those kinds of planets than planets that are actually tethered to stars. And they're not dead. Your first inclination is that a rogue planet without a star would be cold and dead.
Space.com: Do you think there is anything particularly important to keep in mind as we go forward in our search for exoplanets?
Summers: Yes, we're always surprised by what we find, so I think we need to keep an open mind when we're looking at new data. The things that seem unusual are the things that are going to be particularly interesting, and the things that don't quite make sense are going to be things that we should pay extra close attention to.
This interview has been edited for length and clarity. You can buy "Exoplanets" on Amazon.com.