Here on Planet Earth, there's life everywhere we look. But does life exist beyond Earth, and if so, where might we find it? Join the Adler as we meet alien-seeking scientists, and explore the quest to understand life in the universe.
A New Field in a Longstanding Quest
For millennia, humans have wondered whether life might exist beyond Earth, somewhere amongst the stars. Though cultures across the world are filled with examples of people imagining what life on other planets might be like, for most of human history, actually detecting life beyond Earth has been beyond our grasp. But beginning in the 20th century, a new scientific field-- astrobiology-- arose out of the long-standing human curiosity to find (or even communicate with!) alien life.
Uncovering the Limits of Life
Astrobiology, or the study of and search for life in the universe, brings together many different kinds of science. Some astrobiologists study life on Earth-- especially life in extreme environments-- to uncover the limits of where life might exist. Some of their investigations have led to life in such surprising places, it may as well seem alien to humans. (Image Credit: NASA/JPL-Caltech/T. Pyle (SSC))
Living on the Edge
From the depths of the ocean, to the water near nuclear reactors, to boiling acidic pools in Yellowstone National Park (pictured here), life thrives in all kinds of places that would be deadly for human beings! These kinds of life, named “extremophiles” because of their love of extreme conditions, can provide scientists with hints as to the limits of life. Understanding what kinds of places we find life on Earth gives us clues as to where life could potentially exist on other planets.
Exploring Early Mars in the Andes: Dr. Nathalie A. Cabrol
Astrobiologist Dr. Nathalie A. Cabrol explores high altitude lakes in the Andes as analogs to early Mars. She documents life’s adaptation to extreme environments, the effect of rapid climate change on lake ecosystems and habitats, its geobiological signatures, and relevance to planetary exploration. Dr. Cabrol is the Director of the Carl Sagan Center for Research at the SETI Institute, where she leads the strategic vision for science and exploration. She heads projects in planetary science and astrobiology, develops science exploration strategies for Mars, Titan, and the Outer Solar System icy moons, and designs robotic field experiments. (Image courtesy SETI Institute)
More Life in the Solar System?
Some astrobiologists study life on other planets in our own solar system. Of the planets in our solar system, only Mars, Venus, Earth, and Mercury are what are known as terrestrial planets: small, rocky worlds that we think might have environments suitable for life. Of the terrestrial planets, Mars and Venus have received the most attention. Mars and Venus are prime candidates for finding life because, while they are each very different from Earth today, they may have been more hospitable environments in the past. Other worlds in our solar system that are good candidates to have life are some of the moons of the giant planets, for example Europa (a moon of Jupiter), Enceladus, and Titan (moons of Saturn). (Image Credit: NASA/JPL-Caltech)
A Whiff of Life?
Just recently, a stinky molecule known as phosphine was detected in the atmosphere of our neighboring planet, Venus. You might wonder why anyone would care about a smelly gas on another planet-- but phosphine is an example of a biosignature, a chemical compound that we think might point to the existence of life. Finding phosphine doesn’t necessarily mean that life exists on Venus-- there need to be follow up investigations to be sure-- but it’s definitely an exciting hint that something interesting might be happening there. (Image Credit: ISAS, JAXA, Akatsuki)
Decoding the Signatures of Life: Dr. Clara Sousa-Silva
Dr. Clara Sousa-Silva is a quantum astrochemist whose favorite molecular biosignature is phosphine. She investigates how molecules like phosphine interact with light, so that they can be detected on faraway worlds. Dr. Sousa-Silva spends most of her time studying molecules that life can produce so that, one day, she can detect an alien biosphere. When she is not deciphering exoplanet atmospheres, Dr. Sousa-Silva works hard to persuade the next generation of scientists to become an active part of the astronomical community. (Image Courtesy of Melanie Gonick)
Worlds Beyond the Solar System
Besides studying life on Earth, and looking at neighboring planets and moons, many astrobiologists hope to discover life on planets outside our solar system, orbiting other stars. These planets, known as exoplanets, were first discovered in the mid-1990s, but for over 20 years after, finding them was hardscrabble work. It was especially hard to find planets that might be small and rocky, like Earth, Venus, and Mars are. Since those planets are some of the prime places to look for life in our solar system, astrobiologists hoped to find them around other stars, too. (Image Credit: NASA-JPL/Caltech)
Kepler's Search for Terrestrial Planets
In 2009, NASA launched a new space telescope: the Kepler mission. Kepler was specially designed to look for small, rocky planets around stars like our Sun. At that point, no one knew whether terrestrial planets were common or rare-- most of the planets that had been discovered were bigger than Earth, more like the giant planets in our own solar system. Kepler’s goal was to solve the mystery of how many planets could even potentially be like Earth. Because of Kepler, we now know that there are billions of rocky worlds in our galaxy! (Image Credit: NASA/Kepler mission/Dana Berry)
What does it take to be like Earth
Astronomers look for planets that are similar to Earth, because Earth is still the only planet we know definitely has life on it. But if life on Earth can live in a lot of different environments, how do we figure out where alien life might be? And what does it mean for a planet to be “like Earth”? Astronomers start with looking for planets that are roughly the same size as the terrestrial planets in our solar system that have (or might have had) life: Earth, Venus, and Mars. Ideally, they also want to find planets in what’s called the habitable zone. (Image Credits: NASA Ames/SETI Institute/JPL-Caltech)
Follow the Water
The habitable zone is a region where a planet would receive about the same amount of light from its star as Earth receives from the sun. Here on Earth, warmth from our sun provides much of the energy life needs to survive, and also helps make our planet warm enough to have liquid water on its surface. While many factors influence a planet’s suitability for life, and life can survive in lots of different environments, most of the places life thrives on Earth have liquid water. For that reason, astrobiologists sometimes joke that they have an unofficial motto: “Follow the Water”. (Image Credits: NASA Ames/SETI Institute/JPL-Caltech)
Different kinds of stars emit different amounts and energies of light, so the location of the habitable zone changes depending on what kind of star a planet is orbiting. Stars that are smaller than our sun emit only a little bit of light, and their light is mostly red, low-energy light, while stars that are bigger than our sun emit lots of highly energetic, blue light. You can think of the location of the habitable zone like where you’d sit around a campfire to be warm: if you are around a big, blazing bonfire, you’d want to sit farther away to be warm (but not too hot!), and if your fire was small, you’d need to sit closer to the fire to be warm enough. (Image Credit: NASA/Kepler Mission/Dana Berry)
Common Stars, Uncommon Life?
Stars that are like our sun are not the most common kinds of star: That honor belongs to stars that are smaller than our sun, known as “M dwarfs” (or sometimes just red dwarfs), which make up 70% of the stars in our Milky Way Galaxy. M dwarfs also have lots of small planets, so that means that most habitable zone planets are close to their stars, and have very red light. How might life be different on these alien worlds? (Image Credit: NASA/JPL-Caltech/T. Pyle (IPAC))
Exploring Exoplanet Climates: Dr. Aomawa Shields
Dr. Aomawa Shields is a Clare Boothe Luce Associate Professor in the Department of Physics & Astronomy at UC Irvine, and an astrobiologist whose research focuses on exploring the possible climates and potential habitability of Earth-sized planets orbiting cool, low-mass stars. Because exoplanets are so far away, it can be challenging to know what their climate might be like-- so Dr. Shields uses computer models, along with observational data from space- and ground-based observatories, to carry out her research. In addition to her award-winning scientific research, Dr. Shields is also the Founder and Director of the organization Rising Stargirls, which encourages girls of all colors and backgrounds to explore and discover the universe using theater, writing, and visual art. She is the proud mother of 3 year old rising stargirl Garland-Rose, who she and husband Steven have deemed the most extraordinary lifeform in their universe. (Image Courtesy of Ryan Lash/TED)
Probing the atmospheres of far-flung worlds
The habitable zone is one useful concept for figuring out whether a planet could host life or not, but it’s only one piece of the puzzle. After all, look at Earth, Venus, and Mars, which are all rocky planets, and receive roughly similar amounts of light from the Sun. While Venus and Mars could possibly have life, only the Earth has life in abundance. The difference between these planets is their atmospheres, and so astrobiologists are excited to be able to figure out what exoplanet atmospheres are made of. The James Webb Space Telescope, launching in the next few years, will be able to get chemical fingerprints for some planets-- will we find biosignatures? (Image Credit: NASA)
Lessons from Tiny Organisms
Of course, finding signs of life from biosignatures might tell us life is there, but it doesn’t tell us everything we might want to know. After all, the organism that has arguably had the biggest effect on Earth’s atmosphere-- cyanobacteria-- isn’t the kind of life people often picture when they think of aliens. You owe a lot to cyanobacteria, whether you know it or not: without them, the Earth’s atmosphere wouldn't be full of the oxygen human beings need to breathe. But when most people picture “aliens”, they often think about life that we might be able to communicate with.
Tuning in to the Cosmos
That’s where the Search for Extraterrestrial Intelligence, or SETI, comes in. SETI scientists take a different approach to the search for life: rather than look for their chemical signatures in planetary atmospheres, SETI tries to detect transmissions, either intentional or unintentional, from life that has enough technology to broadcast those signals. The original SETI search was conducted in 1960 by astronomer Frank Drake, who, inspired by humans’ use of radio waves to communicate around the globe, used a radio telescope to try to find broadcasts from space.
Looking For Alien Civilizations: Dr. Jill Tarter
Dr. Jill Tarter has spent the majority of her professional career attempting to answer the old human question “Are we alone?” by searching for evidence of technological civilizations beyond Earth-- you may even be familiar with her work as portrayed by Jodie Foster in the movie Contact! Dr. Tarter is the Emeritus Chair for SETI Research at the SETI Institute in Mountain View, California. She served as Project Scientist for NASA’s SETI program, the High Resolution Microwave Survey and has conducted numerous observational programs at radio observatories worldwide. She is a Fellow of the AAAS, the California Academy of Sciences, and the Explorers Club, she was named one of the Time 100 Most Influential People in the World in 2004, and one of the Time 25 in Space in 2012, received a TED prize in 2009, two public service awards from NASA, multiple awards for communicating science to the public, and has been honored as a woman in technology. Since the termination of funding for NASA’s SETI program in 1993, she has served in a leadership role to design and build the Allen Telescope Array and to secure private funding to continue the exploratory science of SETI. (Image Courtesy of Seth Shostak).
Hot on the Trail
Whether it’s in our solar system, or on an alien planet around another star, astrobiologists are hot on the trail of finding life beyond our world. Like our ancestors long before us, we also want to understand humanity’s place in the universe, and we have much to learn about what might be out there. But unlike eons ago, we now have a wide variety of ways that we can look for life-- and that means we don’t just have to wonder, we can search. (Image Credit: NASA/JPL-Caltech)
Thank you to the staff of the Adler Planetarium for their assistance in creating this exhibition, and to NASA/JPL for the use of their images.
The Adler also thanks Dr. Nathalie A. Cabrol, Dr. Clara Sousa-Silva, Dr. Aomawa Shields, and Dr. Jill Tarter for their permission and assistance in creating this exhibition.