Forty years ago, the Voyager 2 spacecraft launched from Florida’s Cape Canaveral. Over the next decade, it swept across the solar system, sending back images of Jupiter’s volcanoes, Saturn’s rings, and for the first time, the icy atmospheres of Uranus and Neptune.
The mission was more than enough to encourage Stephen Kane, a teenager growing up in Australia, to study planetary science in college. By the time he’d graduated, scientists had detected the first planet outside our solar system, known as an exoplanet, inspiring him to join the hunt and look for more.
Over the past two decades, Kane, now an associate professor of planetary astrophysics at UC Riverside, has discovered hundreds of alien planets. At first, he focused on identifying giant Jupiter-like planets, which he describes as “low-hanging fruit” due to their large sizes. But in 2011, the Kepler Space Telescope identified the first rocky planet — Kepler 10b. Unlike gas giants such as Jupiter, rocky planets could potentially harbor life.
With the discovery of more Earth-sized planets on the horizon, Kane realized that astrophysicists would struggle to understand the data they were receiving about terrestrial planets and their atmospheres.
“During the course of the ongoing Kepler mission, I sought out planetary and Earth scientists because they’ve spent hundreds of years studying the solar system and how the Earth’s atmosphere has been shaped by biological and geophysical processes, so they have a lot to bring to the table,” Kane said.
In 2017, Kane formalized that collaboration by joining an interdisciplinary research group led by Tim Lyons, a distinguished professor of biogeochemistry in the Department of Earth Sciences and director of UCR’s Alternative Earths Astrobiology Center. Backed by roughly $7.5 million from NASA, the center, one of only a handful like it in the world, brings together geochemists, biologists, planetary scientists, and astrophysicists from UCR and partner institutions to search for life on distant worlds using a template defined by the only known planet with life: Earth.
Since its formation more than 4.5 billion years ago, Earth has undergone immense periods of geological and biological change.
When the first life appeared — in the form of simple microbes — the sun was fainter, there were no continents, and there was no oxygen in the atmosphere. A new kind of life emerged around 2.7 billion years ago: photosynthetic bacteria that use the sun’s energy to convert carbon dioxide and water into food and oxygen gas. Multicellular life evolved from those bacteria, followed by more familiar lifeforms: fish about 530 million years ago, land plants 470 million years ago, and mammals 200 million years ago.
“There are periods in the Earth’s past that are as different from one another as Earth is from an exoplanet,” Lyons said. “That is the concept of alternative Earths. You can slice the Earth’s history into chapters, pages, and even paragraphs, and there has been life evolving, thriving, surviving, and dying with each step. If we know what kind of atmospheres were present during the early stages of life on Earth, and their relationships to the evolving continents and oceans, we can look for similar signposts in our search for life on exoplanets.”
While it might seem impossible to characterize ancient oceans and atmospheres, scientists can glean hints by studying rocks formed billions of years ago.
“The chemical compositions of rocks are determined by the chemistry of the oceans and their life, and many of the gases in the atmosphere, through exchange with the oceans, are controlled by the same processes,” Lyons said. “These atmospheric fingerprints of life in the underlying oceans, or biosignatures, can be used as markers of life on other planets light years away.”
The search for alien biosignatures typically centers on the gases produced by living creatures on Earth because they’re the only examples scientists have to work with. But Earth’s many chapters of inhabitation reveal the great number of possible gas combinations. Oxygen gas, ozone, and methane in a planet’s biosignature could all indicate the presence of life — and seeing them together could present an even stronger argument.
The center’s search for life is different from the hunt for intelligent life. While those researchers probe for signs of alien civilizations, such as radio waves or powerful lasers, Lyons’ team is essentially looking for the byproducts of simple lifeforms.
“As we’re exploring exoplanets, what we’re really trying to do is characterize their atmospheres,” he said. “If we see certain profiles of gases, then we may be detecting microbial waste products that are accumulating in the atmosphere.”
The UCR team must also account for processes that produce the same gases without contributions from life, a phenomenon researchers call false positives. For example, a planetary atmosphere with abundant oxygen would be a promising biosignature, but that evidence could be misleading without fully addressing where it came from. Similarly, methane is a key biosignature, but there are many nonbiological ways to produce this gas on Earth. These distinctions require careful considerations of many factors, including seasonal patterns, tectonic activity, the type of planet and its star, among other data.
False negatives are another concern, Lyons said. In previous research on ancient organic-rich rocks collected in Western Australia and South Africa, his group showed that about two billion years passed between the moment organisms first started producing oxygen on Earth and when it accumulated at levels high enough to be detectable in the atmosphere. In that scenario, a classic biosignature, oxygen, could be missed.
“It’s also entirely possible that on some planets oxygen is produced through photosynthesis in pockets in the ocean and you’d never see it in the atmosphere,” Lyons said. “We have to be very clever to consider the many possibilities for biosignatures, and Earth’s past gives us many to choose from.”
‘Following the Water’
With several hundred terrestrial planets confirmed and many more awaiting discovery, the search for life-bearing worlds is an almost overwhelming task.
Astronomers are narrowing down their search by focusing on habitable zones — the orbital region around stars where it’s neither too hot nor too cold for liquid water to exist on the surface.
“We know that liquid water is essential for life as we know it, and so we’re beginning our search by looking for planets that are capable of having similar environments to Earth. We call this approach ‘following the water,’” Kane said.
While the habitable zone serves as a target selection tool, Kane said a planet nestled in this region won’t necessarily show signs of life — or even liquid water. Venus, for example, occupies the inner edge of the Sun’s habitable zone, but its scorching surface temperature has boiled away any liquid water that once existed.
“We are extremely fortunate to have Venus in our solar system because it reminds us that a planet can be exactly the same size as Earth and still have things go catastrophically wrong,” Kane said.
Equally important, being in the habitable zone doesn’t mean a planet will boast other factors that make Earth ideal for life. In addition to liquid water, the perfect candidate would have an insulating atmosphere and a protective magnetic field. It would also offer the right chemical ingredients for life and ways of recycling those elements over and over when continents collide, mountains lift up and wear down, and nutrients are swept back to the seas by rivers.
“People question why we focus so intently on Earth, but the answer is obvious. We only know what we know about life because of what the Earth has given us,” said Lyons, who has spent decades reconstructing the conditions during which life evolved.
“If I asked you to design a planet with the perfect conditions for life, you would design something just like Earth because it has all of these essential features,” he added. “We are studying how these building blocks have been assembled in different ways during Earth’s history and asking which of them are essential for life, which can be taken away. From that vantage point, we ask how they could be assembled in very different ways on other planets and still sustain life.”
Kane said a distant planetary system called TRAPPIST-1, which NASA scientists discovered in 2017, could provide clues about the ingredients that are necessary for life.
Although miniature compared to our own solar system — TRAPPIST-1 would easily fit inside Mercury’s orbit around the sun — it boasts seven planets, three of which are in the habitable zone. However, the planets don’t have moons, and they may not even have atmospheres.
“We are finding that compact planetary systems orbiting faint stars are much more common than larger systems, so it’s important that we study them and find out if they could have habitable environments,” Kane said.
At about 40 light-years (235 trillion miles) from Earth, the TRAPPIST-1 system is relatively close, but we’re never going to go there.
“The fascinating thing about astronomy as a science is that it’s all based on remote observations,” Kane said. “We are trying to squeeze every piece of information we can out of photons that we are receiving from a very distant object.”
While scientists have studied the atmospheres of several dozen exoplanets, most are too distant to probe with current instruments. That situation is changing. In April, NASA launched its Transiting Exoplanet Survey Satellite, known as TESS, which will seek Earth-sized planets around more than 500,000 nearby stars. In May 2020, NASA plans to launch the James Webb Space Telescope, which will perform atmospheric studies of the rocky worlds discovered by TESS.
Like Kepler, TESS detects exoplanets using the transit method, which measures the minute dimming of a star as an orbiting planet passes between it and the Earth. Because light also passes through the atmosphere of planets, scientists will use the Webb telescope to identify the blanket of gases surrounding them through a technique called spectroscopy.
Kane and Lyons are working with NASA to design missions that will directly image exoplanets in ways that will ensure that interdisciplinary teams such as theirs can properly interpret a wide variety of planetary processes.
“As we design future missions, we must make sure they are equipped with the right instruments to detect biosignatures and geological processes, such as active volcanoes,” Kane said.
UCR’s astrobiology team is one of only a few groups in the world studying ancient Earth to create a catalog of biosignatures that will inform mission design in NASA’s search for life on distant worlds. With quintillions — think the number of gallons of water in all of our oceans — of potentially habitable planets in the universe, Lyons said he is optimistic that we’ll find signs of life in the future.
“Just like the Voyager missions were important because of what they taught us about our solar system — from the discovery of Jupiter’s rings to the first close-up glimpses of Uranus and Neptune — the TESS and James Webb missions, and more importantly the next generation of telescopes planned for the coming decades, are very likely to change our understanding of distant space,” Lyons said. And perhaps nestled in those discoveries will be an answer to the most fundamental of all questions, ‘are we alone?’
Alternative Earths Astrobiology Center
- Founded in 2015
- One of 12 research teams funded by the NASA Astrobiology Institute, and one of only two using Earth’s history to guide exoplanet exploration
- Awarded $7.5 million over five years to cultivate a “search engine” for life on planets orbiting distant stars using Earth’s evolution over billions of years as a template
- Builds on existing UCR strengths in biogeochemistry, Earth history, and astrophysics
- Unites 66 researchers and staff at 11 institutions around the world, including primary partners led by former UCR graduate students now on the faculty at Yale and Georgia Tech
Through the Looking Glass
In April, the Transiting Exoplanet Survey Satellite (TESS) Mission launched with the goal of discovering new Earths and super-Earths around nearby stars. As a guest investigator on the TESS Mission, Stephen Kane will use University of California telescopes, including those at the Lick Observatory in Mt. Hamilton to help determine whether candidate exoplanets identified by TESS are actually planets.
By studying the planet mass data obtained from the ground-based telescopes and planet diameter readings from spacecraft observations, Kane will also help determine the overall composition of the newly identified planets.
Visit astrobiology.ucr.edu to learn more about the Alternative Earths Astrobiology Center.