Aliens could be hiding on almost any of the Milky Way’s roughly 100 billion planets, but so far, we haven’t been able to find them (dubious claims to the contrary notwithstanding). Part of the problem is that astronomers don’t know exactly where to look or what to look for. To have a chance of locating alien life-forms — which is like searching for a needle that may not exist in an infinitely large haystack — they’ll have to narrow the search.
Astronomers hoping to find extraterrestrial life are looking largely for exoplanets (planets outside Earth’s solar system) in the so-called “Goldilocks zone” around each star: a distance range in which a planet is not too hot and not too cold, making it possible for liquid water to exist on the surface. But after studying our own world and many other planetary systems, scientists have come to believe that many factors other than distance are key to the development of life. These include the mix of gases in the atmosphere, the age of the planet and host star, whether the host star often puts out harmful radiation, and how fast the planet rotates — some planets rotate at a rate that leaves the same side always facing their star, so one hemisphere is stuck in perpetual night while the other is locked into scorching day. This makes it a complex problem that scientists can start to tackle with powerful computers, data and statistics. These tools — and new telescope technology — could make the discovery of life beyond Earth more likely.
Two teams of astronomers are proposing different methods of tackling these questions. One argues that we should try to identify trends in the data generated by surveys of thousands of planets, while the other favors focusing on a handful of individual planets to assess where they’d lie on a scale from uninhabitable to probably populated.
Jacob Bean, an astronomer at the University of Chicago, advocates for the broader approach in a paper he and two other researchers published this spring. It’s not possible to know for sure if a distant planet is friendly to life, Bean says, so he and his colleagues aim to compare lots of planets to figure out which are most likely to host the conditions thought to be important to produce and sustain life. Determining how the amount of water or carbon dioxide in the atmosphere is correlated with distance from the star, for example, could help inform future, more targeted searches that use new space telescopes to look for worlds with hospitable climates. “How many planets do we need to look at to find the number of ‘Earth-like’ ones? That’s the multibillion-dollar question,” he said.
Data that’s already available from NASA’s Kepler space telescope could help astronomers figure out what percentage of planets might be habitable. The Kepler mission revolutionized the study of exoplanets: It has allowed astronomers to analyze thousands of planets and their host stars, rather than the mere dozens or hundreds of extraterrestrial bodies — most of which are uninhabitable gas giants — on which we had data in the pre-Kepler period. In all, Kepler scientists have found 2,335 confirmed exoplanets, plus many more candidates waiting to be verified. With this information, researchers can get a better handle on how many solar systems have rocky planets circling at the right distance from the star or stars at the center, how often those stars zap the planets with radiation, how many planets are likely to have water, and how many feature other indications of a habitable climate. From there, scientists could deduce which of these factors are most important to the formation of planets that could develop life as we know it and determine which kinds of planets and stars are most worth focusing on.
That’s the big-picture strategy for the search for life. The other research, which was led by University of Washington astrobiologist David Catling and which will soon be undergoing peer review, claims that we’re ready to zoom in, going from questions about whether the conditions are right for life to whether life has actually developed on planets we’re interested in. His team proposes a statistical framework to evaluate these worlds.
In addition to a planet’s location and size, it matters whether its star gives off tons of radiation that could scorch off the atmosphere, leaving the planet with nothing to protect it from space weather. For example, the planets circling TRAPPIST-1 and Proxima Centauri, two red dwarf stars, exist in just such a threatening environment, and a new study by Harvard astrophysicists gives them a very low chance of supporting life. If a planet does have an atmosphere, then it matters what’s in it, as oxygen could be a sign of alien beings on the surface — even if they’re only tiny — and water vapor means it’s more likely that the climate is friendly to life. Methane, ozone and carbon dioxide could be positive signs too, but they can be produced by processes that don’t necessarily signify life, such as volcanoes.
To put together as complete a picture as possible about a planet, astronomers need both high-resolution images of the solar system and a light spectrum of the planet, which reveals what gases are present in the planet’s atmosphere based on what wavelengths of light from the star appear or fail to appear after passing the planet. If they had access to more powerful telescopes than those in use today, astronomers would want to collect even more information, including details about the age and activity of the star; the planet’s size and distance from its star; the composition and pressure of the atmosphere; whether there were signs of water, such as glints of light reflecting off oceans; and what signs there were of geological processes such as tectonic or volcanic activity. Catling eventually hopes to be able to use this information to categorize planets so that you could say Planet Y has a 20 to 40 percent chance of having life, while Planet Z has an 80 percent chance.
But at the moment, his plan is largely theoretical.
“We’re not at the point where we can really calculate the frequency or probability of life, but it’s a useful exercise,” said Eric Ford, an astrophysicist and astrostatistician at Penn State University who was not involved in either study. “As in, ‘Here’s what we’d like to do, and, given our limitations, what’s the least-bad assumptions we can make about our prior knowledge?’ It turns an impossible problem into one we can gain a foothold in answering.”
Catling and his team proposed an approach that characterizes the chance that there’s life on a planet based on what’s known about the planet and its star, updating the chances as more data comes in. Distinguishing between the knowns and unknowns helps reduce the biases affecting the system and allows it to produce fewer false positives — but only if the humans doing the characterization have a good understanding of how likely it is that a set of planetary features indicates an inhabited planet versus a lifeless one. Since we haven’t yet found life beyond Earth, even in our own solar system, it’s hard to estimate these things with any confidence.
Catling’s approach evokes the famous “Drake equation,” put forth by astronomer Frank Drake in the 1960s as a way to figure out a ballpark number of extraterrestrial civilizations in the galaxy. The idea is to estimate how many stars there are, how many of those have planets, how many of those planets could support life, how many actually develop life, how many of those life-forms evolve into intelligent life, and so on. Starting with the simpler pieces and then building up to more complex ones helps us better understand the puzzle as a whole, even if some big pieces are still missing.
“This is a wish list,” Catling said of his group’s method, noting that we don’t have the technology to make it happen. “It’s like trying to find microbes before microscopes in the 16th century. We’re at that point now.”
Catling’s team is anticipating data from new telescopes, like the Transiting Exoplanet Survey Satellite and the James Webb Space Telescope, both set to launch next year. But they’re also looking beyond these to more sophisticated telescopes that may be built in the 2030s and 2040s. Those will likely have the capability to detect more potential signs of life from many more exoplanets.
Both approaches use a lot of data and tell scientists quite a bit about how planets form and whether they harbor the conditions that we think allow life to develop. But at least until those next-generation telescopes are finished, we will probably have to wait to find out if we’re alone in the universe.
“Even if we had an ‘Earth twin’ and detected oxygen and methane and glinting from oceans, we’ll never be 100 percent sure,” Catling said. “The only thing truly 100 percent would be [an alien] signal. … That would be a slam dunk.”