In 1977, the New York Times published an article titled “Seeking an End to Cosmic Loneliness,” describing physicists’ attempts to capture radio messages from aliens. The initiative, known as the Search for Extraterrestrial Intelligence (SETI), was still in its early stages and its proponents were struggling to convince their colleagues and Congress that the idea was worth funding.
The quest to determine whether someone or something is out there has gained greater scientific footing in the nearly half century since that article was published. At that time, astronomers had not yet found a single planet outside our solar system. We now know that the galaxy is filled with a diversity of worlds. Our planet’s oceans were once considered exceptional, while current evidence suggests that several moons in the outer solar system harbor groundwater.
Our notion of the range of environments where life could exist has also expanded thanks to the discovery of extremophile organisms on Earth that can develop in places much hotter, saltier, acidic and radioactive than previously thought possible, including creatures that live around of underwater hydrothermal vents.
We are now closer than ever to knowing how common living worlds like ours are. New tools, including machine learning and Artificial Intelligence, can help scientists overcome their preconceived notions about what constitutes life. Future instruments will sniff the atmospheres of distant planets and examine samples from our local solar system to see if they contain telltale chemicals in the right proportions for organisms to thrive.
“I think in our lifetime we will be able to do this,” says Ravi Kopparapu, a planetary scientist at NASA’s Goddard Space Flight Center in Maryland. “We will be able to know if there is life on other planets.”
Although humans have a long history of speculating about distant worlds, for much of that time actual evidence was scarce. The first planets around other stars — known as exoplanets — were discovered in the early 1990s, but it took until the launch of NASA’s Kepler space telescope in 2009 for astronomers to understand how common they were. Kepler carefully monitored hundreds of thousands of stars, looking for small dips in their brightness that could indicate planets passing in front of them. The mission helped increase the number of known exoplanets from a mere handful to more than 5,500.
Kepler was built to help determine the prevalence of Earth-like planets orbiting Sun-like stars at the right distance to have liquid water on their surface (a region often nicknamed the Goldilocks zone). While no extraterrestrial world has been a perfect twin of ours to date, researchers can use the wealth of discoveries to make educated guesses about how many might be out there. The best current estimates suggest that between 10% and 50% of stars similar to the Sun have planets like ours, leading to numbers that make astronomers’ minds.
“If it’s 50%, that’s crazy, right?” says Jessie Christiansen, an astrophysicist at Caltech in Pasadena, California. “There are billions of sun-like stars in the galaxy, and if half of them have Earth-like planets, there could be billions of habitable rocky planets.”
Is there anyone at home?
Determining whether these planets actually contain organisms is no easy task. Researchers need to capture an exoplanet’s faint light and split it into its constituent wavelengths, looking for signatures that indicate the presence and amount of different types of chemicals. Although astronomers want to focus on stars similar to the Sun, doing so is technically challenging. NASA’s powerful new James Webb Space Telescope (JWST) is currently training its 6.5-meter mirror and unparalleled infrared instruments on worlds around stars smaller, cooler and redder than our sun, known as M dwarfs. These places may be habitable, but so far, no one is sure.
For liquid water to exist on their surfaces, the planets around M dwarfs would need to orbit close to their stars – which tend to be more active than the Sun, emitting violent explosions that could remove atmospheric gases and likely leave the ground as a dry bark. JWST is investigating Trappist-1, an M dwarf 40 light-years away, with seven small rocky worlds, four of which are just the right distance to potentially have liquid water. The two closest exoplanets have already been shown to be devoid of atmospheres, but scientists are eagerly awaiting the results of JWST observations of the next three. They want to know whether even those outside the habitable zone can have atmospheres.
There is a special interest in looking for other planets around M dwarf stars, because they are much more common than Sun-sized stars. “If they discover that they have atmospheres, it will increase the habitability of the galaxy by a hundredfold.” , says Christiansen.
When we find a planet that looks a lot like Earth, we’ll want to start looking for chemical signs of life on its surface. JWST is not sensitive enough to do this, but future ground-based instruments such as the Extremely Large Telescope, the Giant Magellan Telescope and the Thirty Meter Telescope — which are expected to begin collecting data in the 2030s — could discover the chemical components of nearby Earth-like worlds. Information on more distant targets will have to wait for NASA’s next planned flagship mission, the space-based Habitable Worlds Observatory, which is scheduled to launch in the late 2030s or early 2040s. The telescope will use an external star shadow or an instrument called a coronagraph to block the bright light from a star and focus the dimmer planetary light and its possible molecular imprints.
Which particular chemicals astronomers should look for is still up for debate. Ideally, they want to find what are known as biosignatures — molecules like water, methane and carbon dioxide present in quantities similar to those found on Earth. What this means in practice is not always clear, as our planet has gone through many periods when it contained life, but the amounts of different chemicals varied greatly.
“Do you want it to detect an Archaic Earth, like 2 or 3 billion years ago?” asks Kopparapu. “Or the Neoproterozoic, where there was a snowball Earth? Or do you want to detect the current Earth, where there is a lot of free oxygen, ozone, water and CO2?
” Recently, there was much excitement when JWST detected dimethyl sulfide, a molecule that in our world is produced only by living things, on an exoplanet nearly nine times the size of Earth, located 120 light years away. The results, which still need to be confirmed, highlight the complexity of such methods. If dimethyl sulfide is indeed present in the planet’s atmosphere, then starlight should also break it down to form ethane, a molecule that has yet to be seen. “No single gas is a biosignature,” says Kopparapu. “You need to see a combination of them.” Last year, he and other members of the community published a report emphasizing that any specific discovery must be placed in the context of its stellar and planetary environment, as there may be many findings that seemingly point to life but that have alternative explanations.
What counts as life?
This problem – how to definitively differentiate between life and non-life – is perennial, whether it concerns distant planets or even phenomena here on Earth. Researchers may soon be helped by algorithmic techniques that can reveal associations too complex for the human brain to understand. In recent experiments, Robert Hazen and his colleagues took 134 living and non-living samples (including oil, carbon-rich meteorites, ancient fossils, and a wasp that flew into the laboratory), vaporized them, and sprayed their chemical constituents. Around 500,000 different attributes were identified in the molecular composition of each sample and run through a machine learning program.
“When we look at these 500,000 attributes, there are patterns that are unique to living things and patterns that are unique to nonliving things,” says Hazen, a mineralogist and astrobiologist at the Carnegie Institution for Science.
After the software was trained on 70% of the specimens, the technique was able to recognize with 90% accuracy which of the remaining samples had a biological origin. The device used to spread the chemical components of the samples is about seven inches long, small enough to be sent on missions to nearby ocean worlds such as Europa on Jupiter or Enceladus on Saturn. NASA’s Perseverance rover has taken a similar instrument to Mars, so Hazen believes his team’s machine learning algorithm could be adapted to examine its data and look for organisms past or present there. And because it relies on molecular relationships rather than detecting specific organic chemicals, like DNA or amino acids, that may not be used in other biospheres, the method could allow scientists to look for life entirely different from what we have on Earth.
These machine learning applications are also beginning to be used in SETI, which in recent years has focused on searching for a broader set of visible evidence for extraterrestrial species. Most professionals in the field are aware of these technosignatures, defined as “some remotely detectable technology signature that we can characterize with astronomical instrumentation,” says Sofia Sheikh of the SETI Institute. This could be a radio signal, but other evidence could include things like optical laser pulses, giant space-based engineering projects, atmospheric pollution, or even artificial probes reaching our solar system.
At the Zwicky Transient Facility near San Diego, California, which continuously scans the entire night sky for brief flashes of light coming from unknown sources, engineers are teaching Artificial Intelligence to identify features that would not be expected from natural phenomena. “This is where we can start asking questions,” says Ashish Mahabal, an astronomer and data scientist at Caltech. The answers to these questions could help reveal new astronomical events or, perhaps, a star surrounded by huge solar panels that powers an energy-hungry alien society.
SETI researchers hope that by using these tools they can help overcome some of their anthropocentric biases. Most recognize that our expectations of otherworldly beings are limited by our own experience. For example, the search for signs of massive alien solar panels is often “based on the assumption that there will always be an exponential need for energy,” says Sheikh.
Because of all the avenues currently being explored, many scientists believe that the answers to our questions about extraterrestrial life are not far away. Yet ultimately the question of our cosmic loneliness is a philosophical one.
For most of human history, we didn’t believe we were alone. We fill the skies with gods, monsters and mythical creatures. It was only in the modern era that our species began to worry about its place in the universe. But regardless of whether or not any other part of the universe harbors life, the cosmos is our home. We can choose to be alone or embrace the beauty and wonder around us.
( fonte: MIT Technology Review )
