Calvin, from the infamous Calvin and Hobbes comic, once declared the only proof of intelligent life on other planets was that it didn’t want to contact us. But that hasn’t stopped us from searching for it.
Using large-wave antennas, scientists at Search for Extraterrestrial Intelligence (SETI) eavesdrop on the stars, listening for radio waves in space. If intelligent life has evolved, then it will have some kind of technology similar to what is seen on Earth — at least that’s the theory, according to Seth Shostak, a senior astronomer at SETI.
“They probably have some form of radio that can be detected from space, just as our radios can,” Shostak said.
With NASA’s recent discovery of the planet Kepler-22b, among others, the amount of known planets in the habitable zone — the zone where a planet can theoretically support life — is on the rise. With an increase of known habitable planets comes an increase in the odds that some of these planets are already inhabited. This is what SETI was designed to discover, according to a NASA press release.
SETI chooses which stars to listen to by whether or not they have planets, according to Shostak. If it’s Earth-like life they’re looking for, they can also judge which stars might be broadcasting depending on how large they are. Large stars give off more heat, and produce massive amounts lethal ultraviolet rays, according Peter Watson, a physics professor at Carleton. Small stars don’t burn hot enough. Either way, it becomes impossible for life to evolve on a planet orbiting a star much bigger or smaller than the sun.
That’s assuming this life is anything close to what is seen on Earth. The moment that expectation is gone, all boundaries for what constitutes life are blown sky high, Watson said. Scientists have limited themselves to Earth-like life and planets, if only to help them find somewhere to start, Shostak said.
Finding these alien life forms is like a game of finding the radio wave in the extraterrestrial haystack, even with the search limited to Earth-like planets. There are no hints of success, or clues to guide future satellite placement.
“It would be like asking Christopher Columbus if he’d seen a hint of a new continent,” Shostak said. “Either you succeed or you don’t.”
To guess that one planet out of 1,000 had life on it would be conservative, considering there are at least a few hundred billion planets in the galaxy, Shostak said.
There are probably planets with bacteria life, but Watson said he wouldn’t hold out for anything intelligent.
“It’s just wildly improbable that there are no planets somewhere in the Milky Way that are sufficiently Earth-like [for life to] to evolve,” he said. “But it’s extremely unlikely there is intelligent life out there.”
Right now, scientists are only looking at Earth-like planets in the habitable zone for signs of life. To be part of this club of potentially life-hosting planets, a planet must be in a certain temperature range and it must have liquid water. In terms of our own solar system, Earth and Mars fall within this zone but anything outside of it, like Venus or Jupiter, is either much too hot or much too cold to support life as we know it.
That said, nothing is stopping life from existing on one of the planets outside this zone, Watson said. There’s a huge amount of science fiction full of creatures that don’t subscribe to our rules.
“You can imagine animals, or entities, that support themselves by floating in the atmosphere of gas giants,” Watson said.
These creatures would be totally different from anything found on Earth — they wouldn’t even be supported by oxygen — but their existence can’t be ruled out entirely, he said. On our own planet, there are many creatures living in extreme temperatures or under significant pressure.
Most are simple, single-cell creatures, but complex organisms also live in these alien environments, said Andrew Pelling, a biophysics professor at the University of Ottawa. Crabs and sponges don’t seem too tough, but some of their habitats say otherwise, according to Pelling.
“They live around these deep, extremely hot hydrothermal vents that are very sulphur-rich,” he said.
Crabs and sponges might not count as intelligent life. But if these creatures weren’t aquatic, they might be able to survive on the uninhabitable Venus. Originally thought to be habitable, it was ruled too hot and sulphurous for any Earth creatures by scientists in the 1970s, Watson said.
But Earth is full of extreme environments that fall outside the norms of Earth-like environments, Pelling said. These places should be barren and devoid of life. But they are not. They are testaments to how organisms can survive where existence should be impossible.
There are amoebas in the Mariana Trench — the deepest part of the ocean — so big that they live outside the rules of physics for single-cell organisms, Pelling said. Some bacteria can eat arsenic and live on top of mountains. There are fish without eyes that have evolved in total darkness and under enormous water pressure. All these creatures evolve to suit their extreme environments — a testament to the adaptability of life.
“We can find life in unconventional places,” Pelling said. “Why not find evidence for life on a desert planet or a planet that’s frozen, perhaps even a gas planet?”
Extreme environments on Earth can be used to guess what kind of life might be found on different kinds of planets, Watson said, adding that the atmosphere at the top of Mount Everest is similar to the atmosphere on Mars. The problem is some of the extreme environments on Earth are difficult enough just to study, let alone use to make guesses about life on other planets.
“We don’t actually know if there are bacteria on the top of Mount Everest because no one stays there long enough,” Watson said.
It was on Mount Everest’s atmospheric cousin, Mars, that for decades, scientists and researchers were certain they would find extraterrestrial life. Eroded river valleys and ravines that looked like they had been carved out by water made Mars seem like a prime candidate, Watson said.
However, there isn’t that much water in the Martian atmosphere. Also any ice on the surface is actually just frozen carbon dioxide. But the water that might have carved the ravines and filled the valleys could still be buried under the surface, Watson said.
Water aside, Mars is also in the temperature range to sustain life. Of course, as our neighbour in the solar system, it was also the easiest and most convenient place to look for alien life forms.
So scientists like those at SETI keep their antennas tuned to planets similar to Earth and Mars and now, Kepler-22b, hoping to catch a strain of some alien signal crackling from a faraway star. Maybe one day someone will answer.
