The answer to the question can never be “yes”, for not every nook and cranny of the universe can ever be searched. Instead, the day we can say “no” for sure is nearing.
The chief scientist of the US space agency NASA, Ellen Stofan, recently said, “I believe we are going to have strong indications of life beyond Earth in the next decade, and definitive evidence in the next 10 to 20 years.”
This new optimism among astronomers is buoyed by many factors. Mainly made possible because of the increasing capabilities of astronomers to peer into the sky and collect lots of useful information.
Take for instance the explosion in the discovery of exoplanets—that is, planets found around stars other than our sun. We have nearly 2,000 confirmed possible alien worlds and another 2,000 waiting to be approved. These exoplanets, detected by the faint decrease in starlight as a planet crosses it, come in many shapes and sizes. But a recent estimate suggests that as many as one in five sun-like stars have planets that can host life.
The famous Drake equation—a thought experiment that estimates the number of detectable civilisation in our galaxy—has “average number of planets that can potentially support life per star that has planets” as one among its seven determining factors. The higher the average, the greater the number of detectable civilisations in the Milky Way. And the data is saying that the favourable conditions required to create life are more common than we previously thought.
Fiction no more
But there is a counter argument, too. Almost all the exoplanets discovered are in the Milky Way, and at least some of the habitable ones have existed for as long as 10 billion years. That is twice the length of time the Earth has had to create us.
Thus, if there are indeed some alien civilisations on any of these planets, at least one ought to have developed the capability of space travel. And, even if their spacecrafts travelled at a fraction of the speed of light, they should have colonised the entire galaxy in a few million years. But that hasn’t happened. So perhaps life is not all that common as it may seem.
So, if the Milky Way is barren, what is it that we can do to look farther to find life? Looking for life outside the Milky Way is much more difficult. So far, we have confirmed the existence of only two exoplanets outside our galaxy. But there are good reasons to believe that the average number of habitable exoplanets in other galaxies would be similar to that in the Milky Way.
A recent study, published in the Astrophysical Journal, suggests that if we are to believe in the hypothesis that any sufficiently advanced civilisation, once evolved, will take over an entire galaxy, then we may have a plausible way of looking for life at the intergalactic scale. To come to this conclusion, the study marries science fiction and cutting edge research.
In a 1937 novel, Olaf Stapledon proposed that an advanced civilisation will try to capture as much energy as is produced by its star so as to continue to grow. This it could do by building a sphere enclosing the home star, so as to capture every photon emitted. Thus, if such a civilisation spreads through the galaxy, we may have entire habited galaxies become dull to our eyes on Earth because they emit less light.
But, in a twist Stapledon would be pleased to hear, in his study Roger Griffith at Pennsylvania State University suggests that such “dark galaxies” would emit a peculiar infrared signal and could perhaps be found. He scoured the data of 100,000 galaxies collected by NASA’s Wide-field Infrared Survey Explorer and found about 50 galaxies that fit the criteria.
So, even if the Milky Way is barren, the answer to the age-old question of whether we are alone in the universe may not be negative. And, yet, it is quite possible that Griffith’s findings may also have a mundane explanation, such as large swathes of interstellar dust blocking the galaxy from Earth. We need to work harder to find out.