The search for life on other worlds is, to say the least, a daunting and overwhelming task.We’ve been searching for only about one human generation and have yet to find anything in our own solar system, let alone the vast expanse of our galaxy.
To increase our chances of finding life on other planets, a systematic approach needs to be adopted, one that applies reasonable assumptions that maximize our success. So, an important question becomes, where should we point our telescopes?
It’s probably reasonable to assume that simpler, microbial life is more abundant that advanced intelligence with its attendant technology. On Earth, for example, life emerged within a few hundred million years of the planet’s formation. This suggests that if the right conditions exist on other planets, life could arise there as well.
So if life is abundant, it is also old because there have been a lot of planetary systems formed in the history of our galaxy, most of them before our own solar system existed. Astronomers estimate that within 326 light years of the Sun, there may be as many as 10,000 rocky planets in the habitable zone. The formation of these planets in the solar neighborhood was likely episodic, coming in fits and starts from as far back as 10 billion years ago.
Our solar system came about five billion years ago when a satellite galaxy which consists of primarily four globular clusters in its body called the Sagittarius dwarf spheroidal galaxy, went straight into the Milky Way’s stellar disk. Astronomers believe the formation of our own solar system was triggered by the first passing of this galaxy. It’s happened two more times since.
So here the stage is set: from about 10 to 5 billion years ago, most of the planets within about 300 light years were formed. This makes them older than the solar system. We do not know which fraction of these terrestrial planets were habitable or earth like in order for microbial life to eventually arise there.
But, if microbial life did arise as soon as it did on Earth in greater than, say, one percent of the 10,000 or so terrestrial planets around k-type stars, then astronomers estimate the closest ‘life-harboring Earth-like planet’ would be 65 light years away. And it would have had up to a 10 billion year head start.
The problem here, of course, is knowing the frequency with which simple life arises from non-life, and we can only guess at that since we haven’t found any life anywhere else yet. But assuming that microbial life did arise as fast as it did on Earth and that it eventually arises in all planets that are in the habitable zone, then the closest, life-harboring Earth-like planet could be less than 16 light years away.
The best way to test this, is to look for biosignatures within a sphere with a radius of 16 light years. Something that future surveys, like the Vera Rubin Telescope and the Habitable Worlds Observatory are uniquely capable of.
There is so much we do not know about life on exoplanets, but we are steadily making progress. If we could just get a handle on how easy or hard it is for life to arise from primordial organic building blocks these estimations would be a lot more accurate. To do that, we’d need several data points, and right now, we have only one. Earth.
If it is hard for life to arise, then we are likely alone, or at best, isolated. But if it is easy, then life is everywhere.