Habitable Exoplanets – Who Else is Out There?

In his groundbreaking Cosmos, now nearly 40 years old, Carl Sagan speculated at length about life beyond the Earth. He even whisked us off on a journey through the galaxy in his ship of the imagination, letting us peer at the “Book of Worlds,” which catalogued dozens of habitable exoplanets.

This was amazing stuff in 1980, but in the four decades since, Carl Sagan has been proven partially correct. We now know that the cosmos is brimming with exoplanets, just as he imagined all those years ago. Nevertheless, the grandest stuff hasn’t yet been demonstrated. After a systematic search for decades, no signs of extraterrestrial life, let alone extraterrestrial intelligence, have been detected.

The failure of the radio signals never particularly bothered me. Many of the exoplanets discovered, even the ones that are potentially habitable, are hundreds or even thousands of light years away. Even at the speed of light, the signals would take hundreds of years to reach us, and even then, we would detect them only if they were beamed directly at us. When we consider that we’ve been sending out signals into space through our radio and TV broadcasts for less than a century, we get the idea of the nature of the problem.

Instead, the more damning indictment about extraterrestrial life has been the absence of any chemical detection suggesting the presence of life. One of the most fascinating things about the universe is that we can tell what something is made of by seeing how it absorbs light. Carl Sagan, as ever, explains it well.

So why haven’t we heard any news of exoplanets with atmospheres containing water vapor, oxygen, nitrogen, or some other substance that might suggest that life is there? Maybe there just hasn’t been enough time to gather all the data, or our instruments aren’t yet adequate to detect something so fine. Exoplanets don’t give off light on their own, after all, and detecting their reflected light from such a distance is daunting.

Habitable exoplanets
Kepler 452-b is the most promising candidate in the list of potentially habitable exoplanets.

Even so, the list of habitable exoplanets seems rather small, and only one, Kepler 452-b, has been discovered orbiting an actual sun-like star. The other potentially habitable exoplanets (that is, planets which are in the habitable zone of their star, where they have a temperature where liquid water can exist) mostly orbit red dwarf stars.

That’s important, because life might not be able to come about without a certain level of ultraviolet light. As early as Cosmos, Carl Sagan was talking about this – that ultraviolet light from the sun and lightning in the atmosphere potentially brought the molecules of life together on the early Earth.

Red dwarf stars give off much less ultraviolet light than the sun does, because they emit far less energy. The sun, it turns out, isn’t an average star after all. It’s larger and brighter than about 88% of the stars in the universe! The implication here is that, while Carl Sagan was right when he noted that “the stuff of life is common throughout the cosmos,” the energy sources needed to do the work of assembling it in the right way, and under the right conditions, might be much rarer.

Some scientists are now describing an “abiogenesis zone,” where it isn’t enough for exoplanets to simply be in a “habitable zone” where liquid water can exist. Instead, they also need to be in a place where they receive enough energy to kickstart the chemical reactions that lead to life. If true, that would reduce the number of potentially habitable exoplanets even further. The scientists behind the “abiogenesis zone” theory created a chart, showing that, of all the potentially habitable exoplanets discovered so far, only a scant few examples fall in the “abiogenesis zone.”

Abiogenesis zone

So, are we alone after all? Is the earth truly a freakish planet?

It’s still a giant universe. Just because of the sheer size of the cosmos, it’s still far more unlikely that Earth is the only inhabited planet, or even that we’re the only civilization. There are certainly more than a few habitable exoplanets in the “abiogenesis zone” out there. We’ve only begun to look. Nevertheless, their comparative rarity does raise some troubling questions, not the least of which is that we’re still very ignorant.

We need to do more and we’ll undoubtedly get better at this thing. When the James Webb Space Telescope launches in 2021, a lot of questions should begin getting answered – and more opportunities will be open for you to make a name for yourself.

The search for habitable exoplanets and extreterrestrial life is one of the prime grounds where glory might be won. You want your name to last through the ages? Whoever is first to discover life on another world is going to be immortalized forever, no matter what kind of life it is. A single bacterium on a planet beyond our own would be one of the biggest scientific breakthroughs in history. Astronomy is one of the few scientific fields available where amateurs can still make a big impact. In many respects, the hunt for habitable exoplanets now resembles what archaeology was back when Heinrich Schliemann began his search for Troy. Professional scholars at the time scoffed at Schliemann’s theories, but he’s the one who reaped the glory. The search for extraterrestrial life, though obviously even harder, is similar.

All my life, I’ve loved space – maybe not like Carl Sagan did, but I’ve loved it nevertheless. I’ve thought more about life lately, and its seeming rarity in the universe has made me contemplate it still more often. As I’ve left my 20s and entered my 30s, that feeling of youthful invincibility has eroded. I know now I don’t have all the time in the world. The days pass quickly and you don’t get them back. Tomorrow always looks like it will come, but sometime,  it won’t. Eventually, even the stars will die. What do you do in that time? Think of all those graves you drive by on the highway. Do you eventually fade in with them, anonymously? Can you make the kind of impact that Carl Sagan made? There might be such a way.

Amateur astronomers have been hunting for habitable exoplanets for a long time. There are even practical guides to doing it. Apart from your other work, it’s something you should consider, as I strongly am. Is it delusional? Quite possibly. But 19th century scholars said the same about the work Schliemann was doing. Some of the work of Carl Sagan was scoffed at.

There’s more than one road to glory. Each field has its own chance of success. Even if you do something great, you might not be remembered. If you discover extraterrestrial life, you surely will be. Even if you don’t, if you discover habitable exoplanets that go on to some other kind of notoriety, your name will be in contention. It’s worth a try, right? Carl Sagan would certainly encourage you.

While you’re at it, be nice to me for giving you these ideas by reading Stumped.