Well, Where Are They?

BY AUSTIN GLOCK

Over the summer, I was working in the university’s observatory to detect exoplanets, which are essentially any planet outside of our solar system. This concept usually inspires the same kind of questions in people, some variation of “Do you believe in aliens?” I always gave a generic response. Something like “I don’t believe aliens have been to Earth if that’s what you mean.” We would continue talking about aliens and, probably, science fiction media, and eventually move on. Once, after I gave my usual answer, I was asked, “Well, where are they?”

Honestly, it’s a good question. Scientists have discovered over 5200 exoplanets as of the end of 2022 (1). You would think that, on one of those planets, there would be signs of life. In the grand scheme of the universe, it’s not as easy as it may sound. There are four main classifications of exoplanets that range in size and in type. The first two are the gas giant and the Neptune-like. These are both planet types with gaseous compositions that make up over half of all discovered exoplanets (1). But life as we know it wouldn’t be able to exist on these planets. Even though there’s a solid core, the gaseous planets don’t have a surface like other planets. Instead, we’ll need to look instead, to the other two classifications.

First is the super-Earth. Don’t let the name fool you, all this means is that these planets are smaller than Neptune but larger than Earth. They can have a gaseous atmosphere like Neptune or a rocky surface like Mars. Finally, and arguably most importantly, is the terrestrial planet. As the name implies, these are planets that would have a solid surface similar to Earth, Venus, or Mercury. They make up only about four percent of all confirmed exoplanets, but these are the planets that would be able to host life somewhat recognizable to us (1).

We know what planets to look at. Now we need to figure out what to look for. Astronomers will look for planetary signatures that would only appear if life was present. Biosignatures are a great example. The idea is that life changes the composition of the planets, whether intelligent or not. If the Earth were being observed from a different galaxy, those scientists would see how society has changed the composition of our atmosphere. There are three types of biosignature detection methods. Gaseous detection, where things such as photosynthesis would lead to the production of oxygen from carbon dioxide. Then there is surface detection, where the formation of forests, cities, and other physical structures can affect the reflection and absorption of light. Finally, temporal detection, where time-dependent factors are used. Let’s say aliens several galaxies away were observing Earth, the changing of seasons could be detected using the temporal method (2).

If astronomers wanted to look for advanced civilizations, it becomes even harder. A planet with flora and fauna would be detectable with biosignatures, but what if scientists are looking for a response? Well luckily, there are technosignatures.

They’re exactly as they sound, a signature left by the existence of technology. This is an emerging method of detection but the idea is simple. Radio waves can travel great distances, and if the source is strong enough, it could reach other planets (2). With that in mind, we’ve begun listening to the universe for a signal.

Astronomers know where to look and they know what to look for, so why haven’t we found anything? Furthermore, why hasn’t anyone found us? This is the basis of the Fermi Paradox, which quite literally asks “Where is everybody?” (3). We have been looking, but really not for that long. Telescopes were invented in the 17th century, or about 400 years ago. The universe is somewhere around 13.7 billion years old, which makes 400 years look pretty measly.

Our short viewing period is just one theory. There are many more theories explaining the Fermi Paradox, ranging from sociopolitical reasons to widespread galactic laziness. One of the scariest theories is the Dark Forest. This was proposed by David Brin who imagines the universe as one giant dark forest. In this forest, there is a hierarchy, similar to a food chain. If there was an intergalactic superpower, smaller planets such as ourselves would never stand a chance against it. In order to avoid the larger predator, the smaller planets stay quiet and try to stay hidden (4). It may seem far-fetched, something that you would only see in a science fiction novel, but it turns out science fiction has gotten some things right. 245 lightyears from Earth exist two stars, Kepler-16A and Kepler-16B. While these stars are too dim for the naked eye to see, astronomers have discovered a gaseous planet orbiting these two stars. Astronomers have nicknamed it Tatooine, after the planet in Star Wars that also orbited a binary star system (1). If we can find planets that resemble the worlds from science fiction, we may one day discover the creatures found in them too. For now, we’re still asking the question, “Well, where are they?”

Recommended Readings

THE PLANET FACTORY

Elizabeth Tasker Bloomsburry, 2017

THE UNIVERSE IN YOUR HAND

Christophe Galfrad Flatiron Books, 2017

THE ZOOLOGISTS GUIDE TO THE GALAXY

Arik Kershenbaum Viking-Penguin, 2020