Are we alone? 

Whether it be from exposure to alien sci-fi movies, online conspiracy theories, alleged UFO sightings, or simply from marveling at the night sky from one’s backyard, most of us have pondered on the potential of extraterrestrial life. 

For astronomers, this question is not just a fleeting thought, but a genuine topic of interest that was made all the more pressing when astronomers Aleksander Wolszczan and Dale Frail discovered the first exoplanet thirty years ago. 

The prefix “exo” is derived from the Greek word for “outside.” Therefore, the term “exoplanet” refers to worlds beyond our solar system. Since 1992, astronomers have confirmed over 6,000 exoplanets exist and estimate that there could be billions more within the Milky Way galaxy alone.

Given the sheer abundance of exoplanets, you would think that the likelihood of life existing elsewhere would be pretty high. And yet, the question “Are we alone?” remains a complete mystery.

“I honestly would say that we’re not confident at all,” said Jessica Spake, a staff scientist at Carnegie Observatories, during a recent interview with The Lab Report. “It’s a roaring debate right now.”

Habitability

The first obstacle stems from our lack of understanding of life in general. 

You may have been introduced to the infamous “what came first: the chicken or the egg?” paradox. It is unsure how life began, and if you consider life to be a mere product of chemical reactions, it’s puzzling why scientists haven’t been able to recreate it.

Since the study of the origins of life remains an ongoing journey, astronomers resorted to the next best approach: defining what life looks like— a hot topic within the science community.

Despite the myriad of differing opinions, one thing most exoplanet scientists do agree upon, is the existence of the habitable zone, or “Goldilocks zone.” This is the hypothetical region around a star that maintains just the right temperature for water to be in its liquid form.

Liquid water is a natural solvent, enabling complex chemical reactions while stabilizing temperatures due to its ability to absorb significant amounts of heat. Many scientists believe it is the key ingredient for life.

Another important indicator of habitability is the existence of an atmosphere.

Without an atmosphere, all humans and animals would asphyxiate. The lack of atmospheric pressure would cause all the oceans to boil and evaporate into space. Extreme temperature fluctuations would mean that daytime temperatures could be hundreds of times higher than nighttime temperatures. Additionally, the Earth’s surface would be bombarded by solar radiation, harmful UV rays and meteors, which would make life nearly impossible as we know it.

While these two criteria — having liquid water and a viable atmosphere — do not guarantee life, they are surely a decent starting point.

Studying exoplanets

Measurements present another area of difficulty in exoplanet research. 

The reason exoplanets are so difficult to study is the presence of its host star — the star it is orbiting. A common analogy for this is trying to see a firefly next to a massive, blinding lighthouse beacon from hundreds of miles away. The fact that stars are billions of times brighter than their planets means that astronomers often must refer to indirect methods of observation.

One such technique is called the Transit Method, in which astronomers capture the periodic dimming of a star’s light as a planet passes between its host star and the observer. Another way to measure an exoplanet is through the Radial Velocity Method, which involves observing a star “wobbling” in space due to the gravity of its orbiting planet. 

There are many more creative solutions astronomers have developed over the past decades, and while direct imaging is becoming an increasingly feasible option, the existence and properties of exoplanets are nearly always determined without even seeing them.

Top candidates for life

Although the obstacles in finding extraterrestrial life seem discouraging, there is also reason to celebrate humanity’s perseverance and progress. Below, we list some of the four most promising potentially habitable exoplanets, along with what we have discovered about each of them.

Kepler-186f: the first

Kepler-186f may not be the closest or the easiest to study, but it holds a special place in history. More specifically, it was the first Earth-sized planet ever discovered within the habitable zone of another star.

Prior to this discovery, the only known exoplanets were large and gaseous, similar to Jupiter in our solar system. This is not due to a population abundance, but rather the bias in our detection methods revealing bigger, more massive planets. 

At the time, Kepler-186f transformed the search for life from speculation into something tangible. It proved that Earth was not an anomaly, but something that could exist elsewhere. 

While located in the habitable zone of its star, Kepler-186f is also a lengthy 500 light-years away from Earth, making it difficult to obtain precise measurements of it. Regardless, Kepler-186f holds its title as one of the most significant milestones in the exoplanet field, as well as a candidate to look out for in future missions.

Proxima Centauri b: the neighbor

In contrast to the distant world of Kepler-186f, we have the closest known exoplanet to Earth: Proxima Centauri b.

While four light years — or 25 trillion miles — may seem impossibly far away, to astronomers, this is practically next door. Proxima Centauri b is slightly more massive than our planet and located within its star’s habitable zone. When it was found, it immediately became one of the most exciting discoveries in modern astronomy.

But proximity does not equal habitability. Proxima Centauri — the host star of Proxima Centauri b — is a small yet powerful red dwarf star. Red dwarf stars in particular are known for their intense magnetic activity and strong stellar flares. These bursts of radiation could make it difficult for a planet to hold onto its atmosphere, or even support life at all. As much as we’d love for Proxima Centauri b to be the habitable planet, it is important to consider other options as well.

TRAPPIST-1e: the Earth-like

A bit further from Earth — roughly 40 light-years away — lies the star TRAPPIST-1. 

This star’s claim to fame is its seven Earth-sized planets in orbit around it — in other words, seven potential opportunities for life.

In our solar system, there are eight planets in orbit. Within the TRAPPIST-1 solar system, TRAPPIST-1e stands out as the most Earth-like. It is rocky, Earth-sized and sits comfortably within the habitable zone. For years, it has been considered one of the best candidates for habitability.

Yet, the limits of our technology persist. Astronomers are still working to determine whether TRAPPIST-1e has a viable atmosphere. According to the latest results from the James Webb Space Telescope, it is possible that the planet has a thin atmosphere, or none at all.

Ultimately, TRAPPIST-1e sits as a prime example of what exoplanet research looks like today: a world that seems incredibly promising on paper, but remains out of reach.

LHS 1140b: the water world

LHS 1140b takes a slightly different approach to habitability. Larger and more massive than Earth, it falls into the category of a “super-Earth.” The extra mass could be an advantage, allowing it to hold onto a thick atmosphere for longer periods of time.

Some models even suggest that it could be covered in a deep, global ocean. If true, life on this planet might not resemble the land creatures typically imagined in our sci-fi depictions of aliens, but rather water or even microbial species. This world acts as a reminder that life elsewhere might not look like life here on Earth, and to not eliminate potentially habitable candidates prematurely.

The future

To put the rapid progression of exoplanet science into perspective, Kepler-186f, the first Earth-sized exoplanet, was discovered in 2014. Up until that point, less than 2000 exoplanets had been confirmed, and scientists believed none of them resembled Earth. 

From discovering the first exoplanet thirty years ago, to having the ability to study rocky, Earth-sized exoplanets in today’s era, it is incredibly exciting to think of the possibilities for the next few decades in this area of research.

Technology is largely what governs what is possible and what is not. The most recent mission, the James Webb Space Telescope, is actively contributing to the observation of distant worlds. In addition, the Habitable Worlds Observatory, aimed for launch in the early 2040s, is a telescope designed specifically to search for signs of life on Earth-like exoplanets. It will directly image planets in ultraviolet, optical and infrared light, analyzing their atmospheres for chemical compounds such as oxygen and methane, which astronomers believe are likely indicators of life.

In anticipation of the future, Spake reminds people of why we study astronomy. “My number one thing is to try and enjoy it,” she said.

Perhaps the reason sci-fi and aliens remain a favored Hollywood trope is due to the very human desire for knowledge and purpose. As science continues to advance and many questions remain unanswered, it is important to stay curious and look up.

While it can be easy to slip into stress over what knowledge we don’t have just yet, Spake reminds us not to take what we do have for granted.

“We’re asking if we’re alone in the universe… [but] we’re not alone when we do science.”