Alien Earths: The Hunt for Life in the Cosmos

By Lisa Kaltenegger

It’s a staggering statistic: one out of every five stars has a planet that could resemble Earth. That’s 40 billion potential worlds in our Milky Way galaxy alone. And with the advent of the James Webb Space Telescope, we’re finally peering into the atmospheres of these distant worlds, searching for signs of life.

I’m part of the team that designed the Near Infrared Imager and Slitless Spectrograph (NIRISS), one of the Webb’s instruments. We’re focusing on the Trappist-1 system, a group of seven rocky planets orbiting a dim, red star just 40 light-years away. These planets are tantalizingly close to the habitable zone, the range of distances from a star where liquid water can exist on a planet’s surface.

But finding life on another planet is not as simple as detecting the presence of water. We need to look for the gases that living organisms breathe in and out. It’s like finding footprints in the forest. You might see a deer’s hoofprint and say, “That’s a deer.” But if you see something else, like a smudge in the dirt, it could be a sign of life, but it could also be something else.

So we’re training our telescopes to recognize the gases that indicate life as we know it. But we’re also keeping an open mind. We know that life on Earth depends on water, but we can’t assume that life on other planets will be the same. There might be other types of biochemistry, based on different solvents or even different elements.

To help us narrow down our search, I’ve created models of Earth’s atmosphere throughout its geological history. These models show that for about 2 billion years, Earth’s atmosphere carried a distinct fingerprint of life: a combination of oxygen and methane. This suggests that if we find a planet with similar atmospheric gases, it could be a strong indication of life.

The James Webb Space Telescope is giving us an unprecedented opportunity to study the atmospheres of distant planets. With its powerful infrared capabilities, Webb can detect the subtle changes in light that occur when starlight passes through a planet’s atmosphere. By analyzing these changes, we can identify the gases that are present.

We’re still in the early stages of our search, but we’re already making exciting discoveries. In 2023, the Webb telescope detected carbon dioxide in the atmosphere of a planet orbiting the star LHS 475. This was the first time that carbon dioxide had been detected in the atmosphere of an exoplanet, and it opened up new possibilities for studying the chemical compositions of other worlds.

The next generation of telescopes, such as the Habitable World Observatory, will be even more powerful than Webb. These telescopes will allow us to study the surfaces of exoplanets, look for signs of vegetation, and even measure their atmospheric temperatures.

If we find a planet with the right conditions for life, what would that mean for humanity? It would be the biggest scientific discovery in history. It would challenge our understanding of our place in the universe and our uniqueness as a species. It would also open up new possibilities for exploration and colonization.

But finding life on another planet is not just about satisfying our curiosity. It’s also about learning more about ourselves. By studying other worlds, we can gain insights into the origins of life on Earth and the factors that make our planet habitable.

The search for life beyond Earth is one of the most ambitious and exciting endeavors in human history. It’s a journey that will take us to the stars and beyond, and it’s a journey that we’re only just beginning.