Why Finding Alien Life Requires Telescopes on Earth and in Space

Detecting alien life hinges on analyzing exoplanet atmospheres using spectroscopy, a task requiring a complementary approach from both space-based telescopes like JWST and powerful ground-based ELTs. While JWST excels at identifying molecules like CO2 and water, ELTs are crucial for the high-resolution search for oxygen, a key biosignature.

Marcus Cole

October 3, 2026 · 4 min read

To find out if we are alone in the universe, scientists must learn to read the atmospheres of planets orbiting distant stars. The primary tool for this extraordinary task is spectroscopy, a technique that decodes the chemical composition of a world by analyzing the light that passes through its air. This search relies on a crucial partnership between two different types of observatories: space-based telescopes like the James Webb Space Telescope (JWST) and the next generation of massive ground-based observatories known as Extremely Large Telescopes (ELTs). Neither can complete the mission alone; their distinct capabilities are complementary, and together they offer our best hope for identifying the potential signs of life.

The fundamental method astronomers use is called transit spectroscopy. When an exoplanet crosses in front of its star from our perspective, a minuscule amount of starlight is filtered through the planet’s atmosphere. Molecules within that atmosphere, such as water vapor or carbon dioxide, absorb very specific wavelengths, or colors, of light. This process leaves a unique pattern of dark lines in the star's spectrum, like a chemical barcode. By analyzing this barcode, scientists can identify the gases present and begin to understand the climate and potential habitability of a world light-years away.

The View from Space: JWST's Infrared Precision

Operating from its vantage point in space, the James Webb Space Telescope has revolutionized the study of exoplanet atmospheres. According to NASA, its position beyond Earth's atmosphere is critical because our planet's air blocks most of the infrared light that carries rich details about exoplanets. JWST's large mirrors and advanced instruments are designed to capture this infrared light with unprecedented stability and precision, far surpassing the capabilities of previous space observatories like Hubble and Spitzer.

This power has already yielded groundbreaking results. NASA has confirmed that JWST detected the clear presence of carbon dioxide in the atmosphere of the exoplanet WASP-39 b, the first such indisputable evidence on a planet outside our solar system. The telescope has also detected water vapor in the planet-forming disk of a young star, PDS 70, showcasing its ability to identify the building blocks of habitable worlds. These detections of key molecules demonstrate JWST's role as a premier tool for initial atmospheric reconnaissance. However, for all its power, the current generation of space-based instruments has a significant blind spot: it lacks sensitivity in the specific light bands absorbed by molecular oxygen (O2), a gas strongly associated with life on Earth.

Comparing Exoplanet Atmosphere Observatories

The decision to use a telescope in space or on the ground is driven by the specific scientific question being asked. Each platform offers a unique set of advantages and is vulnerable to different limitations. JWST's stable, space-based environment is ideal for broad infrared surveys, while the sheer size and high-resolution technology of ground-based ELTs are tailored for targeted searches of specific molecules like oxygen.

A comparison of the distinct capabilities and limitations of space-based and ground-based telescopes for analyzing the atmospheres of exoplanets.
Feature	James Webb Space Telescope (JWST)	Extremely Large Telescopes (ELTs)
Observational Environment	Operates from space, free from the filtering and distorting effects of Earth's atmosphere.	Operates from the ground, looking up through Earth's turbulent and chemically active atmosphere.
Key Advantage	Offers exceptional stability and precision, with access to a broad range of infrared wavelengths that are blocked by Earth's atmosphere.	Will possess the high-resolution capability to detect key molecules like oxygen (O2), which are challenging for current space-based instruments.
Primary Limitation	The current generation of its spectroscopic instruments lacks sensitivity in the specific bands of light absorbed by oxygen (O2).	Signals can be distorted by atmospheric turbulence and obscured by absorption from molecules in Earth's own atmosphere (telluric features).
Example Target Molecules	Has successfully detected carbon dioxide and water vapor in the atmospheres of exoplanets.	A primary future target is oxygen (O2), a potential biosignature, in the atmospheres of terrestrial exoplanets.