Unveiling Exoplanets: The Journey from Cosmic Mysteries to James Webb’s Revolutionary Insights

Detecting exoplanets was once such a formidable challenge that scientists discovered black holes, detected the lingering radiation from the Big Bang, and captured images of numerous distant galaxies before spotting the first planet beyond our solar system in 1992. Many astronomers who now specialize in exoplanets began their careers before exoplanet astronomy was even recognized as a field.

Today, however, astronomers have identified over 5,000 exoplanets, with new discoveries continuing to emerge. Despite this flood of findings, it’s easy to overlook how much we still don’t know about these distant worlds. The vivid images of colorful volcanoes, oceans, and cloud-streaked atmospheres seen in artists’ renditions are often speculative, based on the scant data scientists have: typically, a planet’s mass, radius, and distance from its star.

The advent of the James Webb Space Telescope (JWST) is transforming our understanding of exoplanets. Since its launch at the end of 2021, JWST has been used to study the atmospheres of hundreds of worlds, from gas giants to rocky planets about the size of Earth. As an exoplanet orbits its star, the starlight passing through its atmosphere gets absorbed at specific wavelengths depending on the gases present. This absorption leaves telltale signatures in the starlight’s spectrum, allowing scientists to identify the atmospheric chemicals. JWST is particularly sensitive to the infrared wavelengths where these signatures are most prominent, already detecting water, carbon dioxide, methane, and more on other worlds.

Given these capabilities, Laura Kreidberg of the Max Planck Institute for Astronomy in Heidelberg, Germany, notes a common question she encounters about JWST’s potential. “There’s a lot of excitement about finding signatures of alien life,” she says. “And I’m excited about that also.” However, she cautions that there is still much to learn about exoplanets before scientists can confidently detect life. Due to technical limitations, JWST’s observations will primarily focus on very hot or very large exoplanets—conditions not considered favorable for life.

Our current understanding of exoplanets is largely informed by the eight planets in our solar system. JWST’s planned 10-year mission could revolutionize this understanding, potentially answering fundamental questions about exoplanet composition, formation, and the uniqueness of our solar system within our galaxy.

Why Do Some Rocky Planets Have Atmospheres and Others Don’t?

A rocky planet’s ability to host life hinges on its atmosphere. Yet, scientists are still uncertain what factors enable a rocky body to retain a gaseous outer shell. Astronomers are searching for a “cosmic shoreline,” a theoretical boundary distinguishing worlds with atmospheres from those without. In 2017, researchers identified such a shoreline within our solar system, determined by the balance between solar radiation and a planet’s surface gravity. Sunlight energizes gas particles, potentially allowing them to escape, while gravity retains the atmospheric gases.

To test the existence of this cosmic shoreline throughout the galaxy, scientists need to determine which exoplanets have atmospheres. This seemingly basic question is only now becoming answerable thanks to JWST.

Renyu Hu, an astronomer at NASA’s Jet Propulsion Laboratory in Pasadena, California, reports that his team has resolved the atmosphere question for 55 Cancri e, a super-Earth orbiting a sun-like star about 40 light-years from Earth. In a study published in Nature on May 8, Hu and colleagues revealed JWST data indicating that 55 Cancri e has an atmosphere composed of carbon monoxide, carbon dioxide, or a mix with nitrogen—the first detection of an atmosphere surrounding a rocky exoplanet.

However, scientists remain skeptical about atmospheres on other rocky worlds observed by JWST, particularly those orbiting M-dwarf stars. These stars, easier for JWST to observe, frequently emit bursts of atmosphere-stripping radiation. Consequently, many researchers doubt that rocky planets around these stars can maintain atmospheres.

For instance, JWST observations of LHS 3844b, a super-Earth orbiting an M-dwarf star, suggest it is likely a bare rock. Similarly, observations of TRAPPIST-1b and TRAPPIST-1c, planets orbiting the M-dwarf TRAPPIST-1, indicate these worlds are also likely devoid of substantial atmospheres. However, astronomer Elsa Ducrot of the Paris Observatory notes the possibility of very thin atmospheres on these planets, a hypothesis further JWST observations will explore.

As JWST continues to examine more rocky planets with and without atmospheres, the concept of a cosmic shoreline will undergo rigorous testing, potentially reshaping our understanding of planetary atmospheres.

The James Webb Space Telescope is not just a tool for discovery; it is a revolutionary instrument reshaping our understanding of the cosmos and our place within it. Through its observations, we stand on the brink of answering some of the most profound questions about the nature of exoplanets and the potential for life beyond Earth.