Science

How to Search for Alien Planets - with Nikku Madhusudhan

Royal Institution
Royal Institution•Mar 24, 2026

Why It Matters

Detecting credible biosignatures on nearby exoplanets would transform our understanding of life’s universality and guide the design of next‑generation telescopes for astrobiology.

Key Takeaways

  • •Only ~10 exoplanets currently observable for atmospheric study
  • •K2-18b emerges as prime hydrogen‑rich ocean world candidate
  • •Transmission spectroscopy reveals methane, carbon dioxide, tentative dimethyl sulfide
  • •Bio‑signature detection hinges on confirming dimethyl sulfide signals
  • •Future telescopes must target nearby habitable‑zone planets for life search

Summary

The video outlines how astronomers prioritize exoplanets for life‑search missions, emphasizing the blend of theoretical habitability criteria and practical observational limits. With over 6,000 known worlds, only about ten lie close enough and within the right temperature range for current instruments to probe their atmospheres.

Researchers first filter planets by size, stellar type, and orbital distance to define a habitable zone where liquid water could exist. They then assess whether the system is close enough for telescopes like JWST to capture transmission spectra during transits. This dual filter narrows the field dramatically, focusing resources on a handful of promising targets.

K2-18b, a nine‑Earth‑mass planet orbiting within its star’s habitable zone, exemplifies this approach. Theoretical models suggested a thin, hydrogen‑rich atmosphere could still support surface oceans, prompting JWST observations that detected methane and carbon dioxide, and a tentative dimethyl sulfide (DMS) signal—an Earth‑based microbial biosignature. The alignment of prediction and measurement marks a milestone for the emerging class of "Hishen" worlds, planets with global oceans and hydrogen‑rich skies.

These findings highlight both the promise and the limits of current technology. Confirming DMS or other biosignatures could revolutionize astrobiology, but definitive proof remains elusive. Future, more sensitive telescopes must target nearby habitable‑zone planets to expand the sample size, refine atmospheric models, and move from tentative hints to robust evidence of extraterrestrial life.

Original Description

Are we alone in the universe? An astrophysicist answers one of the oldest and most primal questions of humankind.
This talk was filmed at the Royal Institution on the 27th of October 2025
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But we may be closer to finding an answer, thanks to a recently-proposed new class of habitable exoplanets called Hycean worlds. Hycean planets are expected to be temperate ocean-covered worlds with H2-rich atmospheres, and thanks to their larger sizes and more extended atmospheres, compared to rocky planets of similar mass, Hycean worlds are significantly more accessible to atmospheric spectroscopy. The James Webb Space Telescope (JWST) recently observed the atmospheric spectrum of a possible Hycean world for the first time, named K2-18 b, which led to inferences of multiple carbon-bearing molecules in its atmosphere with hints of a potential biomarker.
Astrophysicist and exoplanet expert Nikku Madhusudhan will present the observations from K2-18b, and what they’ve taught us about the atmospheric composition and physical processes, along with the possibility of a habitable ocean underneath the atmosphere. Discover the latest future prospects in the search for habitable environments, and life beyond the solar system in the next decade.
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Nikku Madhusudhan is a Professor of Astrophysics and Exoplanetary Science at the Institute of Astronomy at the University of Cambridge. He is credited with pioneering atmospheric retrieval methods for determining the atmospheric properties of exoplanets using spectroscopic observations with large telescopes, besides other important developments in the understanding of the atmospheres, interiors, formation conditions, and habitability of exoplanets.
Chapters:
00:00 Why Finding Alien Life Changes Everything 🌌
00:42 Meet the Astrophysicist Behind the Discovery 👨‍🔬
01:05 6,000 Exoplanets… But Which Ones Matter? 🪐
02:10 What Makes a Planet Habitable? (Habitable Zone Explained) 🌍
03:25 Why Distance Matters: Limits of Our Telescopes 🔭
04:05 Only 10 Planets We Can Study Right Now 🎯
04:32 How Scientists Detect Alien Atmospheres 🌫️
05:05 What Are Biosignatures? Signs of Life Explained 🧬
05:40 Transmission Spectroscopy Made Simple 🌈
06:55 K2-18b: The Most Promising Planet for Life? 🪐
08:05 The Big Surprise: A “Non-Habitable” Planet Might Be Habitable 💡
09:10 Hycean Worlds: Ocean Planets with Hydrogen Atmospheres 🌊
10:05 JWST Discoveries: Methane, CO₂ & Missing Molecules 🔬
10:45 Dimethyl Sulfide (DMS): A Potential Biosignature 🧪
11:40 Are We Close to Detecting Alien Life? 🚨
12:05 What Could Life on K2-18b Actually Look Like? 🦠
12:40 The Big Unknown: Why We Must Be Careful ❓
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