Exoplanets Explained: How Scientists Find and Study Potentially Habitable Worlds

No telescope has ever taken a close-up photograph of an Earth-like planet around another star. Instead, almost everything known about the thousands of worlds beyond our solar system comes from indirect clues: a flicker of starlight, a tiny shift in a star's spectrum, or a brief spike in brightness from a distant background star. NASA's exoplanet science division confirms the current tally at more than 6,000 planets, with thousands of additional candidates awaiting confirmation.

What counts as an exoplanet

An exoplanet is simply a planet that orbits a star other than the Sun. The first confirmed exoplanet around a Sun-like star, 51 Pegasi b, was discovered in October 1995 using the radial-velocity method, according to NASA. Three decades later, NASA's science division notes that researchers now believe there are more planets than stars in the Milky Way galaxy.

The main detection methods

Because planets are faint and sit close to much brighter stars, astronomers mostly rely on indirect signals rather than direct images. NASA's exoplanet science team lists four principal techniques, each suited to different types of planets and distances.

A fifth technique, transit spectroscopy, is not used to find planets but to study them once they are known. Starlight filtering through a planet's atmosphere during a transit leaves chemical fingerprints that telescopes such as Hubble and the James Webb Space Telescope can read, according to NASA's science division.

What makes a world 'potentially habitable'

Researchers generally focus on planets in a star's so-called habitable zone — the range of distances where temperatures could allow liquid water to persist on a planet's surface. Being in that zone does not prove a planet is habitable; it only means the question is worth asking with further observation. NASA points out that many of the rocky planets accessible to current atmosphere studies orbit relatively dim, cooler stars, because those planets produce a stronger and more detectable signal.

The K2-18b case: how a biosignature claim is tested

The clearest recent example of this search in action is K2-18b, a planet about 8.6 times the mass of Earth located roughly 120 to 124 light-years away, depending on the source. It orbits in its star's habitable zone and has been proposed as a possible 'Hycean' world — a planet covered by a liquid-water ocean under a hydrogen-rich atmosphere, according to NASA's science division.

Using James Webb Space Telescope data, a team led by Nikku Madhusudhan of the University of Cambridge's Institute of Astronomy reported chemical signatures consistent with dimethyl sulfide (DMS) and possibly dimethyl disulfide (DMDS) in K2-18b's atmosphere. On Earth, these compounds are produced almost exclusively by living organisms such as marine phytoplankton, according to the University of Cambridge.

The detection reached a statistical confidence of three sigma — meaning roughly a 0.3% chance the pattern occurred by random noise — which the Cambridge team says falls well short of the five-sigma standard (about a 0.00006% chance of a fluke) normally required to call a result an established scientific discovery. The researchers estimated that 16 to 24 additional hours of Webb observing time could reach that threshold.

Madhusudhan also urged caution, noting that unknown non-biological chemistry could in principle produce the same molecules, and that further theoretical and laboratory work is needed before any firmer claim can be made. NASA's science division echoes this, stating that a single molecule detection would not amount to a discovery of life; scientists require multiple, converging lines of evidence, and results can be complicated by how a star and planet's atmosphere change over time.

Why this search is difficult

NASA notes that confirming biosignatures can require hundreds of hours of Webb observing time for a single planet — a scarce and heavily requested resource. The signals involved are faint, the stars being studied are often small and prone to flares or variability, and any chemical fingerprint must be checked against both biological and non-biological explanations before scientists will treat it as evidence of life.

• More than 6,000 exoplanets have been confirmed, according to NASA's science division, with thousands of additional candidates awaiting confirmation
• Habitable-zone status refers to orbital distance and potential surface temperature, not proof of habitability
• A detected molecule associated with life on Earth is not, by itself, proof of life elsewhere
• Statistical confidence (sigma level) indicates how unlikely a result is to be a chance fluke, not how certain a biological explanation is
• NASA's Nancy Grace Roman Space Telescope, launched in August 2026, is expected to find around 100,000 more exoplanets, mainly via microlensing

What comes next

NASA's science division describes exoplanet characterization as an ongoing process rather than a single moment of discovery: each new observation either strengthens or weakens candidate biosignature explanations. Additional James Webb observing time for K2-18b, along with the newly launched Roman Space Telescope, are expected to add both new candidate planets and sharper atmospheric data in the coming years.