How Astronomers Find Exoplanets and Judge Which Ones Could Support Life

Confirming a planet around a distant star takes indirect evidence almost every time, because the star's glare drowns out the planet's own faint light. NASA's science division lists four main detection techniques in current use: the transit method, radial velocity, gravitational microlensing, and direct imaging, each suited to different kinds of planets and distances.

Four ways to spot a planet you cannot see

The transit method looks for a small, regular dip in a star's brightness when a planet passes in front of it. NASA's Kepler Space Telescope, which operated from 2009 to 2018, watched about 150,000 stars in one patch of sky for four years using this technique; NASA's Transiting Exoplanet Survey Satellite (TESS) has continued the approach since its 2018 launch and, as of early October 2026, has contributed to more than 1,000 confirmed planets, according to the NASA Exoplanet Archive maintained by Caltech's IPAC.

Radial velocity, sometimes called the wobble method, measures tiny shifts in a star's light spectrum caused by the gravitational tug of an orbiting planet. NASA notes this technique revealed the first confirmed exoplanet around a Sun-like star, 51 Pegasi b, in 1995.

Gravitational microlensing relies on Einstein's prediction that gravity bends light: when a star with a planet passes in front of a more distant background star, the foreground system briefly magnifies the background star's light in a distinctive pattern. NASA's planned Nancy Grace Roman Space Telescope is designed to use this method to find planets far from their stars.

Direct imaging captures light from the planet itself rather than inferring its presence from effects on starlight. It mainly works for young, self-luminous gas giants far from their stars; NASA cites the James Webb Space Telescope's direct image of HIP 65426 b across several infrared wavelengths as a recent example.

Habitable zone: necessary, not sufficient

Once a planet is confirmed, researchers check whether it sits in its star's habitable zone, the range of orbital distances where surface temperatures could allow liquid water, given the right atmospheric pressure. NASA describes this as varying by star: hotter stars have wider, more distant habitable zones, while small, dim red dwarf stars have narrow zones close in.

Being in the habitable zone does not guarantee habitability. NASA points to TRAPPIST-1, a red dwarf system with seven Earth-sized rocky planets, several within its habitable zone; James Webb Space Telescope observations nonetheless indicate that TRAPPIST-1 d appears to have no detectable atmosphere, which NASA's own materials cite as evidence that location alone is not enough. Star type matters too: NASA notes that planets around M-dwarf red dwarfs, the most common stars in the galaxy, face elevated X-ray and ultraviolet radiation that can strip away atmospheres, while K-dwarf stars are considered by some researchers to offer longer-lived, more stable conditions.

Reading atmospheres: the biosignature problem

For planets that transit their star, astronomers can use transmission spectroscopy: starlight filtering through the planet's atmosphere during a transit is absorbed at specific wavelengths by specific gases, producing a spectral pattern researchers compare to a barcode.

The sub-Neptune planet K2-18b illustrates how contested these readings can be. A team led by Nikku Madhusudhan at the University of Cambridge first reported a weak, low-confidence hint of dimethyl sulfide (DMS) — a gas mainly produced by marine life on Earth — using Webb's NIRISS and NIRSpec instruments in 2023, according to EarthSky. In 2025, the same team reported a statistically stronger signal of DMS or the related compound dimethyl disulfide, using Webb's MIRI instrument, which covers a different wavelength range than the earlier observations, according to EarthSky.

The claim drew swift pushback. Planetary scientist Sarah Hörst and exoplanet researchers Chris Lintott and Ryan MacDonald argued that DMS can form through non-biological chemistry and that a single molecule cannot establish life, EarthSky reported. Madhusudhan himself has said researchers must remain "deeply skeptical" of their own results, according to EarthSky.

What comes next

The European Space Agency's PLATO mission, built with contributions from Germany's national aerospace research center DLR, is scheduled to launch in late 2026 on an Ariane 6 rocket to a Sun-Earth Lagrange point about 1.5 million kilometers from Earth. PLATO will use 26 cameras to monitor at least 200,000 stars for transiting terrestrial planets around Sun-like stars, specifically targeting Earth-sized worlds in their habitable zones, according to DLR.

As of October 8, 2026, the NASA Exoplanet Archive listed 6,445 confirmed exoplanets; that week's update also marked the point at which data from NASA's TESS mission alone had contributed to over 1,000 confirmed planets, according to the archive maintained by Caltech's IPAC.