The Search for Exoplanets: How Astronomers Find Worlds Around Other Stars

Every star you see likely hosts at least one planet. The search for exoplanets has revealed thousands of worlds, some potentially habitable. Photo: Brett Sayles / Pexels

Billions of Worlds Waiting to Be Found

For most of human history, we had no idea whether planets existed beyond our solar system. We could speculate, and many scientists and philosophers did, but we had no evidence. The first confirmed detection of an exoplanet orbiting a Sun-like star came in 1995, when Swiss astronomers Michel Mayor and Didier Queloz discovered a Jupiter-mass planet orbiting the star 51 Pegasi. That single discovery opened a floodgate. As of 2025, astronomers have confirmed more than 5,700 exoplanets in over 4,300 star systems, with thousands more candidates awaiting confirmation.

The numbers suggest something profound: planets are not rare. Statistical analyses of Kepler mission data indicate that, on average, every star in our galaxy hosts at least one planet. That means the Milky Way alone contains hundreds of billions of planets, many of them rocky, and a significant fraction orbiting in the habitable zones of their stars where liquid water could exist on the surface. The question is no longer whether other worlds exist. It is what they are like and whether any of them harbor life.

How Do You Find a Planet Around a Distant Star?

Exoplanets are extraordinarily difficult to detect directly. A star like the Sun is roughly a billion times brighter than a planet like Earth, and the planet orbits so close to the star (from our distant perspective) that the star’s glare overwhelms it completely. It is like trying to see a firefly next to a searchlight from a thousand miles away. Astronomers have developed ingenious indirect methods to reveal these hidden worlds.

The Transit Method

The transit method detects planets by watching for the tiny dimming of a star’s light when a planet passes in front of it (transits). If a planet’s orbit is aligned so that it crosses our line of sight to the star, it blocks a small fraction of the star’s light, typically 0.01% to 1% depending on the planet’s size relative to the star. By measuring this precise dimming, astronomers can determine the planet’s size.

The transit method has been the most prolific planet-finding technique, largely thanks to two space missions:

Kepler (2009-2018): NASA’s Kepler space telescope stared at a single patch of sky containing about 150,000 stars for four years, watching for transits. It discovered over 2,600 confirmed planets and revealed that small, rocky planets are far more common than gas giants. Kepler fundamentally transformed our understanding of planetary systems.

TESS (2018-present): The Transiting Exoplanet Survey Satellite surveys nearly the entire sky, focusing on bright, nearby stars. Unlike Kepler, which studied distant stars, TESS finds planets around stars close enough for detailed follow-up study. TESS has already discovered thousands of planet candidates, with more being confirmed regularly.

The Radial Velocity Method

Also called the Doppler wobble method, this technique detects planets by measuring tiny shifts in a star’s spectral lines caused by the gravitational tug of an orbiting planet. As the planet orbits, it pulls the star in a small circle (or ellipse). When the star moves toward us, its light is slightly blue-shifted; when it moves away, it is red-shifted. By measuring these shifts over time, astronomers can determine the planet’s orbital period, its minimum mass, and the shape of its orbit.

This was the method used to discover 51 Pegasi b, the first exoplanet around a Sun-like star. It remains essential for measuring planet masses (transits give sizes, radial velocity gives masses, and combining both gives density, which reveals composition).

Direct Imaging

The most challenging but most visually compelling method is direct imaging: actually photographing the planet. This requires blocking the overwhelming starlight using a coronagraph or starshade and then detecting the faint reflected or emitted light from the planet. Direct imaging works best for large, young, hot planets on wide orbits far from their host stars.

The most famous directly imaged system is HR 8799, which has four massive planets that have been photographed multiple times, allowing astronomers to track their orbital motion. Direct imaging is currently limited to giant planets, but future telescopes may be able to image Earth-sized worlds.

Gravitational Microlensing

Gravitational microlensing occurs when a foreground star with planets passes in front of a more distant background star. The foreground star’s gravity bends the background star’s light, acting as a natural magnifying glass. If the foreground star has a planet, the planet creates a brief, distinctive spike in the magnified light curve. This method can detect planets at enormous distances and is sensitive to planet masses as small as Earth, but each event is one-time and unrepeatable.

The Diversity of Exoplanets

One of the biggest surprises of exoplanet research is the sheer diversity of worlds we have found. Our solar system, with small rocky planets close to the Sun and gas giants farther out, turns out to be just one possible arrangement among many.

Hot Jupiters: Gas giant planets orbiting extremely close to their stars, with orbital periods of just a few days. These were the first exoplanets discovered because they are the easiest to detect (large size, strong Doppler signal). 51 Pegasi b is a hot Jupiter. Their existence was a complete surprise since there is no equivalent in our solar system, and they cannot have formed where they currently orbit. They must have migrated inward from more distant formation locations.

Super-Earths: Rocky or icy planets with masses between about 1.5 and 10 times Earth’s mass. There is nothing like them in our solar system, but they are among the most common planet types in the galaxy. Some may have thick atmospheres and deep oceans.

Mini-Neptunes: Planets roughly 2 to 4 times Earth’s radius with thick hydrogen-helium atmospheres. They are extremely common, perhaps the most common type of planet in the galaxy, yet our solar system has none.

Rogue planets: Planets that have been ejected from their star systems and wander through interstellar space. Gravitational microlensing surveys suggest there may be billions of rogue planets in the Milky Way.

The Habitable Zone and the Search for Life

The habitable zone (sometimes called the Goldilocks zone) is the range of distances from a star where conditions could allow liquid water to exist on a planet’s surface, not too hot and not too cold. The boundaries depend on the star’s luminosity and the planet’s atmosphere.

Several potentially habitable exoplanets have been identified:

TRAPPIST-1 system: Seven roughly Earth-sized planets orbiting an ultracool red dwarf star just 40 light-years away. Three of them (TRAPPIST-1e, f, and g) orbit within the habitable zone. The system is one of the primary targets for atmospheric characterization by JWST.

Proxima Centauri b: A roughly Earth-mass planet orbiting the nearest star to the Sun, just 4.24 light-years away, within its habitable zone. However, Proxima Centauri is an active red dwarf that produces intense flares, which may strip away any atmosphere the planet might have.

Kepler-442b: A super-Earth orbiting a K-type star about 1,200 light-years away, considered one of the most Earth-like exoplanets found by Kepler based on its size and orbit.

JWST: Reading Exoplanet Atmospheres

The James Webb Space Telescope is revolutionizing exoplanet science by characterizing the atmospheres of transiting planets. When a planet transits its star, some starlight filters through the planet’s atmosphere, and different atmospheric gases absorb specific wavelengths. By comparing the star’s spectrum during and outside transit, JWST can identify atmospheric components.

JWST has already detected carbon dioxide, water vapor, and sulfur dioxide in the atmospheres of several hot gas giant exoplanets. The ultimate goal is to detect atmospheric biosignatures, gases like oxygen, ozone, and methane, that could indicate biological processes on rocky worlds in habitable zones. This is at the very edge of JWST’s capabilities for the nearest targets, but it represents the beginning of a search that will define astronomy for decades to come.

The Drake Equation and the Big Question

In 1961, astronomer Frank Drake formulated an equation to estimate the number of communicating civilizations in the galaxy. The Drake Equation multiplies factors like the rate of star formation, the fraction of stars with planets, the fraction of planets that could support life, and the probability that life develops intelligence and technology. When Drake first wrote it, most of these factors were unknown. Today, thanks to exoplanet discoveries, we know that the first few terms are very favorable: most stars have planets, and many of those planets are in habitable zones.

The remaining unknown factors, particularly the probability that life develops and persists, are still completely open questions. But the sheer number of potentially habitable worlds in our galaxy (estimated at tens of billions) makes the question of whether we are alone one of the most compelling in all of science.

What Comes Next

The search for exoplanets is accelerating. ESA’s PLATO mission, planned for launch in 2026, will search for transiting Earth-like planets around Sun-like stars with unprecedented precision. Future concepts like the Habitable Worlds Observatory would use advanced coronagraphs to directly image and characterize Earth-like planets around nearby Sun-like stars.

We are living through the transition from knowing that exoplanets exist to understanding what they are like and whether any of them harbor life. The first generation of exoplanet discoverers found thousands of new worlds. The next generation will study them in detail. And perhaps, if we are fortunate, one of those studies will reveal something that changes everything: evidence that we are not alone in the universe.

The search continues. Every star you see tonight likely has its own family of worlds. Pair your exoplanet knowledge with our guide on gravitational wave astronomy for a fuller picture of how we study the cosmos today. The question of what is on those worlds is one of the deepest questions humanity has ever asked, and we are closer to answering it than ever before.