What Is an Exoplanet?
Planets orbiting a star other than the Sun are called “exoplanets.” The first exoplanet was confirmed in the mid-1990s, and astronomers have since detected thousands of new worlds. But these planets are so distant that photographing them directly is nearly impossible, even with the most powerful telescopes — like trying to spot a firefly next to a candle from many kilometers away. That is why astronomers developed methods that measure a planet's indirect effects on its star, rather than trying to see the planet itself.
Why Is It So Difficult?
A star is millions of times brighter than any planet orbiting it. From Earth, this brightness difference completely drowns the planet in the star's glare. According to NASA, this is why most scientists detect exoplanets not by directly imaging them but by watching for small changes in the star's behavior.
The Transit Method
The most productive technique is known as the “transit method.” When a planet passes between Earth and its star, it creates a tiny but regular dimming in the star's light — much like the Moon partially blocking the Sun during a solar eclipse. Telescopes continuously monitor a star's brightness for weeks, months, or even years, searching for these repeating dips.
NASA's Kepler mission and its successor K2 confirmed more than 3,000 planets using this method. The currently active TESS satellite similarly scans bright stars near Earth, identifying candidate planets for more detailed follow-up study. The size of the dimming reveals information about the planet's size, while the interval between dips indicates how close the planet orbits its star and how long its “year” lasts.
The Radial Velocity Method
As a planet orbits its star, both actually move around a shared center of mass — meaning the star itself wobbles slightly, even if only by a tiny amount. This wobble causes a measurable shift in the color of light coming from the star: as the star moves toward us its light shifts slightly toward blue, and as it moves away, toward red. This is called the “Doppler effect,” the same physical principle behind why an ambulance siren sounds higher-pitched as it approaches and lower-pitched as it recedes.
This method was used in some of history's earliest exoplanet discoveries and is especially effective at detecting large planets orbiting close to their star. When combined with the transit method, scientists can calculate both a planet's size and its mass — and from these two figures, determine its density, revealing whether it is rocky or gaseous.
Direct Imaging and Microlensing
In some cases, astronomers can actually “see” the planet. To do this, telescopes are fitted with a type of mask called a coronagraph, which blocks the star's bright light while revealing the fainter points around it. However, this method only works for very large, young planets (and therefore still hot and relatively bright) that orbit far from their star; to date, fewer than a hundred planets have been detected this way.
Another, rarer technique called “gravitational microlensing” involves the light of a distant star being temporarily magnified by the gravity of another star-planet system passing in front of it. A small spike in this magnification curve can indicate the presence of a planet in the background system. This method has the advantage of detecting much more distant planets than other techniques, but since the same alignment never repeats, confirming the detection is difficult.
Atmospheric Analysis with James Webb
Once a planet is discovered, the burning question usually becomes: what is actually on it? This is where spectroscopy comes in. As a planet passes in front of its star, some of the starlight filters through the planet's atmosphere. This light is subtly absorbed at different wavelengths by different molecules in the atmosphere, leaving behind a kind of “chemical fingerprint.”
The James Webb Space Telescope is among the most sensitive instruments ever built for examining these traces. Thanks to it, substances like water vapor, carbon dioxide, methane, and sodium have been detected in the atmospheres of some exoplanets. One goal of future missions is to extend this method to biological signatures associated with life.
Future Missions
Most of the exoplanets discovered so far lie within a narrow patch of sky surveyed by missions like Kepler. The upcoming Nancy Grace Roman Space Telescope is designed to combine the transit and microlensing methods to scan a much wider area, likely revealing thousands of new candidates. The European Space Agency's PLATO mission similarly focuses on finding Earth-like rocky planets, particularly those located in their star's habitable zone.
The shared goal of this new generation of instruments is not just to answer “how many planets are there,” but to identify in advance which planets are worth more detailed study — especially atmospheric analysis with telescopes like James Webb. There is still a large gap between knowing a planet exists and understanding what is actually on it — and astronomy today is working precisely to close that gap.
A Well-Known Example: The TRAPPIST-1 System
To see how these methods work in practice, the TRAPPIST-1 system offers a good example. Around this cool dwarf star, roughly 40 light-years from Earth, seven rocky planets were detected using the transit method — one of the most crowded known exoplanet systems. Several of these planets lie within the star's so-called “habitable zone,” the distance range where liquid water could theoretically exist. The James Webb Space Telescope continues observing this system to determine whether these planets have atmospheres; findings so far suggest the innermost planets lack thick atmospheres, though research on the outer planets continues.
Why Does This Matter?
Exoplanet research does more than catalog distant worlds; it helps us understand how typical or exceptional our own Solar System is, how planets form, and how often conditions favorable to life might arise in the universe. Every new detection method adds one more piece of data bringing us slightly closer to answering the question: “Are we alone?”