An exoplanet is any planet beyond our solar system. Most orbit other stars, but some drift through space untethered to any star at all. NASA has confirmed more than 6,000 exoplanets so far, out of the billions believed to exist in our galaxy alone. That number keeps climbing as telescopes get more powerful and detection methods improve.
How Astronomers Find Exoplanets
Exoplanets are extraordinarily difficult to see. Stars are billions of times brighter than the planets orbiting them, so astronomers have developed clever indirect methods to detect worlds they can’t photograph.
The most productive technique is the transit method. When a planet crosses in front of its host star from our perspective, it blocks a tiny fraction of the star’s light. Telescopes measure that periodic dip in brightness, and from it, scientists can calculate the planet’s size and how long its orbit takes. This method works best for planets orbiting close to their stars, since those cross our line of sight more frequently and block a larger proportion of light relative to their distance.
The second major technique is the wobble method, also called radial velocity. A planet’s gravity tugs on its star as it orbits, causing the star to move slightly toward and away from us in a repeating cycle. That motion squeezes and stretches the wavelengths of the star’s light in a detectable pattern. The size of the wobble reveals the planet’s mass, while the timing reveals its orbital period. Heavier planets produce a stronger signal, so this method initially favored the discovery of gas giants.
For planets that don’t orbit any star, neither technique works. The only way to find these “rogue planets” is through gravitational microlensing. When a free-floating planet drifts between Earth and a distant background star, its gravity bends and magnifies the background star’s light like a lens. The brief brightening alerts astronomers that something massive passed through. Calculating the rogue planet’s actual mass from this signal is tricky, though, because the same pattern of magnification can result from different combinations of mass and distance.
Types of Exoplanets
Exoplanets come in a striking range of sizes and compositions, and scientists group them into four broad categories.
- Terrestrial planets are Earth-sized or smaller, made of rock, silicate, water, or carbon. These are the worlds most likely to resemble our own.
- Super-Earths are more massive than Earth but lighter than Neptune, typically 2 to 10 times Earth’s mass. They can be rocky, gaseous, or a mix of both. Interestingly, nothing like them exists in our own solar system, yet they appear to be common elsewhere. Data from NASA’s Kepler spacecraft showed a curious gap: planets between 1.5 and 2 times Earth’s diameter are surprisingly rare, suggesting something about planet formation prevents worlds from settling at that particular size.
- Neptunian planets are similar in size to Neptune or Uranus and are typically wrapped in thick hydrogen and helium atmospheres with possible rocky cores.
- Gas giants range from Saturn-sized to far larger than Jupiter. Some orbit scorchingly close to their stars and are called “hot Jupiters,” a type of planet that was among the first ever discovered because their size and proximity to their stars make them easy to detect.
The Habitable Zone
The habitable zone is the distance from a star where liquid water could exist on a planet’s surface. It’s sometimes called the “Goldilocks zone” because conditions there are not too hot and not too cold. Every star has one, but its location and width depend on the star’s size and brightness. Hotter, more luminous stars have wider habitable zones that sit farther out. Smaller, dimmer red dwarfs have much tighter habitable zones very close in, as seen in the TRAPPIST-1 system, where seven rocky planets are packed into orbits smaller than Mercury’s.
Being in the habitable zone doesn’t guarantee a planet is livable. A world still needs the right atmospheric pressure, composition, and magnetic protection to keep liquid water stable on its surface. But the habitable zone is the starting point for identifying the most promising candidates in the search for life.
Some astronomers consider a middle class of stars called K dwarfs to be the true Goldilocks stars. They’re dimmer than our Sun but brighter and more stable than red dwarfs, and they live for tens of billions of years. That long, steady energy output gives life more time to develop on any planets in their habitable zones.
The Nearest Known Exoplanet
The closest confirmed exoplanet is Proxima Centauri b, orbiting the nearest star to our Sun at about 4 light-years away. It completes one orbit every 11.2 days, placing it far closer to its star than Mercury is to ours. Because Proxima Centauri is a dim red dwarf, though, that tight orbit actually puts the planet near the edge of the habitable zone. Whether it truly has conditions suitable for liquid water remains an open question, largely because red dwarfs are prone to intense stellar flares that could strip away a planet’s atmosphere over time.
Studying Exoplanet Atmospheres
Knowing a planet exists is one thing. Understanding what it’s actually like requires studying its atmosphere, and that’s where the James Webb Space Telescope has made major progress. In 2024, Webb captured the strongest evidence yet for a thick atmosphere on a rocky exoplanet, a super-Earth called TOI-561 b. This world orbits so close to its star that its surface is a global ocean of molten rock, but Webb’s measurements showed its dayside temperature was around 3,200°F, far cooler than the roughly 4,600°F expected if the planet had no atmosphere. The difference points to a thick blanket of gases, possibly rich in water vapor, absorbing and redistributing heat. Researchers described it as essentially a “wet lava ball,” far more volatile-rich than Earth.
Webb detects atmospheres by splitting a planet’s light into a spectrum. Different gases absorb specific wavelengths, leaving telltale fingerprints. Water vapor, carbon dioxide, and methane each leave a distinct signature, letting scientists inventory a distant world’s air from trillions of miles away.
Directly Photographing Exoplanets
Most exoplanets are found through indirect clues, but a next-generation technology called a coronagraph aims to photograph them directly. A coronagraph is a system of masks, prisms, detectors, and self-flexing mirrors designed to block out a star’s overwhelming glare so that the faint light of orbiting planets becomes visible. NASA’s upcoming Roman Space Telescope will carry one that is expected to be 100 to 1,000 times more capable than any coronagraph previously flown in space.
The challenge is almost absurdly precise. Inside the instrument, thousands of tiny actuators push and pull on deformable mirrors in real time, correcting for optical imperfections smaller than the width of a strand of DNA. These mirrors work alongside specialized masks that suppress diffraction, the bending of light waves around edges inside the instrument. Roman’s first targets will be Jupiter-sized planets around Sun-like stars, but the technology it demonstrates could eventually lead to instruments capable of imaging smaller, rockier worlds.
How Exoplanets Get Their Names
Most exoplanets carry alphanumeric designations based on the star they orbit. The first planet found around a star gets the letter “b” appended to the star’s catalog name (for example, Proxima Centauri b), with subsequent planets labeled “c,” “d,” and so on in order of discovery. These scientific designations are never replaced, but the International Astronomical Union periodically runs public naming campaigns where people can propose common names for select exoplanets and their host stars, following a common theme for each pair.