In Search of Other Worlds: What Is an Exoplanet?

Imagine a planet where molten diamond rains from the sky, hurled sideways by winds that scream at thousands of miles per hour. Another world swelters in an orbit so close to its star that its surface is scorched bare, stripped of any atmosphere, like a real-life Crematoria. These aren’t scenes from science fiction—they’re real planets orbiting stars far beyond our own.

These distant worlds are called exoplanets: planets that exist outside our solar system. Most of the exoplanets scientists have discovered so far orbit stars within the Milky Way, but some lie even farther afield, seen only in fleeting glimpses with the help of powerful space telescopes.

Exoplanets come in all shapes and sizes. Some are familiar enough to earn nicknames like super-Earth, hot Jupiter, and mini-Neptune. Others are so bizarre they defy easy classification. Scientists have found gas giants so improbably huge they should have ignited as stars, but they didn’t, and we don’t know why. There are icy planets locked in perpetual winter, water worlds with hydrogen skies, rocky little planets with twin or triple suns, and “rogue” exoplanets drifting through interstellar space without any sun at all. Astronomers have discovered planets covered in ice from pole to pole and planets so close to their star that they’re baked to a crisp, with no atmosphere to speak of. Our next-door neighbor, Proxima Centauri—the nearest star system to Earth, at just over four light-years away—hosts at least two confirmed exoplanets, including one that might be rocky and temperate enough to support life.

But how do we know what these planets are like when we can’t visit them or collect samples? When studying planets in our own solar system, we can collect physical samples. To study exoplanets, instead of sending spacecraft, scientists use light—captured, split, and analyzed—to decipher the secrets of these alien worlds from afar.

How Are Exoplanets Found?

The most frequently used exoplanet-hunting method is transit photometry, or just the transit method for short. Telescopes watch for tiny dips in a star’s brightness—caused when a planet that orbits “edge-on” with respect to Earth crosses in front of its parent star. When it does, it dims the star’s light ever so slightly, allowing scientists to measure the difference with telescopes. NASA’s Kepler Space Telescope revolutionized this approach, confirming over 2,700 exoplanets during its mission. Its successor, TESS (Transiting Exoplanet Survey Satellite), continues the hunt, scanning nearly the entire sky for planetary candidates.

In 2001, astronomers using the Hubble Space Telescope announced the first confirmed detection of an exoplanet’s atmosphere. About 150 light-years away, a yellow, Sun-like star called HD 209458 is visible through amateur telescopes, dubbed HD 209458. That star has an exoplanet, a gas giant, and here’s how we know: As the planet passed in front of its star, the starlight had to filter through its atmosphere to reach us. Traces of sodium in its nitrogen-rich atmosphere left their telltale mark on the yellow star’s light.

Not all systems are aligned edge-on. For off-axis star systems, astronomers use the radial velocity method, also known as the “wobble” method. A planet’s gravity tugs on its star, making the star wobble slightly in space. (This happens in our own solar system. Jupiter is so massive that it drags the Sun around, detectably, in a little circle whose diameter is just larger than the Sun’s actual diameter.)

The radial velocity method is best for finding exoplanets that are relatively close by because it requires us to be able to resolve relatively fine features on the star. For this method, astronomers need to see more detail than just a single pixel of light. The way planets tug on their stars doesn’t just move the star physically around a common center of gravity; it also subtly affects the speed at which the star spins. Just like the rotation of our Sun, one side of a spinning star will be approaching Earth, while the other will be rotating away from us. Consequently, light from different regions of a star will be Doppler shifted toward the blue or the red, depending on whether the star is coming toward us or moving away. Gravitational pull from an orbiting planet changes the radial velocity of the star’s rotation. Telescopes like Webb can detect this motion and use it to infer the presence of a planet.

Exoplanets also reflect light that’s Doppler shifted according to their own rotation. That’s how we know LP 791-18 d, a rocky planet just smaller than Earth, is tidally locked—with one side permanently facing toward its star and the other forever in darkness. One hemisphere is a frozen wasteland, and the other is a searing hellscape.

Finally, there’s direct imaging, which captures actual pictures of exoplanets. In 2013, scientists using the Pan-STARRS telescope directly imaged PSO J318.5-22, a rogue planet just 80 light-years away. It doesn’t orbit a star—it just drifts through the “Big Empty” of interstellar space. (All nightlife, all the time.) And yet, this inhospitable world has observable weather, raining down droplets of liquid metal from clouds of iron vapor. Where did it come from? How does it stay so hot?

We don’t know. Yet.

How We Study Exoplanets

Telescopes use optical lenses to capture light from the targets they observe. Some modern telescopes, like the JWST and the upcoming Nancy Grace Roman telescope, record their sky views with a grid of sensors called a charge-coupled device (CCD). (The same sensor technology powers the digital camera on NASA’s Ingenuity Mars helicopter and the one in your phone.)

Scientists turn to spectroscopy to learn what exoplanets are made of. Spectroscopes, like the NIRSpec on Webb, split incoming light into a rainbow of wavelengths, revealing a spectral fingerprint. Different molecules absorb and reflect light in unique ways, so we can identify gases like nitrogen, carbon dioxide, or methane—sometimes from hundreds of light-years away.

Spectroscopy also helps us estimate surface temperatures. Many molecules behave differently at different pressures and temperatures. Molecules behave differently depending on pressure and heat: Nitrogen gas reflects light differently than nitrogen ice, which in turn differs from carbon dioxide ice (“dry ice”). These subtle shifts let us distinguish between icy comets and rocky exoplanets, even when they’re just a few pixels wide.

The way a planet makes its parent star wobble also says something about the planet’s mass. A planet’s gravitational pull causes its star to wobble, especially if the planet is massive or orbits close in. These “hot Jupiters“ leave a detectable signature in the star’s motion and light.

By combining a planet’s mass (from its wobble) and size (from its transit), we can calculate its density—a clue to its composition. Rocky planets like Earth are dense; gas giants like Neptune are not. How do we distinguish between a cold super-Earth and a warm mini-Neptune? Neptune, an ice giant (a cold gas giant with a rocky, icy core), has an average density not unlike a Slurpee. Earth, in contrast, is a rocky planet with a density like a tumbled river rock—more than five times the density of water. That’s how we tell a cold super-Earth from a warm mini-Neptune, even if we’ve never seen them up close.

Spotlight: Proxima b

The closest known exoplanet to Earth is Proxima b, orbiting Proxima Centauri, a red dwarf star just over four light-years away. It’s part of the Alpha Centauri system: a trio of stars that includes Alpha Centauri A and B, both Sun-like, and Proxima Centauri, the smallest and faintest of the three. Proxima Centauri has at least two confirmed exoplanets, but Proxima b is of great interest to researchers because it lies in the star’s habitable zone—the orbital sweet spot where temperatures might allow liquid water to exist on a planet’s surface. Scientists sometimes call this the Goldilocks zone: not too hot, not too cold. Just right. And where there’s water, there might be life.

But there’s a catch. Proxima Centauri is a red dwarf, a type of star prone to violent flares and bursts of radiation. These outbursts—X-rays, gamma rays, even low-energy UV—could strip away a planet’s atmosphere over time, leaving it exposed and barren. Unlike the Sun, a relatively even-tempered star midway through its life, red dwarf stars may have a habitable zone that’s too close to the star to be safe from its radiation.

Proxima b is too far away for us to visit on human timescales, at least with the technology currently available to us. (Sadly, no warp drives just yet.) For a sense of scale, it would take the Voyager probes about 75,000 years to make it to Proxima b. Undaunted, projects like Breakthrough Listen and Breakthrough Starshot are research initiatives that aim to study Proxima b with telescopes, and eventually launch ultra-fast probes in hopes of one day taking direct and detailed readings from the exoplanet’s surface. One day, we may have the most direct evidence we can get: observations by human visitors with boots on the ground. One small step for man; one giant leap for mankind.

Recent Exoplanet Research Highlights

Here are a few of our favorite exoplanet research advances from the past year:

• Over 1,000 atmospheric spectra now catalogued: The Exoplanet Archive surpassed a major milestone in 2025, with more than 1,000 transmission spectra available for comparative atmospheric analysis.

• Proxima Centauri d confirmed: A newly discovered planet in the Alpha Centauri system was confirmed in July 2025. Proxima Cen d joins the growing family of nearby exoplanets discovered via the ESO Near InfraRed Planet Searcher (NIRPS).

• Signs of a gas giant in Alpha Centauri A’s habitable zone: Using mid-infrared imaging, researchers spotted a candidate planet orbiting Alpha Centauri A. While it’s likely a gas giant, its location hints at the potential for smaller, rocky companions nearby.

And although it happened in January of last year, here’s our honorable mention: Astronomers observed a small exoplanet, HD 63433 d, shedding its atmosphere in a dramatic comet-like tail of gas, offering a rare chance to explore how planetary atmospheres evolve under intense stellar radiation.

Exoplanets by the Numbers

Every confirmed world adds a new data point to our understanding of how common exoplanets really are, how varied these distant worlds can be, and how many of them might be capable of supporting life.

• As of July 2025, 5,983 exoplanets had been confirmed. 2,784 of them have been confirmed by Kepler alone, with another 1,687 confirmed by TESS.

• Of those nearly six thousand planets, 180 are thought to orbit in their parent star’s habitable zone.

• For every confirmed exoplanet, there’s another point of light in the sky that has been identified as a candidate exoplanet. Between Kepler and TESS, 7,634 candidate exoplanets await confirmation.

From molten skies to frozen wastelands, twin suns to nightbound rogues, the universe of exoplanets is as wild as it is vast. In just a few decades, we’ve gone from wondering if other planets exist at all to having a confirmed catalogue of thousands—and learning what they’re made of, how they behave, and whether they might host life. With new telescopes coming online and fresh discoveries arriving almost weekly, the search is only accelerating. Each new world we find is more than a point of light somewhere in the great darkness. It’s a clue, a challenge, and a reminder that our solar system is just one chapter in a much bigger story.