An exoplanet is a planet that exists outside our solar system. The first exoplanet was confirmed in 1992, and since then, scientists have discovered about 6,000 such planets. However, researchers estimate that there could be trillions more across the universe. According to University of Chicago Professor Jacob Bean, “The amount of information we have gotten about exoplanets just in the last 10 years is incredible. We’re writing whole new chapters on exoplanets all the time.” Despite these advances, no definitive signs of life have been found yet.
Exoplanets are challenging to observe because they are far away and often too small or faint for direct imaging with telescopes. Scientists use indirect methods such as monitoring a star's light for dips when a planet passes in front of it (the transit method) or detecting slight wobbles in a star's movement caused by orbiting planets (the radial velocity method). Additional techniques include spectroscopy to analyze starlight filtered through planetary atmospheres and gravitational microlensing to find distant worlds.
University of Chicago Associate Professor Leslie Rogers noted, “If you look up at the night sky, most—if not all—of the stars you see have planets around them.” NASA’s Exoplanet Catalog has over 5,000 entries, but many more likely remain undiscovered.
Scientists prioritize finding liquid water when searching for potentially habitable exoplanets. Prof. Bean explained: “All life on Earth needs liquid water, no matter how different it looks; that’s one of the unifying principles of all life here. It’s also a good way to narrow down planet candidates, because liquid water implies the planet must be the just-right distance from its star to be warm but not boiling.”
Habitability depends on factors such as having an atmosphere and a stable star. Many discovered planets have extreme conditions that make them unsuitable for human life.
The first confirmed exoplanet was detected around a pulsar in 1992 by Aleksander Wolszczan and Dale Frail using observations of radio wave pulses affected by orbiting planets. In 1995, another technique identified 51 Pegasi b orbiting a regular star—a discovery considered pivotal in modern exoplanet research.
Exoplanets come in various forms: gas giants larger than Jupiter or Saturn; terrestrial planets like Earth made mostly of rock and metal; Neptune-like rocky worlds with hydrogen-rich atmospheres; and mini-Neptunes which are between Earth and Neptune in size but lack equivalents in our solar system.
New technologies continue to advance research. The James Webb Space Telescope has enabled significant progress following earlier missions like Hubble and Kepler. Ground-based instruments also play a role—for example, University of Chicago scientists developed MAROON-X at Hawaii’s Gemini Observatory to help identify exoplanets using precise measurements.
Planet formation remains an important area of study. Current theories suggest planets form from dust disks surrounding young stars as material clumps together over time—a process thought responsible for Earth's origin as well.
As Professor Bean said: “From what we’ve seen, it looks like star and planet formation are intrinsically linked.”
Several ongoing space missions aim to expand knowledge further—including ESA’s GAIA mission and upcoming projects like the Nancy Grace Roman Space Telescope.
Among frequently asked questions:
- The closest known exoplanets orbit Proxima Centauri at 4.2 light-years away.
- Traveling to these distant worlds would require technology far beyond current capabilities.
- While no exomoons have been definitively identified yet due to their small size, some may exist around large gas giants.
- The largest known exoplanets are gas giants even bigger than Jupiter.
- Exoplanet names typically derive from their parent stars’ survey designations followed by letters indicating discovery order.
- Although some rocky worlds similar in size to Earth have been found—such as those orbiting TRAPPIST-1—none have been confirmed as true "Earth twins."
- NASA offers citizen science programs where members of the public can help search telescope data for new discoveries.