As of October 2023, astronomers have identified over 5,500 exoplanets. Data indicates that there are more planets than stars in the galaxy, with an average of at least 1.1 planets per star. While gas giants are more common around stars rich in elements heavier than helium, Earth-sized planets appear to be present around every star on average.
Current detection methods include radial velocity, transit photometry, and direct imaging. The radial velocity method, which led to the 2019 Nobel Prize in Physics, identifies massive planets with small orbits. Transit missions like NASA's Kepler have provided data on thousands of worlds, while direct imaging remains limited to young, hot gas giants.
Researchers estimate that approximately 1% of exoplanets are potentially habitable. However, identifying biosignatures remains a significant challenge due to the vast diversity of planetary conditions. Future missions, such as the NASA Roman observatory scheduled for 2027, aim to improve the detection of mature gas giants and potential exomoons.
The diversity of discovered systems includes types absent from our solar system, such as 'hot Jupiters' and planets with masses between those of Earth and Neptune. Additionally, astronomers have identified rogue planets, such as those found in the Orion Nebula, which do not orbit a host star.
