As of now, no exoplanets have been directly explored by spacecraft. However, numerous exoplanets have been discovered and studied through telescopic observations and indirect methods. Here are some key milestones and methods used in the exploration and discovery of exoplanets:
1. **1992**: The first confirmed exoplanets, PSR B1257+12 b and PSR B1257+12 c, were discovered orbiting a pulsar (a type of neutron star) by Aleksander Wolszczan and Dale Frail.
2. **1995**: The first exoplanet orbiting a main-sequence star (51 Pegasi b) was discovered by Michel Mayor and Didier Queloz using the radial velocity method. This discovery marked a significant milestone in exoplanet research.
3. **Kepler Space Telescope (2009-2018)**: Launched by NASA, Kepler was designed to find Earth-sized planets orbiting other stars. It used the transit method, detecting small dips in a star's brightness caused by planets passing in front of them. Kepler discovered thousands of exoplanets, greatly expanding our knowledge of planetary systems.
4. **Transiting Exoplanet Survey Satellite (TESS)** (2018-present): Launched by NASA, TESS continues the work of Kepler, using the transit method to discover exoplanets around the nearest and brightest stars. TESS has identified numerous exoplanet candidates.
5. **Radial Velocity Method**: This technique measures the wobbling motion of stars caused by the gravitational pull of orbiting planets. Ground-based observatories, such as the HARPS (High Accuracy Radial velocity Planet Searcher) spectrograph, have used this method to confirm many exoplanets.
6. **Direct Imaging**: A few exoplanets have been directly imaged using advanced telescopes equipped with coronagraphs to block out starlight. Examples include:
- **HR 8799**: A system with four directly imaged giant planets, discovered in 2008.
- **Beta Pictoris b**: Directly imaged in 2008, orbiting the young star Beta Pictoris.
7. **Gravitational Microlensing**: This method detects exoplanets by observing the bending of light from a distant star as a planet passes in front of it. Notable discoveries include OGLE-2005-BLG-390Lb, a cold super-Earth discovered in 2005.
8. **Transit Spectroscopy**: This technique studies the atmospheres of exoplanets by analyzing the starlight passing through their atmospheres during transits. The Hubble Space Telescope and other observatories have been used for this purpose, providing insights into the atmospheric composition of several exoplanets.
While no exoplanets have been directly explored by spacecraft, these methods and missions have provided a wealth of information about the diversity, composition, and potential habitability of exoplanets beyond our Solar System. Future missions, such as the James Webb Space Telescope (launched in December 2021) and the planned LUVOIR and HabEx missions, aim to further advance our understanding of exoplanets.
