Exoplanet

What Are Exoplanets?

An exoplanet is a planet outside our Solar System. The idea of planets beyond our own has been a fascinating concept for centuries, with early speculations dating back to the sixteenth century by Giordano Bruno and Isaac Newton. But it wasn’t until 1917 that the first evidence was noted, and in 1992, the first confirmed detection around a pulsar. As of March 2025, there are 5,849 confirmed exoplanets in 4,367 planetary systems.

How Are Exoplanets Detected?

Detection methods include transit photometry and Doppler spectroscopy, but these favor planets near the star. The first published detection was made by Canadian astronomers Bruce Campbell, G. A. H. Walker, and Stephenson Yang in 1988 for Gamma Cephei. Follow-up observations solidified the results, confirming a third planet in 1994.

Michel Mayor and Didier Queloz announced the first definitive detection of an exoplanet around a main-sequence star in 51 Pegasi on October 6, 1995. This discovery marked the beginning of the modern era of exoplanetary discovery and was recognized with a share of the 2019 Nobel Prize in Physics.

Techniques for Detection

Improved techniques allowed for the confirmation of the existence of planets orbiting Gamma Cephei in 2003. Astronomers were initially surprised by the presence of ‘hot Jupiters’ but more diverse types of exoplanets have since been discovered.

Kepler-16 contains the first discovered planet that orbits a binary main-sequence star system. On February 26, 2014, NASA announced the discovery of 715 newly verified exoplanets around 305 stars by the Kepler Space Telescope. These planets are mostly between the size of Neptune and Earth.

On July 23, 2015, NASA announced Kepler-452b, a near-Earth-size planet orbiting the habitable zone of a G2-type star. On September 6, 2018, NASA discovered Wolf 503b, an exoplanet twice the size of Earth orbiting an Orange Dwarf star.

In January 2020, scientists announced the discovery of TOI 700 d, the first Earth-sized planet in the habitable zone detected by TESS. As of January 2020, NASA’s Kepler and TESS missions had identified 4374 planetary candidates yet to be confirmed.

Characteristics of Exoplanets

The least massive exoplanet is twice the mass of the Moon, while the most massive is about 30 times that of Jupiter. Some exoplanets are far away from their stars, and there’s evidence of extragalactic planets.

Direct imaging is challenging due to the faintness of exoplanet light and glare from parent stars. Indirect methods such as transit method, radial velocity or Doppler method, and transit timing variation are used to detect exoplanets.

Planetary Systems

There is one planet on average per star, with about 1 in 5 Sun-like stars having an ‘Earth-sized’ planet in the habitable zone. Most known exoplanets orbit main-sequence stars similar to the Sun. Lower-mass stars (red dwarfs) are less likely to have massive planets detectable by current methods.

Correlations have been found between star metallicity and planetary detection, with higher metallicity stars more likely to host giant planets. Some planets orbit binary or triple star systems, while others are circumbinary, orbiting both members of a binary system.

Habitability and Atmospheres

The study of planetary habitability is ongoing to search for life beyond Earth. The IAU defines an exoplanet as an object with a mass below the deuterium fusion threshold (13 Jupiter masses), that orbits a star, brown dwarf or stellar remnant, and has a mass ratio below a certain instability point.

Direct imaging is challenging due to the faintness of exoplanet light and glare from parent stars. Indirect methods such as transit method, radial velocity or Doppler method, and transit timing variation are used to detect exoplanets.

Atmospheric Features

The first atmosphere was detected around HD 209458 b in 2001. As of February 2014, more than fifty transiting and five directly imaged exoplanet atmospheres have been observed, resulting in detection of molecular spectral features; observation of day–night temperature gradients; and constraints on vertical atmospheric structure.

Kepler-1520b is a small rocky planet, very close to its star, that is evaporating and leaving a trailing tail of cloud and dust like a comet. In June 2015, scientists reported that the atmosphere of GJ 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14 million km (9 million mi) long.

Future Prospects

The search for exoplanets continues with projects like ANDES, CoRoT, ESPRESSO, HARPS, Kepler, and TESS. The discovery of potentially habitable planets such as Kepler-62f, Kepler-186f, and Kepler-442b, with Proxima Centauri b being a notable candidate about 4.2 light-years away, brings us closer to understanding the potential for life beyond our Solar System.

Exoplanets are often part of planetary systems with multiple planets, interacting gravitationally and sometimes forming resonant systems. The study of these distant worlds continues to expand our knowledge of the universe and the possibility of extraterrestrial life.