Since the discovery of the first exoplanet in 1996, scientists have confirmed the existence of more than 4,000 planets around other stars, and hundreds more await confirmation. Which of these 4,000 planets has been discovered the most?
Search for exoplanets
Astronomers mainly use two methods for detecting exoplanets: the radial velocity method and the transit method. The radial velocity method uses the gravitational interaction between a star and a planet, along with the Doppler effect, to search for exoplanets. Stars pull on planets by gravity, but planets also exert a gravitational pull on their parent star. Thus, the planets cause their stars to wobble slightly. When a star moves, the wavelength of the light it emits changes depending on which direction it is moving relative to us. If a star moves towards us, its light shrinks and shifts to blue. If it moves away from us, its light will be stretched and redshifted. This is the Doppler effect, and by observing the Doppler shift of a star’s light, astronomers can determine if a planet is exerting a gravitational pull on the star.
The radial velocity method was the main method for detecting the earliest exoplanets. In recent years, the transit method has become the most widely used method for searching for exoplanets. The transit method is quite simple. By observing the light of a star, astronomers look for any reduction in the amount of light emitted by a passing planet. Just as the moon sometimes blocks sunlight during a solar eclipse, a planet blocks some of the star’s light as it passes in front of it. Astronomers use planet-finding telescopes, such as the Kepler space telescope, to observe distant stars and look for any changes in their light caused by a planet. While the radial velocity method has been used to search for the earliest exoplanets, the transit method has been used to search for the vast majority of known exoplanets.
Bias in the search for exoplanets
Both the radial velocity method and the transit method have an obvious error in the types of planets they typically detect. The radial velocity method tends to detect only massive gas giants that orbit close to their star, while the transit method tends to detect planets that also orbit close to their star. The vast majority of the earliest exoplanets discovered were Jupiter-sized worlds that orbited very close to their parent star, called hot Jupiters, and so for many years astronomers assumed these types of worlds were the most common type of planet. However, as telescopes began to use the transit method, it became apparent that the radial velocity method was very biased towards certain types of planets. Of the more than 4,000 known exoplanets, the most common type are planets with a mass between Earth and Neptune, worlds that could be either super-Earths or mini-Neptunes. However, even the transit method is biased, and therefore these types of planets may not actually be the most common. The current data on the most common types of planets is far from complete, and it could be years before we really know which type of planets is the most common in our galaxy. However, at the moment, the most common type of planets discovered are both super-Earths and mini-Neptunes.