What Is The Most Common Type Of Planet?

Since the discovery of the first exoplanet in 1996, scientists have confirmed the existence of over 4,000 planets around other stars, with hundreds more awaiting confirmation. Of those 4,000 planets, what is the most common type discovered?

Finding Exoplanets

Radial velocity method
Visual illustration of the radial velocity method 

Astronomers primarily 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 planet, along with the Doppler Effect, to find exoplanets. Stars pull on planets through gravity, yet planets also exert a gravitational force on their parent star. Thus, planets cause their stars to wobble ever so slightly. As a star moves, the wavelength of the light it emits changes depending on what direction it’s moving relative to us. If a star is moving towards us, its light is compressed and blue-shifted. If it is moving away from us, its light will be stretched and red-shifted. This is the Doppler Effect, and by observing the Doppler shift of a star’s light, astronomers can determine whether or not a planet is exerting a gravitational force on the star. 

The radial velocity method was the primary method for detecting the earliest exoplanets. In recent years, the transit method has become the most widely used way to find exoplanets. The transit method is rather simple. By observing a star’s light, astronomers look for any reduction in the amount of light being emitted by a passing planet. Like how the moon will occasionally block the sun’s light during a solar eclipse, a planet will block some of a star’s light as it passes in front of it. Astronomers use planet-hunting telescopes, such as the Kepler Space Telescope, to observe distant stars and look for any changes in their light that is caused by a planet. Although the radial velocity method was used to find the earliest exoplanets, the transit method has been used to find the vast majority of known exoplanets. 

Bias In Finding Exoplanets

Exoplanets Discovered
The total number of exoplanet types discovered as of December 14, 2017. Image credit: NASA

The radial velocity method and the transit method both have some obvious bias in the type of planets they generally detect. The radial velocity method tends to only detect 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 found were all Jupiter-sized worlds that orbit exceedingly close to their parent star, called hot-Jupiters, and thus for years astronomers assumed that these types of worlds were the most common type of planet. When telescopes began using the transit method, however, it became obvious that the radial velocity method was very biased towards specific types of planets. Of the more than 4,000 known exoplanets, the most common type are planets in between the mass of Earth and Neptune, worlds that can either be a super-Earth or a mini-Neptune. However, even the transit method is biased, and so these types of planets may in fact not be the most common. Current data on the most common type of planet is nowhere near complete, and it may still be years before we truly know which type of planet is the most common in our galaxy. For now, however, the most common type of planets discovered are both super-Earths and mini-Neptunes. 

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