The ever growing number of detected exoplanets shows that small planets are far more common than larger ones. A study by Morton & Swift (2014) examines the abundance of terrestrial planets with orbital periods less than 150 days around cool stars with effective temperatures below 4,000 K (i.e. red dwarf stars). These stars make up the majority of stars in the galaxy. For comparison, the Sun has an effective temperature of 5,778 K. The study analysed data from NASA’s Kepler space telescope on exoplanets in the size range between 0.5 to 4.0 Earth radii.
Figure 1: Artist’s impression of a terrestrial planet around a cool red dwarf star.
Figure 2: Distribution of planets orbiting cool stars with orbital periods less than 150 days. The blue horizontal lines represent the standard “occurrence rate per bin” calculations. The vertical red lines represent the number of planets with a particular radius is observed. Morton & Swift (2014).
Results from the study indicate there is an average of 2.00 ± 0.45 planets between 0.5 to 4.0 Earth radii per cool star. Additionally, for planets between 0.5 to 1.5 Earth radii, there is an average of one planet per cool star. The distribution of exoplanets shows a rise with decreasing planetary radius, down to one Earth radius. Below ~0.8 Earth radii, the distribution curve appears to decrease again. If the decrease is indeed a true feature, it could mean that in the formation of terrestrial planets, only a few larger planets typically remain. As a result, planets about the size of Earth could be the most likely outcome of terrestrial planet formation.
However, the drop in abundance below ~0.8 Earth radii is most likely an artefact due to inadequate data on smaller planets since these planets are more difficult to detect (Figure 3). The true distribution is expected to keep rising below 0.5 Earth radii. The distribution of exoplanets between 0.5 to 4.0 Earth radii also indicates that planets larger than ~3 Earth radii are very rare around cool stars. Finally, estimates from the study show there are ~0.25 habitable-zone Earth-sized planets per cool star, and the number could be as high as ~0.8. This suggests that habitable-zone Earth-sized planets are ubiquitous around cool stars.
Figure 3: Discovery efficiency as a function of planet radius. Morton & Swift (2014).
Timothy D. Morton and Jonathan Swift, “The Radius Distribution of Planets around Cool Stars”, 2014 ApJ 791 10