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).
Reference:
Timothy D. Morton and Jonathan Swift, “The Radius
Distribution of Planets around Cool Stars”, 2014 ApJ 791 10