# 3.6 Comparing theory and observation

Figure 10 shows a snapshot of the exoplanet population (as of early 2023). While being a result of the observational biases connected to the various detection techniques, the figure highlights some interesting trends, including the existence of types of planet that are not present in our Solar System. While core accretion successfully explains the bulk of the giant planet population discovered through transit, radial-velocity and microlensing techniques (orange diamonds, red squares and green triangles, respectively, in Figure 10), and disc instability may explain some of the planets discovered by direct imaging (blue triangles in Figure 10), neither scenario is able to easily explain the characteristics of all planets shown here.

Figure 10 Masses of the known exoplanets (in units of Earth mass) plotted against their orbital period in years. The different colours and shapes represent the different detection methods. Solar System planets are plotted as open circles. The dashed-dotted line marks the position of the Earth.

In particular, the core-accretion scenario struggles to explain the formation of giant planets at orbital distances larger than a few astronomical units (corresponding to orbital periods longer than a few years), because of the extended time needed to form big enough cores at these distances. On the other hand, discs are unlikely to fragment at small orbital distances from the central star, because the stellar irradiation tends to stabilise the disc by maintaining high temperature (hence high sound speed and high Q), so the Toomre and cooling criteria cannot be satisfied simultaneously. Therefore, unless planets formed by the core-accretion scenario can migrate or be scattered outward to large distances, then the directly imaged giant planets at large orbital distances must have formed on their current orbits via another mechanism, such as the disc-instability scenario. This suggests that giant-planet formation could be bimodal, with different mechanisms dominating depending on the distance from the central star.

## Activity 8

A current catalogue of known exoplanets is maintained at the website exoplanet.eu. Visit the website now, and you will see two buttons labelled ‘The catalog’ and ‘The plots’. The first of these allows you to explore the current catalogue of known exoplanets, while the second allows you to plot various planet parameters against each other. Try to produce an updated version of Figure 10 by plotting the masses of known exoplanets against their orbital periods. You can click on the axis labels to change the units in which the quantities are displayed.

OpenLearn - The formation of exoplanets
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