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The formation of exoplanets
The formation of exoplanets

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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.

Described image
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.
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The figure illustrates the masses of exoplanets plotted against their orbital periods as a graph. The horizontal axis is labelled ‘orbital period (in units of years)’, and ranges from 10 raised to the negative fourth power to 10 raised to the fourth power in increments of 100 units. The vertical axis is labelled ‘mass (in units of Earth Masses)’, and ranges from 10 raised to the negative second power to 10 raised to the fifth power in multiples of 10 units. In the graph, various types of entities are plotted as clusters of points. Solar System planets are plotted as white circles. Exoplanets detected via different methods are also shown. Those from the radial-velocity method are shown as a red squares, the transit method are yellow diamonds, the microlensing method are green triangles, direct imaging are blue triangles and the astrometry method are dark-blue hexagons. A dashed horizontal line is drawn from 10 raised to the zeroth power on the vertical axis. A dashed vertical line is drawn from 10 raised to the zeroth power on the horizontal axis at. At the intersection of these two lines, a white circle labelled ‘Earth’ is shown. The other planets of the Solar System are shown at the following position on the graph: Mercury at approximately (10 raised to the negative first power, 10 raised to the negative 1.2 power); Venus at approximately (10 raised to the negative 0.2 power, 10 raised to the negative 0.9 power); Mars at approximately (10 raised to the 0.3 power, 10 raised to the negative first power); Jupiter at approximately (10 raised to the 1.1 power, 10 raised to the 2.5 power); Saturn at approximately (10 raised to the 1.5 power, 10 squared); Uranus at approximately (10 raised to the 1.8 power, 10 raised to the first power); and Neptune at approximately (10 squared, 10 raised to the first power). A cluster of red squares is shown between approximately (10 raised to the negative second power, 10 raised to the zeroth power) to approximately (10 raised to the negative second power, 10 raised to the fourth power) and approximately (10 squared, 10 raised to the fourth power). A cluster of yellow diamonds is shown to the left of the dashed vertical line. A cluster of green triangles is shown to the right of the dashed vertical line. A cluster of blue triangles is shown between approximately 10 raised to the zeroth power and 10 raised to the fourth power on the horizontal axis. A small cluster of dark-blue hexagons is shown between (10 raised to the zeroth power, 10 raised to the third power) and (10 raised to the second power, and 10 raised to the fourth power).

Figure 10 Masses of the known exoplanets (in units of Earth mass) plotted against their orbital period in years. The different colours ...

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 [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] . 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.