An exemple of a diagram

Here is a diagram showing the mass distribution as a function of the semi-major axes ? (typical star-planet distance) of exoplanets.

Edges :

The first thing to understand is what defines the four edges of this figure, that is, the lower and upper values ​​of the two parameters:

  1. The lower value of the semi-major axes ? is the smallest distance of an exoplanet from its star. We see that this is approximately 10,0000 km. A planet that finds itself too close to its star will lose its atmosphere and outgas like a comet ? , then it will deform by tidal effect ? before falling into the star. This limit is therefore a physical one.
  2. The upper value of the semi-major axes ? is the greatest distance of an exoplanet from its star. This limit of the catalog is approximately 40,000 AU ? , it is defined by a difficult question: if an object is seen near a star in the sky, is it really near this star (beware of projection effects), is it orbiting around this star or is it on its way somewhere else (we only see its position at an instant)? The answers to these questions will determine if this object is linked to the star and therefore part of its gravitational system..
  3. The lower mass values ​​correspond to the least massive planets in the catalog. With observational advances, the Encyclopedia catalog lists objects with masses on the order of the lunar mass, even though most observational techniques have much higher limits. The example of the Solar System shows that there can be very small objects orbiting their star. The limit of the planetary domain is then a matter of dynamical stability, as is the definition of exoplanets.
  4. The higher mass values ​​correspond to the most massive planets. This is an editor’s choice: the catalog lists objects with less than 60 Jupiter masses. We can find some objects beyond this limit because the condition takes into account the error bar $ \Delta M $ on the knowledge of the mass $ M $. A planet is added to the catalog if $ M-\Delta M < 60 M_J $.

A preliminary remark about this diagram:

Different observational techniques occupy different spaces on the diagram. For example, imaging only detects massive planets far from their stars (often young planets). The timing method detects massive planets very close to their stars (pulsar ? or neutron star ? planets).
Planets accessible to several techniques are particularly useful thanks to cross-referenced data, such as transits and radial velocities ? , which provide insight into the density of exoplanets.
This highlights the sheer difficulty of piecing together the immense puzzle of planetary systems.

In the activity Explore the database with statistical tools other histograms/diagrams are visited.