This figure shows a total of 1932 planets with masses (in Jupiter mass) over distance of the planet to its host star (semi-major axis in Astronomical Units). Different marker symbols (with different colors) stand for the method used to detect the planet. The data is taken from exoplanet.eu.
The transit method cannot derive the planetary mass. This is why I have two different categories for transits: a regular one (black circles) and 'Transit R' (yellow circles). The first category has masses measured with a different technique, usually the radial velocity method. The 'R' category, which are virtually all the small transiting planets, do not have their masses measured yet. So how do I know it then? I know the radius from the transit method and assume a density. So the masses of the yellow circles are probably incorrect to some degree; however, it gives me a rough estimate where they might lie and I can plot all planets in this graph.
One could probably give an entire lecture just on this one picture. I will only give some brief notes.
- Using different symbols illustrates nicely which types of planets are discovered by which method. E.g., the transit method is good in finding planets close to the star down to very small sizes. However, it works bad for long period planets.
- The very far out planets are all detected by direct imaging. However, they are also huge which might in many cases not really qualify them as something we would call a planet - they are just too massive and more like a thing between a planet and a star. Strangely, two close-in planets are marked as directly imaged, too. This is incorrect and caused by an error in the database. These points indicate Kepler-70 b and c which are not imaged planets. Actually, they were not even seen in transits and are, in my opinion, highly disputable.
- The smallest planet is Kepler-37 b which only is about 30 % the radius of the Earth. This means it is smaller than Mercury.
- The solar system planets do not really look like they are a part of the distribution but are located to the lower edge of the exoplanet distribution. However, it is exciting that we slowly start to see exoplanets with roughly the same size and distance than Earth. I think it cannot be emphasized enough that this information alone does not tell us much about whether the conditions on these planets are comparable to Earth at all.
- In the end I should probably point out that the most important method to determine the masses of exoplanets is the radial velocity technique (RV). You can see that it covers a large range of masses and distances. Although its results are extremely important, we should not forget that it does not give us the exact mass of the planet. The result still depends on how well we can determine the mass of the star and, which is the bigger challenge, what the inclination of the planet's orbit around the star is. The latter is in most of the cases completely unknown.
