Saturday, July 18, 2015

Multi-planet systems with at least 3 (

This plot shows the 156 exoplanet systems with at least three planets known to date according to Additionally, the solar system is drawn at the very beginning. The x-axis gives the distance of the planet to its host star (in Astronomical Units); note that it is a logarithmic scale, so planets a little bit further to the right are actually much further away from the star. The thick green line illustrates the distance of the Earth from the Sun. To the left of each system its name is given.

The orange circles at a distance of (almost) zero illustrate the size of the star relative to the Sun, so it's the star's radius compared to the Sun's radius, which is shown at the upper left. All the other circles (green, yellow, pink) to the right of the orange stars indicate the planets, and their sizes are not in proportion to the stellar sizes. The relative planet sizes are only correct when comparing planets but not in relation to the stars. The scale of the planets was arbitrarily chosen so that the large planets are not too big and the small planets are still visible.

Now we have again the complication that planets are detected using different methods. Some of these systems were found with the transit method, others were detected with the RV technique. For the latter we usually only have the masses but not the radii; these systems are colored pink and the circle sizes correspond to the masses. The other systems have known radii derived from transits and they have yellow colors. To be able to roughly compare masses and radii with each other, I again assume a density: I define that all planets with one Jupiter radius must have the mass of Jupiter. Although this probably makes good sense for large planets, it most likely is pretty wrong in the case of small (rocky) planets. One should keep this is mind when comparing pink and yellow circles.

On the right side of the panel the number of planets in the system is given. The solar system is still the one with the most planets known but this will certainly change in the near future. Right below it you can already see one system with seven and four systems with six planets.

I do not want to go into detail on individual systems here, but rather give some general remarks:

The fact that most RV systems have huge planets compared to the transiting ones is not rooted in the way I present it; the RV technique is not yet capable of measuring the masses of the smaller planets detected with transits. You can also see that in my post on detection methods.

Most exoplanets are closer to the star than the solar planets; exoplanets with orbital periods of years and decades, which would be far out in a system, are very tough to detect.

Finally, the smallest planets usually orbit stars smaller than the sun. This again is caused by the transit and RV method, which depend of the relative sizes or masses of host star and planet. Thus, the conclusion that less massive stars have less massive planets is tricky because for the more massive stars they are much harder to find.

In upcoming posts I will present diagrams of - and talk in detail about - several systems shown here individually. As you might already suspect, all of these systems are highly interesting.

(Link to a hi-res pdf version of this diagram.)

Addendum: In this diagram I only considered planets from transits or RVs. This is why HR 8799 (direct imaging) and PSR 1257 12 (timing) are not included. Unfortunately, for quite a number of shown systems planets are missing; this is due to the database not providing the semi-major axis (distance) of these planets.