Of course, these numbers don’t necessarily mean anything. There are large errors if a person has a common name or alternative spellings. However, it’s interesting to see that there is indeed a correlation (which is labeled main sequence in the plot).

Update: I’ve made an improved version in which the google searches include the words “exoplanet”, “astronomy” and “astrophysics”. The sum of the number of results is used in the plot. This should avoid most of the issues for people with common names. You can download the scripts to create these plots here and fine tune all parameters.

If you feel that someone is missing from the list let me know and I’ll add him/her. If you’re unhappy with your score, please send me bribes in forms of cakes or iPhones.

PS: Stephen, beware! I’m heading towards you!

Exoplanet HR Diagram (follow link for an svg version)

(original version)

Hanno Rein, John C. B. Papaloizou

Many small moonlets, creating propeller structures, have been found in Saturn’s rings by the Cassini spacecraft. We study the dynamical evolution of such 20-50m sized bodies which are embedded in Saturn’s rings. We estimate the importance of various interaction processes with the ring particles on the moonlet’s eccentricity and semi-major axis analytically. For low ring surface densities, the main effects on the evolution of the eccentricity and the semi-major axis are found to be due to collisions and the gravitational interaction with particles in the vicinity of the moonlet. For large surface densities, the gravitational interaction with self-gravitating wakes becomes important.
We also perform realistic three dimensional, collisional N-body simulations with up to a quarter of a million particles. A new set of pseudo shear periodic boundary conditions is used which reduces the computational costs by an order of magnitude compared to previous studies. Our analytic estimates are confirmed to within a factor of two.

On short timescales the evolution is always dominated by stochastic effects caused by collisions and gravitational interaction with self-gravitating ring particles. These result in a random walk of the moonlet’s semi-major axis. The eccentricity of the moonlet quickly reaches an equilibrium value due to collisional damping. The average change in semi-major axis of the moonlet after 100 orbital periods is 10-100m. This translates to an offset in the azimuthal direction of several hundred kilometres. We expect that such a shift is easily observable.

Preprint

http://arxiv.org/abs/1006.1643

Movies

Simulation EQ40050DT:

• Stochastic orbital migration of small bodies in Saturn’s rings – Hanno Rein and John Papaloizou
• Migration of a moonlet in a ring of solid particles: Theory and application to Saturn’s propellers – Aurelien Crida, John Papaloizou, Hanno Rein, Sebastien Charnoz and Julien Salmon

This is a video of one of the simulations (EQ40050DT, to be precise). Make sure you watch it in HD if possible.

It shows a three dimensional simulation of a small moonlet which is embedded in the rings. Due to collisions and gravitational interactions with ring particles, it will eventually undergo a random walk.

Update: A new feature “SkyView” lets you see the position of all exoplanets on a sky chart.

I am a member of the Astrophysics Group at the Department of Applied Mathematics and Theoretical Physics in Cambridge. After finishing my PhD, I will go to the School of Natural Sciences at the Institute for Advanced Study in Princeton. I am interested in a broad variety of topics in theoretical astrophysics associated with the formation, evolution, and physical properties of planets and planetary systems.

For more information on my research, have a look at my academic webpages and a selection of my scientific codes and iPhone applications.

Beside of the scientific part, this site is a conglomeration of all kinds of things. Have a look below for the most recent blog posts or use the menu above to find what you’re not searching for.

The picture shows the star HD45364 which is slightly smaller than our sun. There are also two planets orbiting this star. Unfortunately, they are not visible in the picture because they are too small and the star is too bright. But the planets make the star wobble as they go around it. We only know about the planets because of this reflex motion. One of the planets is similar to our Jupiter, the other has the same mass than our Saturn. The most striking fact is that the more massive planet is on the outside. The planets swapped places compared to our solar system, where Jupiter, the more massive one, is interior to Saturn. Furthermore, the planets are in a 3:2 resonance, meaning that one planet orbits the star exactly three times while the other one orbits the star twice. A plot of the orbits of both planets is shown below. Note that, because they are in a 3:2 resonance, their orbits are very close together but nevertheless stable. How could such a system form and why is it not like our own solar system?

To answer these question, we have performed two dimensional hydrodynamic simulations. We simulate the final stages of the planet formation process. The planets have already accumulated all their mass but are still embedded in the proto-planetary disc. This disc contains the material which all planets are made of. In that stage, the planets strongly interact with the proto-planetary disc. Due to the exchange of angular momentum, the planet can move within the disc. They usually spiral inwards on very long timescales. That is called planetary migration. In special circumstances they can also migrate inwards very quickly. In astronomical terms, quickly still means a few thousand years. But that is very short compared to the millions of years that the whole process of forming a planetary system takes.

The above plot shows a snapshot of a simulation with both planets orbiting the star HD45364. Clearly visible are the long spiral waves created by the planets. The darker, blueish region is a gap in the disc, opened by the Jupiter-mass planet. The planet is so massive, that it can shape the profile of the proto-stellar disc in its vicinity. The smaller planet, just inside of the gap is not massive enough to do that. It turns out that this difference, whether a planet can open a gap or not, is one key to explaining the formation of the whole system.

The goal of the study is to reproduce the observed 3:2 resonance. Resonances are a natural outcome of migrating planets. Whenever planets migrate at different speeds, and this happens almost all the time, they can end up in a resonance. There are many resonances, more precisely, there is an infinite number of resonances. Three important factors determine the final resonance of two migrating planets:

• The planets’ masses
• The initial positions of the planets
• The migration speed

The masses of the planets are well constrained from observations. It’s also not expected that they change during the migration phase. Varying the initial positions would allow us to get the wanted 3:2 resonance. But we can even try to think of a more generic formation scenario by assuming that one of the planets migrates very rapidly. This happens naturally in a massive disc. If the migration speed of the outer planets exceeds a certain value, then the end result is always the 3:2 resonance.

The conclusion from this study is quite interesting. To form the system’s orbital configuration, namely the 3:2 resonance, we assumed a fairly massive proto-planetary disc. This is the only requirement that is put in by hand. Everything else is computed self-consistently. We can even go further and make some predictions!

All our simulations produce a generic pattern of orbital parameters. It is different from what has been known from the observational data alone. Our results show that the eccentricities of both planets should be a factor 4-5 smaller. In other words, we predict that the orbits of the planets are much more circular. In the plot shown above, the two curves show the different solutions: The green one with the higher peaks is the one with large eccentricities; The blue one is the prediction that comes from our formation scenario with more circular orbits. The red points are measurements of the star’s wobbling. It can be seen from the plot that the current observational data is not good enough to choose a clear winner. However, only a few more data points have to be taken whenever the discrepancy between the two models is large. Hopefully, we can confirm the first predicted orbital configuration of an exo-planet within a couple of years!

More details can be found in our scientific article, which has been highlighted in a recent edition of Astronomy and Astrophysics. The article is also available free of charge from the arXiv servers.


javascript:(function(){
document.getElementById('headerpane').style.display='none';
document.getElementById('bodypane').style.top='0px';
document.getElementById('zingspane').style.display='none';
document.getElementById('sidebaradpane').style.display='none';
document.getElementById('mainpane').style.left='0px';
})();


• DPMHD3D
• GravTree (movie!)