DDA 2015 – Using Populations of Gas Giants to Probe the Dynamics of Planet Formation

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Session: Exoplanet Theory I

Ruth Murray-Clay (UC Santa Barbara) (invited)




  • How do giant planets and brown dwarfs form?
  • Architecture of Solar System is atypical.
  • Lots of gas giants at large distances, small distances (“hot Jupiters”), but not much in between a la Solar System. Why?
  • SS: rocky planets (~1 AU), gas giants (~5-10 AU), ice giants (~20-30 AU)
  • Theory: cannot predict numbers, but can predict patters in system architectures and statistical populations
  • How to get companions to stars: 1) turbulent fragmentation, 2) grav. instability, 3) core accretion
  • HR8799: testbed for planet formation theories
    • 4 Jupiter-mass planets
    • turbulent frag.? No: system is not hierarchichal
    • grav. inst.?
      • iffy – minimum fragment distance problems (but could have migrated)
      • Timing – collapse must occur at end of infall or a binary star results
    • core accretion?
      • dynamical (growth) timescale is too long ($t_{grow} > t_{infall}$)
      • $t_{grow} > t_{disk}$
      • cross section regimes — all problematic:
        • physical cross section
        • grav focusing
        • gas drag capture
  • Make gas useful.
    • no gas: particles can orbit inside core Hill radius
    • gas: “wind shear”
      • binary capture
      • particle capture can occur out to Hill radius
      • growth time at 70 AU can be short enough to nucleate an atmosphere
      • turbulent gas: okay
    • accretion cross sections increase by up to $10^4$
  • Gemini Planet Imager could confirm this theory.
  • Metal-rich stars hostmorehotJupiters and highly eccentric planets: signature of planet-planet interactions? Why?
    • Scattering?
    • Secular chaos?
    • Perhaps those systems form many Jupiters.
  • Are the solar system analogs orbiting low metallicity stars?

The Printer and I: A Tale of Spinning Fans, Diseased Hearts, and the Tragedy that is Life

[Click to embiggen.]

This (see photo) is how I spent my afternoon and evening, today. I have a conference to attend next week and must present a poster paper on some recent research results. Because I know by now that both Old Man Murphy and Loki the Trickster always lie in wait, snickering — I hear you, you bastards — I go to check the large-format printer. It is a Beast, and it turns electrons into poster papers. I flip the power switch, and it makes a horrible noise, won’t boot up, freezes, then whines plaintively, “call HP … call HP … please, won’t you call HP ….” Not very encouraging. Screw you, Loki — thou art a Puck.

As with all things computer that misbehave, I keep trying the same thing over and over, hoping for a different result, though I know full well that no different result will … um … result. Indeed, no dice. Run around the building and check with everybody: nobody knows what’s wrong or what happened. Yeah, sure.

What to do? Go find some screwdrivers, of course. The horrible noise emanates from somewhere around the power supply. Sort of. It’s buried in the guts of the Beast, so it’s hard to tell from the outside. It is a place to start, anyway. I roll up the sleeves of my robe, pick up a Holy Implement of Torx, and get to work …

Several hours later, I finally have figured out, cuss word by cuss word (proper ordering is important), how to get past all the barriers cleverly designed by Evil HP Engineers to make rational disassembly near-impossible. (Ever disassemble a laptop computer, down to the bare metal? This is harder, I kid you not.) Sixty screws later (I count them, twice), I get to the power supply fan. The heart of the Beast is diseased, despoiled. It is not turning quite right, and the motor shaft wiggles a little. It is not supposed to wiggle. Even a little. Culprit apprehended at last? Perhaps. Fortunately, it’s just a cheap $8 cooling fan you can pick up at any Radio Shack.

But Radio Shack does not exist anymore. When did that happen?

We have come round to this place again: what to do? Rummage around in the junk spare parts room, of course. It is a glorious room, beloved of tinkerers on staff. Bingo: six salvaged computer power supplies, just lying there on a shelf, calling to me. No, seven! But I am wise to their siren song. One after another, a closer look reveals frightening ugliness — mostly in the form of caked-on dust and dirt and grime. Their hearts spin, but they are Unclean and Decrepit. Sigh … last one: yay, Cleanliness! The Blessed One, Savior of the Beast, is found.

It believes it has been bestowed a new chance at life. I wish I could be happy for it. Little does it know its fate. Surely it deserves to be told of its pending doom? Yet that would crush its new-found hopes. You are perverse and cruel, you Fates! I do not have the heart to tell it.

True to my calling as Lord High Tinkerer, I pick up the Holy Implement of Torx and sacrifice the Blessed One upon the Ancient Altar of Gorthung (a fifty-year-old, government-issue desk, solid and heavy as a tank, with an ice-cold slate top). I flay its body and cut out its heart. I know no mercy.

Fan in bloody hand (a blood blister acquired some time during printer pieces-parts separation has popped), I trundle down the hill to the electronics lab. There, a colleague — the Wizard of Wire, Lord of Circuit — performs minor surgery. Lo, and behold! Upon application of the Lightning of Zoltar (a 12-volt power supply), the heart of the Blessed One lives again, spinning round and round in a most pleasing whir. Back up the hill.

That dreaded niggle squatting in the back of my mind finds a crack and blossoms. It dawns on me: now I have to put it all back together. Sixty screws. I realize I am tired. I’ll never remember where they all go. Come back tomorrow with freshly caffeinated veins? Pffft. Such is for wusses, unbecoming of a Tinkerer. So, since the operation of my memory — even on a good day — resembles most closely that of a sieve, I have little choice but to re-figure out how to take apart the Beast but in reverse. I am reminded of Ginger Rogers. I miss Ann Richards and her rapier wit. Today is not a good day.

Another hour passes by. I wave hi. We do that a lot, Time and I. My finger leaks on the table; I wipe it. And also on the housing of the reassembled printer power supply. I look at the smear, and I do not wipe it. I have left my mark upon this Beast, I think to myself. I shall not remove it. It will be buried amidst your guts; only you and I will ever know it is there. This token of my toil is enough, I decide. I move on.

At last, it is back together, despite all the King’s men staying home, watching TV. I do not want to plug it in. I’m sure you understand. Don’t you? Even so, I still roll the Beast back to its lair. I reattach its stiff black tail. I notice it is dirty, the cord, this conduit of the Lightning of Zoltar.

We have arrived at the moment of truth: I flip the switch. And wait. As with a pot of water that has yet to boil, it is best not to stare at a booting computer, especially one as slow and dumb as the Beast’s. I stare anyway. I wave hi to passing Time again, then it whirs with a pleasing sound. And dies. And tells me to call HP.

Naturally, I turn it off, wait ten seconds (capacitors can be slow to bleed, you know), and then turn it on again. Maybe something different will happen this time.

Null Hypothesis

I write

Into the void.

Transient, ephemeral, aperiodic,



Null hypothesis. By assumption

Not significant. You are

Alone. Free. Leap

Into the ether, the abyss —

Into the void.


math test

Here’s how to get MathJax up and running for your blog: part I, part II. The three tests below are text lifted from elsewhere.

Test 1

Consider first what we shall call the direct geometry case, in which we use only the zenith angle $z$ and bypass the geocentric angle $\theta$. The length of side $\overline{CM}$ follows from the right triangle $\widehat{CMP}$:

$$\begin{equation}\begin{array}[b]{ccl}\left(R+H\right)^{2} & = & \left(D\sin z\right)^{2}+\left(R+h+D\cos z\right)^{2}\\ \\ & = & D^{2}+\left(R+h\right)^{2}+2\left(R+h\right)D\cos z\end{array}\label{eq:R+H-test}\end{equation}$$


\begin{equation}D^{2}+2\left(R+h\right)D\cos z-\left[\left(R+H\right)^{2}-\left(R+h\right)^{2}\right]=0\label{eq:D eqn-test}\end{equation}

with solution

\begin{equation}\begin{array}[b]{ccl}D & = & -\left(R+h\right)\cos z\pm\sqrt{\left(R+h\right)^{2}\cos^{2}z+\left[\left(R+H\right)^{2}-\left(R+h\right)^{2}\right]}\\ \\& = & \left(R+h\right)\left(\sqrt{\cos^{2}z+\dfrac{\left(R+H\right)^{2}-\left(R+h\right)^{2}}{\left(R+h\right)^{2}}}-\cos z\right)\end{array}\label{eq:D soln quadratic ugly-test}\end{equation}

where the geometry of the problem requires the positive root. For convenience, define

\begin{equation}\epsilon\equiv\dfrac{H}{R}\quad\mathrm{and}\quad\xi\equiv\dfrac{h}{R}\label{eq:eps and xsi defs-test}\end{equation}

Then we can write eq. \eqref{eq:D soln quadratic ugly-test} as

\begin{equation}D=\left(R+h\right)\left(\sqrt{\cos^{2}z+\left(\dfrac{1+\epsilon}{1+\xi}\right)^{2}-1}-\cos z\right)\label{eq:D soln quadratic-test}\end{equation}

Eq. \eqref{eq:D soln quadratic-test} has the disadvantage of subtraction of two nearly equal numbers.

Test 2

We would like to know what is the radius $\bar{r}$ of the center of mass

of a grid cell of inner radius $r_{1}$ and outer radius $r_{2}$. In polar coordinates $\left(r,\theta\right)$ an infinitesimal area element is $dA=r\,dr\,d\theta$, so

\begin{equation}\bar{r}=\frac{1}{\Delta A}\intop_{0}^{\Delta\theta}\intop_{r_{1}}^{r_{2}}r\,dA=\frac{1}{\Delta A}\intop_{0}^{\Delta\theta}\intop_{r_{1}}^{r_{2}}r^{2}dr\,d\theta\label{eq: area-weighted r integral-test}\end{equation}

where $\Delta A=\frac{\Delta\theta}{2\pi}\cdot\pi\left(r_{2}^{2}-r_{1}^{2}\right)$.


\begin{equation}\Delta A=\frac{\Delta\theta}{2}\left(r_{2}^{2}-r_{1}^{2}\right)\label{eq: cell area-test}\end{equation}


\begin{equation}\bar{r}=\frac{1}{3}\frac{\Delta\theta}{\Delta A}\left(r_{2}^{3}-r_{1}^{3}\right)=\frac{2}{3}\frac{r_{2}^{2}+r_{1}r_{2}+r_{1}^{2}}{r_{1}+r_{2}}\label{eq: area-weighted r-test}\end{equation}


Thus, we have the bootstrapping scheme

\begin{equation}\begin{array}{rclcrcl}\bar{r}_{0} & = & \dfrac{2}{3\Delta^{2}}\left(r_{2,0}^{3}-r_{1,0}^{3}\right) & & r_{2,0} & = & \sqrt{r_{1,0}^{2}+\Delta^{2}}\\& \vdots & & & & \vdots\\\bar{r}_{k} & = & \dfrac{2}{3\Delta^{2}}\left(r_{2,\,k}^{3}-r_{2,\,k-1}^{3}\right) & & r_{2,\,k} & = & \sqrt{r_{2,\,k-1}^{2}+\Delta^{2}}\\& \vdots & & & & \vdots\\\bar{r}_{N_{r}-1} & = & \dfrac{2}{3\Delta^{2}}\left(r_{2,\,N_{r}-1}^{3}-r_{2,\,N_{r}-2}^{3}\right) & & r_{2,\,N_{r}-1} & = & \sqrt{r_{2,\,N_{r}-2}^{2}+\Delta^{2}}\end{array}\label{eq: bootstrap scheme}\end{equation}

where, again, we start with $r_{1,0}=r_{min}$ .

Test 3

Now, $-\widehat{z}\times{\left(\widehat{z}\times\overrightarrow{r}\right)}=\overrightarrow{r}-{\left(\widehat{z}\cdot\overrightarrow{r}\right)}\widehat{z}$, so


Define a new effective potential




Then we find the satisfying result


The individual components of \eqref{EQUATION.7aeaeb03-1226-46ab-815a-4b28e71a84a5} are

\begin{equation}\begin{aligned}x^{\prime\prime}-2y^{\prime} & =\frac{1}{{1+e_{p}\mathrm{cos}\mathrm{\theta}}}\frac{{\partial\mathrm{\Omega}}}{{\partial x}}\\y^{\prime\prime}+2x^{\prime} & =\frac{1}{{1+e_{p}\mathrm{cos}\mathrm{\theta}}}\frac{{\partial\mathrm{\Omega}}}{{\partial y}}\\z^{\prime\prime}+z\hspace{0.9em} & =\frac{1}{{1+e_{p}\mathrm{cos}\mathrm{\theta}}}\frac{{\partial\mathrm{\Omega}}}{{\partial z}}\end{aligned}\label{}\end{equation}


\begin{equation}\begin{array}{rcl}\overrightarrow{\nabla}\mathrm{\Omega} & = & \left[\begin{matrix}x-\dfrac{1-\mathrm{\mu}}{r_{1}^{3}}\left(x+\mathrm{\mu}\right)-\dfrac{\mathrm{\mu}}{r_{2}^{3}}\left(x-1+\mathrm{\mu}\right)\\y\left(1-\dfrac{1-\mathrm{\mu}}{r_{1}^{3}}-\dfrac{\mathrm{\mu}}{r_{2}^{3}}\right)\\z\left(1-\dfrac{1-\mathrm{\mu}}{r_{1}^{3}}-\dfrac{\mathrm{\mu}}{r_{2}^{3}}\right)\end{matrix}\right]\\ \\& = & \left(1-\dfrac{1-\mathrm{\mu}}{r_{1}^{3}}-\dfrac{\mu}{r_{2}^{3}}\right)\overrightarrow{r}-\mathrm{\mu}\left(1-\mathrm{\mu}\right)\left(\dfrac{1}{r_{1}^{3}}-\dfrac{1}{r_{2}^{3}}\right)\widehat{x}\end{array}\label{}\end{equation}

How I Do MathJax II. Example

To render equations in a WordPress blog, you have several options. The most aesthetically pleasing is MathJax. An earlier post tells you how to install MathJax for your WordPress site. This second post shows a few pointers by way of an example (you’ll probably want to view the page source, then search for “For example”). Here are a few more usage examples.

How to Do Math in a Blog Post

If you’ve installed MathJax in your site, then in a blog post you can trigger the loading of MathJax by putting the shortcode at the top of your post. It will not show up in your readers’ browsers.

That’s it! You can write your post now.

What I usually do, if the document has a lot of equations, is to compose the post in the quasi-WYSIWYG LaTeX editor, LyX. You can, of course, use whatever writing tool you like. When you’re happy with how your article looks, then copy the text to the clipboard. (With LyX, open up the source pane (View→Source Pane) and select the text.) Paste to your WordPress post editor.

You now have to make one change to the pasted text: remove the line breaks inside AMS environments

\begin{...} ... \end{...}

For example,

D & = & -\left(R+h\right)\cos z\pm\sqrt{\left(R+h\right)^{2}\cos^{2}z+\left[\left(R+H\right)^{2}-\left(R+h\right)^{2}\right]}\\
& = & \left(R+h\right)\left(\sqrt{\cos^{2}z+\dfrac{\left(R+H\right)^{2}-\left(R+h\right)^{2}}{\left(R+h\right)^{2}}}-\cos z\right)
\end{array}\label{eq:D soln quadratic ugly}


\begin{equation}\begin{array}[b]{ccl}D & = & -\left(R+h\right)\cos z\pm\sqrt{\left(R+h\right)^{2}\cos^{2}z+\left[\left(R+H\right)^{2}-\left(R+h\right)^{2}\right]}\\\\& = & \left(R+h\right)\left(\sqrt{\cos^{2}z+\dfrac{\left(R+H\right)^{2}-\left(R+h\right)^{2}}{\left(R+h\right)^{2}}} \cos z\right)\end{array}\label{eq:D soln quadratic ugly-how}\end{equation}

Here’s how to refer to the above equation. Write, for example,

eq. \eqref{eq:D soln quadratic ugly}

which renders as eq. \eqref{eq:D soln quadratic ugly-how}.

How I Do MathJax I. Installation

I use equations. To enable equations in a WordPress blog, there are several options. The most comprehensive—and aesthetically pleasing—is to use MathJax. This post tells you how to install MathJax for your WordPress site. A second post has a few pointers. Here are a few usage examples.

1. Edit default.js

I do not use the MathJax CDN since occasionally their site has problems. When that happens, your math stops working and your pages containing math become ugly. So I download MathJax to my WordPress install. Rather than futz with <script> tags in my site’s header, I edit the default configuration file to my liking. Thus:

  • Download the latest version of MathJax: go to https://github.com/mathjax/MathJax/, click on Releases, and download the latest version.
  • Unpack the archive file to your hard drive.
  • Edit default.js in the config directory. My preferences:
    • You’ll probably want to add to your extensions, something like:
      extensions: ["tex2jax.js", "TeX/AMSsymbols.js", "TeX/AMSmath.js"]
    • Scroll down and set messageStyle to your liking (I changed mine to messageStyle: "simple").
    • Scroll down to menuSettings and change these to your liking (I set zoom: "Hover").
    • In the tex2jax section that immediately follows:
      • Under inlineMath uncomment the line with inline delimiters ['$','$']. This enables normal LaTeX inline delimiters. You’ll have to escape actual dollar signs with \\\$.
      • processEscapes: true
      • preview: "[math]"
    • Scroll down to the TeX section.
      • Under equationNumbers, set autoNumber: "AMS".
      • Fiddle with whatever else there catches your fancy.
    • Fiddle with whatever else catches your fancy.
  • Finally, upload your entire MathJax directory to your WordPress site, something like http://yourdomain/mathjax/.

2. Get the WordPress plugin.

Next, get the MathJax-LaTeX plugin and set the settings. The easiest way is to go to your blog administration Dashboard→Plugins→Add New, and type mathjax in the search box. My plugin settings (Dashboard→Settings→MathJax-LaTeX) are

  • Force Load = unchecked
  • Default [latex] syntax attribute = inline (this seems to have no effect with my configuration)
  • Use wp-latex syntax? = unchecked
  • Use MathJax CDN Service? = unchecked
  • Custom MathJax location? = http://yourdomain/mathjax/MathJax.js
  • MathJax Configuration = default

Do not forget to click the Save Changes button!

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