Rarity of Jupiter-like planets means planetary systems exactly like ours may be scarce

Is our little corner of the galaxy a special place? As of this date, we’ve discovered more than 1,500 exoplanets: planets orbiting stars other than our sun. Thousands more will be added to the list in the coming years as we confirm planetary candidates by alternative, independent methods.

In the hunt for other planets, we’re especially interested in those that might potentially host life. So we focus our modern exoplanet surveys on planets that might be similar to Earth: low-mass, rocky and with just the right temperature to allow for liquid water. But what about the other planets in the solar system? The Copernican principle – the idea that the Earth and the solar system are not unique or special in the universe – suggests the architecture of our planetary system should be common. But it doesn’t seem to be.

A mass-period diagram. Each dot marks the mass and orbital period of a confirmed exoplanet.
Stefano Meschiari

The figure above, called a mass-period diagram, provides a visual way to compare the planets of our solar system with those we’ve spotted farther away. It charts the orbital periods (the time it takes for a planet to make one trip around its central star) and the masses of the planets discovered so far, compared with the properties of solar system planets.

Planets like Earth, Jupiter, Saturn and Uranus occupy “empty” parts of the diagram – we haven’t found other planets with similar masses and orbits so far. At face value, this would indicate that the majority of planetary systems do not resemble our own solar system.

The solar system lacks close-in planets (planets with orbital periods between a few and a few tens of days) and super-Earths (a class of planets with masses a few times the mass of the Earth often detected in other planetary systems). On the other hand, it does feature several long-period gaseous planets with very nearly circular orbits (Jupiter, Saturn, Uranus and Neptune).

Part of this difference is due to selection effects: close-in, massive planets are easier to discover than far-out, low-mass planets. In light of this discovery bias, astronomers Rebecca Martin and Mario Livio convincingly argue that our solar system is actually more typical than it seems at first glance.

There is a sticking point, however: Jupiter still stands out. It’s an outlier based both on its orbital location (with a corresponding period of about 12 years) and its very-close-to-circular orbit. Understanding whether Jupiter’s relative uniqueness is a real feature, or another product of selection effects, has real implications for our understanding of exoplanets.

Jupiter as seen by the Hubble Space Telescope.

Throwing its weight around

According to our understanding of how our solar system formed, Jupiter shaped much of the other planets’ early history. Due to its gravity, it influenced the formation of Mars and Saturn. It potentially facilitated the development of life by shielding Earth from cosmic collisions that would have delayed or extinguished it, and by funneling water-rich bodies towards it. And its gravity likely swept the inner solar system of solid debris. Thanks to this clearing action, Jupiter might have prevented the formation of super-Earth planets with massive atmospheres, thereby ensuring that the inner solar system is populated with small, rocky planets with thin atmospheres.

Without Jupiter, it looks unlikely that we’d be here. As a consequence, figuring out if Jupiter is a relatively common type of planet might be crucial to understanding whether terrestrial planets with a similar formation environment as Earth are abundant in the galaxy.

Despite their relative heft, it’s a challenge to discover Jupiter analogs – those planets with periods and masses similar to Jupiter’s. Astronomers typically discover them using an indirect detection technique called the Doppler radial velocity method. The gravitational pull of the planet causes tiny shifts in the wavelength of features in the spectrum of the star, in a distinctive, periodic pattern. We can detect these shifts by periodically capturing the star’s light with a telescope and turning it into a spectrum with a spectrograph. This periodic signal, based on a planet’s long orbital period, can require monitoring a star over many years, even decades.

Are Jupiter-like planets rare?

In a recent paper, Dominick Rowan, a high school senior from New York, and his coauthors (including astronomers from the University of Texas, the University of California at Santa Cruz and me) analyzed the Doppler data for more than 1,100 stars. Each star was observed with the Keck Observatory telescope in Hawaii; many of them had been monitored for a decade or more. To analyze the data, he used the open-source statistical environment R together with a freely available application that I developed, called Systemic. Many universities use an online version to teach how to analyze astronomical data.

Our team studied the available data for each star and calculated the probability that a Jupiter-like planet could have been missed – either because not enough data are available, or because the data are not of high enough quality. To do this, we simulated hundreds of millions of possible scenarios. Each was created with a computer algorithm and represents a set of alternative possible observations. This procedure makes it possible to infer how many Jupiter analogs (both discovered and undiscovered) orbited the sample of 1,100 stars.

Orbit of the newly discovered Jupiter-mass planet orbiting the star HD 32963, compared to the orbits of Earth and Jupiter around the sun.
Stefano Meschiari, CC BY-ND

While carrying out this analysis, we discovered a new Jupiter-like planet orbiting HD 32963, which is a star very similar to the sun in terms of age and physical properties. To make this discovery, we analyzed each star with an automated algorithm that tried to uncover periodic signals potentially associated with the presence of a planet.

We pinpointed the frequency of Jupiter analogs across the survey at approximately 3%. This result is broadly consistent with previous estimates, which were based on a smaller set of stars or a different discovery technique. It greatly strengthens earlier predictions because we took decades of observations into account in the simulations.

This result has several consequences. First, the relative rarity of Jupiter-like planets indicates that true solar system analogs should themselves be rare. By extension, given the important role that Jupiter played at all stages of the formation of the solar system, Earth-like habitable planets with similar formation history to our solar system will be rare.

Finally, it also underscores that Jupiter-like planets do not form as readily around stars as other types of planets do. It could be because not enough solid material is available, or because these gas giants migrate closer to the central stars very efficiently. Recent planet-formation simulations tentatively bear out the latter explanation.

Long-running, ongoing surveys will continue to help us understand the architecture of the outer regions of planetary systems. Programs including the Keck planet search and the McDonald Planet Search have been accumulating data for decades. Discovering ice giants similar to Uranus and Neptune will be even tougher than tracking down these Jupiter analogs. Because of their long orbital periods (84 and 164 years) and the very small Doppler shifts they induce on their central stars (tens of times smaller than a Jupiter-like planet), the detection of Uranus and Neptune analogs lies far in the future.

The Conversation

This article was originally published on The Conversation. Read the original article.

An interactive Barnes-Hut tree

[TL,DR: if you’d like to play with a simple Barnes-Hut octree code, scroll down to the little embedded app.]

Gah! It’s been quite a while since my last post. Despite my best intentions, work (and a lot of feedback from Super Planet Crash!) has taken precedence over blogging. I do have a sizable list of interesting topics that I’ve been meaning to write about, however, so over the next few weeks I’ll try to keep to a more steady posting clip.

Super Planet Crash has been a resounding success. I have been absolutely, positively astounded with the great feedback I received. My colleagues and I have been coming up with lots of ideas for improving the educational value of SPC, add new, interesting physics, and addressing some of the complaints. In order to have the ability to dedicate more time to it, over the past few months, we’ve been furiously applying for educational and scientific grants to fund development. Hopefully something will work out — my goal is to make it into a complete suite of edu-tainment applications.

When Giants Collide

I’ve recently started experimenting with a new  visualization that I think will turn out pretty darn cool. Its draft name is When Giants Collide. When  Giants Collide will address a common request from planetary crashers: “Can I see what happens when two giant planets collide”?

A sketch of the interface.

When Giants Collide will be a super-simple JavaScript app (so it will run in your browser) that will simulate the collision of two massive spheres of gas. The simulation will have to model both gravity and the dynamics of the gas: to address this, I’ve been dusting off and reviewing an old Smoothed-Particle Hydrodynamics (SPH) code I worked on for a brief period in graduate school. SPH is a very simple technique for cheaply simulating gas flows with good spatial accuracy, and is somewhat straightforward to code. There are some shortcuts that have to be taken, too — large time steps, low particle counts, and more (e.g., a polytropic equation of state for the gas giants; more on this in future posts). These shortcuts come at the expense of realism, but will enable fast, smooth animation in the browser.

Gravity with the  Barnes-Hut algorithm

Gravity is an essential ingredient of When Giants Collide! Even with very low particle counts (say, N = 1000), a brute force calculation that just sums up the mutual gravitational force between particles won’t do if you want to run the simulation at 60 frames per second. Direct summing is an N^2 operation:

(this is a simple force accumulator written in R).

A better way that involves only a slightly more complicated algorithm is to use the Barnes-Hut algorithm (a short Nature paper with more than 1,000 citations!). The algorithm involves recursively subdividing space into cubes and loading them with particles, such that every cube contains either 0 or 1 particles. This is represented in code with an oct-tree structure.  Once such a tree is constructed, one can calculate the gravitational force on a given particle in the brute-force way for close particles, and in an approximate way for distant particles; whether to use one or the other is determined by walking the tree down from the top. An excellent explanation (with great visuals!) is provided in this article.

The other advantage is that, once the tree has been already built for the gravity calculation, it can be used to identify the nearest neighbors of a given particle through the same tree-walking procedure. The nearest neighbors are needed for the hydrodynamical part of the SPH algorithm (see, e.g., this review article by Stefan Rosswog or this one by Daniel Price).

An interactive tree

Below is an interactive JavaScript applet that subdivides space with the Barnes-Hut algorithm. You can add new points by clicking on the surface, or using the buttons to add new, random ones.

The code for building the Barnes-Hut tree from an array of 3D positions is available at the GitHub repository for When Giants Collide. I will be developing the code in the open, and post periodically about my progress. Hopefully by the end of summer I will have an attractive app running on any modern device and web browser. Any ideas on how to gamify it?

The Automated Planet Finder, Systemic and Super Planet Crash

[This short article I wrote has been published on The Conversation UK.]

The following is a short article about the Automated Planet Finder, Systemic and Super Planet Crash. We recently announced the first batch of exoplanets that were discovered in the first few months of science operation of APF. The first two systems (HD141399 and Gliese 687) have been submitted and will be available on astro-ph shortly.

Telescope apps help amateurs hunt for exoplanets

Laurie Hatch

People around the world are being invited to learn how to hunt for planets, using two new online apps devised by scientists at the University of Texas at Austin and UC Santa Cruz.

The apps use data from the Automated Planet Finder (APF), Lick Observatory’s newest telescope. The APF is one of the first robotically operated telescopes monitoring stars throughout the entire sky. It is optimised for the detection of planets orbiting nearby stars – the so-called exoplanets.

Systemic is an app that collects observations from APF and other observatories and makes them available to the general public. Anyone can access a simplified interface and follow the steps that astronomers take to tease a planetary signal out of the tiny Doppler shifts collected by the telescope.

Students and amateurs can learn about the process of scientific discovery from their own web browsers, and even conduct their own analysis of the data to validate planet discoveries.

The second app, SuperPlanetCrash, is a simple but addictive game that animates the orbits of planetary systems as a “digital orrery”. Users can play for points and create their own planetary systems, which often end up teetering towards instabilities that eject planets away from their parent stars.

First catch

Despite only being in operation for a few months, APF has already been used to discover new planetary systems.

Night after night, the telescope autonomously selects a list of interesting target stars, based on their position in the sky and observing conditions. The telescope collects light from each target star. The light is then split into a rainbow of colours, called a spectrum. Superimposed on the spectrum is a pattern of dark features, called absorption lines, which is unique to the chemical makeup of the star.

When a planet orbits one of the target stars, its gravitational pull on the star causes the absorption lines to shift back and forth. Astronomers can then interpret the amplitude and periodicity of these shifts to indirectly work out the orbit and the mass of each planet.

This method of detecting exoplanets is dubbed the Doppler (or Radial Velocity) technique, named after the physical effect causing the shift of the absorption lines. The Doppler technique has been extremely productive over the past two decades, leading to the discovery of more than 400 planet candidates orbiting nearby stars – including the first exoplanet orbiting a star similar to our own Sun, 51 Pegasi. To conclusively detect a planetary candidate, each star has to be observed for long stretches of time (months to years) in order to rule out other possible explanations.

The APF has now found two new planetary systems surrounding the stars HD141399 and Gliese 687.

HD141399 hosts four giant, gaseous planets of comparable size to Jupiter. The orbits of the innermost three giant planets are dramatically more compact than the giant planets in our Solar System (Jupiter, Saturn, Uranus and Neptune).

Gliese 687 is a small, red star hosting a Neptune-mass planet orbiting very close to the star: it only takes about 40 days for the planet to complete a full revolution around the star.

Team leader Steve Vogt of the University of California, Santa Cruz has dubbed both of these almost “garden variety” planetary systems, and indeed, they are quite similar to some of the systems discovered over the last few years. However, what look like distinctly unglamorous planetary systems now can still pose a puzzle to scientists.

The new normal

The planetary systems discovered so far are typically very different from our own solar system. More than half of the nearby stars are thought to be accompanied by Neptune-mass or smaller planets, many orbiting closer than Mercury is to the Sun. In our solar system, on the other hand, there is a very clear demarcation between small, rocky planets close to the Sun (from Mercury to Mars) and giant planets far from the Sun (from Jupiter to Neptune). This perhaps suggests that planetary systems like the one we live in are an uncommon outcome of the process of planet formation.

Only further discoveries can clarify whether planetary systems architected like our own are as uncommon as they appear to be. These observations will need to span many years of careful collection of Doppler shifts. Since the APF facility is primarily dedicated to Doppler observations, it is expected to make key contributions to exoplanetary science.

The two apps produced by the APF team make amateur scientists part of the hunt. These applications join the nascent movement of “citizen science”, which enable the general public to understand and even contribute to scientific research, either by lending a hand in analyzing massive sets of scientific data or by flagging interesting datasets that warrant further collection of data.

The Conversation

2,000,000 systems played!

The high scores as of April 13, 2014 for all of posterity. Good job, brave folks.!
The high scores as of April 13, 2014 for all of posterity. Good job, brave folks!

This week has been quite the ride. Super Planet Crash has been featured on io9, Huffington Post, space.com, Motherboard, and other online publications, and it “suffered” from repeated surges of traffic from imgur. Not bad for a game hacked together over the weekend! It overjoyed me to receive emails, and pictures!, by people enjoying the game, especially from the younger generation.

More than 2,000,000 games have been played as of today, and hopefully a fraction of those players will want to know more about exoplanets. I would also encourage everyone who enjoys this little free game to donate to science education funds, such as McDonald Observatory’s Science Education Fund. I would be oh so happy to have bragging rights due to planet crashers donating en masse!

I’m slowly trying to work through some of the feature requests. Not all are feasible on a short timescale (science is my full-time job, after all!), but I will strive to at least try to address the lowest-hanging fruit. One pet peeve shared by many was the inability to see the high-scoring games. In trying to address this, I discovered two bugs in the implementation of the high-scores.

The first is that the server relied too much on trusting the high-scores that were sent from the client (i.e. the Javascript running in the web-browser). Although I had tried to mitigate it somewhat, several fake high-scores were submitted. I added some stricter checks that should further help address the problem. The right solution would be to run the system on the server in order to check for any shenanigans. Unfortunately, this is unfeasible, as too many games are being played: it would place an unduly amount of stress on my server.

The second is a bug in the way systems were recorded and sent to the server. Some of the highest-scoring systems attempt to score high on masses, “crowdedness” (how close are the orbits of the bodies to each other) and habitability. They do that by (a) adding a binary companion (the “dwarf star”) and (b) putting a lot of planets in the same orbit within the habitable zone.


Something like this.
Something like this.

The resulting systems are likely highly chaotic, so any small error in recording the state of the system [ref]The state of the system being the current position and velocity of each body.[/ref] will change the outcome very quickly (the so-called “butterfly effect“). Unfortunately, one bug in Super Planet Crash resulted in this exact scenario happening. Any rounding or truncation of the floating point values for the coordinates will also affect the evolution of the system. The most common outcome is that these high-scoring systems will appear to be unstable when replayed. Grrr.

The decision I reached is to clean up the high-score table. The systems should now be recorded the correct way, and everyone will be able to see how the scores were achieved.

I understand this is sad news for the current record holders, so the screenshot at the top of this page will record the brave folks who reached upwards of 300,000,000 points for all posterity. (Just imagine someone unplugged the arcade machine by mistake…)

Next up on my agenda is releasing the game on GitHub. I am cleaning up the last few bits. If you are a programmer, you’ll be able to create pull requests for new features there.

In my next post, I will go into a bit more detail about how I created Super Planet Crash (and so can you!).

Go Crash Some Planets!

Super Planet Crash
A screenshot of Super Planet Crash playing in Safari


If you enjoyed playing Super Planet Crash, please consider donating to the Science Education Fund at McDonald Observatory. Every little bit counts. Go support science!

Update 2: 2,000,000 systems were created!
: Systemic and Super Planet Crash were featured on io9Space.comGlobalNews, Motherboard, Huffington Post, The Verge, and two press releases by UC Santa Cruz and McDonald Observatory. Thank you!

Super Planet Crash is a little game born out of some of my work on the online version of Systemic. It is a digital orrery, integrating the motion of massive bodies forward in time according to Newtonian gravity. It works on any recent web browser and modern tablets.

The main goal of the game is to make a planetary system of your own creation be stable (i.e. no planet is ejected, or collides with another body). This is of course exceedingly easy when your system comprises of a few Earth-mass planets, but dynamical instability can quickly set in when adding a lot of heavier bodies (from giant planets, all the way to stellar companions).

The challenge is then to fit as many massive bodies as possible inside 2 AUs (twice the distance between the Earth and the Sun), teetering close to instability but lasting at least 500 years. Accordingly, the game rewards a daring player with more points (proportionally to the mass of each body added to the system). A few simple rules are listed under the “Help” button.

The game always starts with an Earth-mass planet in a random location, but you can also have fun overloading known planetary systems! Clicking on the “Template” dropdown brings up a list of planetary systems to use as starting templates, including the compact system Kepler-11 and the super-eccentric planet HD80606 (more systems to come). You can even share your creations with your friends by copying the URL in the “Share” box.

The game is open-source, and still under active development. The entire code will be downloadable from GitHub (as soon as I get a bit of work done!).In the near future, I will be adding integration with Systemic Live, a longer list of template planetary systems and smartphone support. In the meantime, have fun crashing planets!


The game was made possible by the wonderful paper.js library, which let me quickly prototype the app despite having little experience in web gaming. The palette draws from the base16 color set.

Many many thanks to my wonderful testers: Rachael Livermore, Mike Pavel, Joel Green, Nathan Goldbaum, Maria Fernanda Duran, Jeffrey SilvermanAngie Wolfgang, and other cool people.

My work is funded by the W. J. McDonald Postdoctoral Fellowship. If you enjoyed the game, please donate to the McDonald Observatory fund to support science education.

-5 minutes to Kepler teleconference!

Watch here.

Woohoo! 715 new planets in one go were announced during the teleconference.

A few screen captures:

photo 2 photo 1

knownexoplanets (1)

Arc posted two new papers (Lissauer et al., 2014 & Rowe et al., 2014) and a media kit.


Look at that histogram go!

Figure 3 of Rowe et al., 2014
Figure 3 of Rowe et al., 2014

This figure from Rowe et al., 2014 shows the incident flux (normalized to the incident flux on Earth) versus the radius of the planet (in Earth radii). There’s something interesting to be said about it, but it will deserve a blog post on its own…

New release of Systemic 2 (2.14)

I have just released a new version of Systemic 2 (2.14).  Lots of little bug fixes, together with a few features.

My favorite is the “smooth orbit plot”, which recreates plots that will appear on an upcoming planet paper (I will put proper credit here once the paper is out!). The routine takes samples of orbital elements from the output of a Markov-Chain Monte Carlo or bootstrap run, and plots the orbits with some transparency, so that it is visually evident where orbits “crowd up”.

Smooth orbit plot
Smooth orbit plot made with Systemic 2.14

On top of the samples, I plotted the “best-fit” orbit in red (which hopefully will fall on top of the range of possible orbital elements!). This plot can also give some visual sense of multi-modal or non-symmetrical element distributions.

Download Systemic 2.14

Other changes:
– Added a feature for smoother/faster plotting in GUI
– Periodograms now print out the strongest peaks
– Periodograms are zoomable
– Fixed Cross-validation for fits with only one planet
– New menu item to add random Gaussian noise to data
– Smooth orbits plot from an error estimation object
– FIXED: Improved Linux installation instructions (credit for reporting: Thomas Kosvic)
– FIXED: bug in SWIFTRMVS on 32-bit installations (credit for reporting: Thomas Kosvic)

Circumbinary Planet formation: Here be high-speed impacts

[A shorter-form version of this article I wrote has been published on The Conversation UK.]
The Conversation
Last week was a hot week for those interested in circumbinary planets and how they form. Firstly, Kostov et al. discovered Kepler-413b, another Neptune-size circumbinary planet; this one has some interesting dynamical considerations owing to its inclination. Secondly, Lines et al. published a new N-body simulation of the formation environment close to the Kepler-34 binary.

In this post, I will explain in layman’s terms how we think planet formation around binary stars works.

The giant planet Kepler-34b orbits round two stars. Now that’s just greedy. David A. Aguilar

It is quite remarkable to remember that, until less than two decades ago, scientists had no concrete examples of planets orbiting other stars like the Sun. The Solar System was not only the archetype of planetary system, but the only planetary system astronomers knew of.

Since the discovery of a giant planet around the Sun-like star 51 Peg in 1995, the pace of discovery of exoplanets (planets orbiting other stars) has not slowed down. Quite the contrary: the trickle of planet discoveries became a steady stream in the 2000’s, as technology improved and astronomers became more efficient at homing in on stars likely to have planets orbiting them. In 2009, the Kepler space telescope was launched with the express goal of simultaneously surveying hundreds of thousands of stars for the presence of exoplanets.

The advent of Kepler transformed the steady stream into a veritable deluge of planets and planetary systems. Today, more than a thousand exoplanets are known, with thousands of planet candidates waiting for further verification and vetting. Although this quickly expanding “census” of exoplanets has injected new life to the field of planet formation (the study of how planets are born), it has also brought forth many new questions.

A big leap

Our best understanding of how planets form is inextricably linked with our understanding of how stars form. Many different lines of strong evidence point to planets forming inside a thin, gaseous disk surrounding nascent stars. Within this disk, solid particles (evocatively named “dust”) collide and progressively grow to asteroid-sized bodies. These bodies, called planetesimals, are the essential “bricks” of planet formation. Further collisions among planetesimals build protoplanets — rocky, Earth-sized bodies.

Farther out from the central star, water and other compounds “freeze out” and become part of the solid component. At and beyond this location (the “ice line”), protoplanets can grow even larger and amass thick, massive atmospheres. This sharp divide between small, Earth-sized planets close to the central star (Mercury through Mars) and massive giant planets further out (Jupiter through Neptune) is easily recognized in the Solar System, where the ice line is just inside Jupiter’s orbit.

For this theory to work, it demands an incredible feat: through collisions and gravity alone, it requires growth from microscopic dust particles more than a hundred times smaller than a grain of sand, all the way to Jupiter-sized objects. To put things into perspective, it is the same jump in size between a single atom and an average sized human! This is a very delicate process, involving many physical mechanisms, some of which are still poorly understood today.

Double trouble

One of the sticking points is the stage in which planetesimals collide. Planetesimals need to collide surprisingly gingerly in order to accrete; smash them too fast, and they will “break” into smaller rocks. Regions with high-speed collisions become essentially sterile for planet formation, as no further growth happens under normal circumstances.

This is why the recent discovery of circumbinary planets had astronomy theorists raise an eyebrow (or two). Circumbinary planets are planets that orbit a binary star. Such stars are bound together by gravity into an often-tight orbital dance. Kepler-16 ABb, the first circumbinary planet discovered in 2011 by the Kepler spacecraft, orbits both stars A and B (hence the AB monicker; the lowercase b simply means “the first planet discovered in the system”). The recent discovery of Kepler-413b by the team led by Veselin B. Kostov adds another member to the (so far) very exclusive circumbinary club: seven planets have been discovered orbiting binary stars, all giant planets with sizes between Neptune and Saturn.

Everyone likes to bring up Star Wars' Tatooine, but I like Doctor Who more -- so I will show Gallifrey (the home world of the Time Lords) instead. BBC, Peter McKinstry
Everyone likes to bring up Star Wars’ Tatooine, but I like Doctor Who more — so I will show Gallifrey (the home world of the Time Lords) instead. BBC, Peter McKinstry

That these planets, once the purview of sci-fi fare such as “Star Wars” or “Doctor Who”, were detected is alone quite a testament to the power of Kepler and its skilled team: astronomers had to meticulously analyze small variations in brightness of the two stars, caused both by mutual eclipses (each star passing in front of each other) and planetary eclipses (the planet passing in front of one star or the other).

Their very existence, however, is also a testament to the surprising resilience of planet formation. As mentioned before, our models indicate that planetesimals will be destroyed if their impact speeds are too high. But this should be exactly the case around binary stars! Binary stars should gravitationally perturb planetesimals, just like Jupiter perturbs asteroids and comets in the Solar System. These perturbations fling planetesimals into orbits that, when crossing other planetesimals’ orbits, assure high-speed impacts and mutual destruction. Only far away from the central binary, where perturbations are weaker, it is expected that collision speeds would become low enough to resume planetary building.

All things considered, this should be an easily overcome hurdle: as explained in the previous section, far enough from the central star (or stars) is where giant planets form — and all circumbinary planets found so far are giant planets! Is this a spectacular confirmation of theory?

Too close for comfort?

Not so fast, unfortunately: all circumbinary planets discovered so far are also orbiting very close to their parent binary. So close, in fact, that if they were any closer to the binary, their orbit would be destabilized to the point of ejection from the system or collision with one of the two stars. These planets are essentially flirting with disaster, their orbital velocity closely balancing the gravitational attraction of the binary in a dangerous tug of war. Inside their orbit, in the “instability region”, no planet could survive for long.

Reconciling these two apparently incompatible findings (giant planets in a location where they should have never formed) required invoking an old idea: that of planetary migration.

The very first planet discovered, 51 Peg b, was a giant planet orbiting its parent star at a very small distance — smaller than even Mercury. Such a planet could have never formed so close to the star, as the high temperatures would sublimate rocks and ices. Our theory would then predict that there would not be enough bricks (planetesimals) to build a giant planet. Theorists quickly understood what had occurred early on in this system’s history: 51 Peg likely formed further away from the star, and subsequently interacted with the disk in which it was born in such a way to be pushed further in. This process is called “migration”.

Migration could be at play with circumbinary planets as well. Computer models have shown that a giant planet formed far from the central binary will tend to migrate and move inwards. Encouragingly, the migrating planets do not move all the way to the instability region. Rather, they tend to stop at a specific distance, which matches well with their current location.

One last finding of computer models match another observed property of circumbinary planets. While we admittedly discovered only a handful of circumbinary planets, it is tantalizing that all of them are quite a bit smaller than Jupiter. It is surprising as the bigger a planet is, the easier it is to detect: therefore, we should have discovered a few Jupiter-sized circumbinary planets by now. Computer models explain this last piece of the puzzle: migrating Jupiter-sized circumbinary planets end up strongly interacting with the central binary and are subsequently flung out from the system. We would not observe such planets today simply because they did not survive their turbulent beginnings.

Although there are still many details to be worked out, this theoretical framework appears to be in good concordance with Kepler’s discoveries. However, it is possible, even expected, that further planetary discoveries might surprise us once more.