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.

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.

Using Celestia for fun and profit

Celestia is a stunning program for visualizing 3D objects in space, in real-time.  It has a large database of astronomical objects (stars, planets, moons, minor bodies, etc.) that are rendered realistically and are positioned accurately in space and time.

A screenshot of Celestia running on my laptop.
A screenshot of Celestia running on my laptop.

One thing I discovered recently is that Celestia is also eminently programmable. What I mean is that, instead of manually zooming, flying-by and orbiting objects, Celestia can run scripts that execute complex macros. These scripts let you create engaging visualizations — either interactively initiated, or recorded into a video. Panning, zooming, orbiting, accelerating and flying through space with a cinematic flair, which makes for great outreach presentations. The scripting language itself is a fully-featured language (Lua, one of my favorite languages!), so math, looping constructs, data structures are all available.

I am using Celestia to prepare a short talk about how the discovery of exoplanets revolutionized planetary science, and shook a lot of the assumptions that were rooted in centuries of observing our Solar System.

Cover slide of my talk.

Above is the (draft) first slide of the talk. Each planetary slice is a screenshot from Celestia, using one of the planetary objects that will be touched during the talk. The composition of the slide was inspired by a recent posting on Reddit — a beautiful painting of the planets of the Solar System (I would have died of happiness if I got this painting when I was little!)

Another great feature of Celestia is that every single object is defined within a text file (with references to 3D models, textures, etc.) that are easily modifiable and extendable; a bundle of files placed in Celestia’s extras/ folder is loaded at startup, and the astronomical objects come to life.

This spawned another amazing resource for people doing astronomy outreach, the Celestia Motherlode. The Celestia Motherlode is an extensive repository of mods, textures and objects (even fictional ones!) that are freely downloadable. Some are high-resolution textures of the Solar System planets; others are exoplanetary systems that have been discovered since the discovery of Celestia; others still are beautiful renderings of hypothetical systems.

For my talk, I made several videos. Among them, these will be the background as I explain the basics of planet formation.

A zoom into the inner parts of a protoplanetary disk reveals planetesimals and embryos embedded in it…

[videojs poster=”http://www.stefanom.org/wp-content/uploads/2014/01/planetesimals.png” mp4=”http://www.stefanom.org/wp-content/uploads/2014/01/planetesimals.mov”]

…while outside the ice line, giant cores accrete massive atmospheres from the disk.

[videojs poster=”http://www.stefanom.org/wp-content/uploads/2014/01/giantplanets.png” mp4=”http://www.stefanom.org/wp-content/uploads/2014/01/giantplanets.mov”]

(Please attribute this website if you’d like to reuse them!). These shots were accomplished using scripts and custom textures and models from Motherlode (this add-on which I heavily modified, and this model for the impacted protoplanet).

Since I found Celestia such an useful, little-know tool,  I decided to write a series of blog posts on how to use its scripting facilities and create custom planetary systems. Hopefully they will be useful to fellow astronomers!

In the next post in this blog series, I will show how to create this simple animation that shows the orbits of the planets in the Solar System, and then rotates the view to show that the Solar System is rather flat (planet sizes not to scale, of course!):

[videojs poster=”http://www.stefanom.org/wp-content/uploads/2014/01/solar.png” mp4=”http://www.stefanom.org/wp-content/uploads/2014/01/solar.mov”]

Some useful resources to get started: