Are Solar Systems Eccentric?

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Eccentricity Distribution Beyond the Snow Line and Implications for Planetary Habitability
Authors: S. Kane and R. Wittenmyer
First Author’s Institution: University of California, Riverside
Status: Published in ApJL

A fundamental question in exoplanet science is to discern how common or rare our solar system is compared to the other planetary systems across the galaxy. While we have found many thousands of exoplanetary systems, so far, none “look” like our own, with small, rocky planets interior to gas giant planets. One reason we haven’t found any lookalikes is an observational bias that effectively prevents us from being sensitive to these systems. For example, small planets comparable to Earth in size are very difficult to detect through planetary transits (which give the planet’s radius) and impossible currently to detect with radial-velocity measurements (which give the planet’s mass). Therefore, we don’t expect to be finding systems like our own solar system, at least not yet (we’re getting closer every day!). So how can we begin to answer this question of solar system–like architecture occurrence rate in the meantime?

Another special feature of our solar system is Jupiter’s low eccentricity. As a quick refresher, eccentricity is the measure of how elliptical a planet’s orbit is. The more elliptical, the more eccentric. The more circular the orbit, the less eccentric. In our solar system, all planets have orbital eccentricities of less than 0.2 (most are near 0.05), which means all orbits are very nearly perfect circles. But we have found exoplanets with very high eccentricities that have interesting implications for planet formation and the dynamical evolution of exoplanetary systems, as explored in this astrobite. Jupiter’s low eccentricity of 0.04 has big implications for how Jupiter likely scattered material into the inner solar system during its formation, potentially seeding the inner solar system with the very material needed for our own planet Earth to form and become habitable to life as we know it — in particular, ices, which in planetary science can be water ice, things like CO₂ ice aka “dry ice,” or even things like methane that condense into solids at the very cold temperatures of the outer solar system. Today’s daily article summary looks at a study that attempts to investigate how efficient this scattering of ices to the inner system is for Jupiter-like planets in hypothetical planetary systems.

First, the authors look to quantify how common or rare it is for known Jupiter-like planets to have low eccentricities like our own Jupiter. They took all planets with well-measured masses and cut out all those with masses below 0.3 Jupiter mass, or about the mass of Saturn. They then split this sample of 846 planets into those interior and exterior to the “snow line.” In a planetary system, the snow line is the minimum distance from the host star where it is cold enough that water exists only as ice. For each star, the snow line exists at a different semi-major axis, based entirely on the temperature of the host star. Giant planets beyond the snow line can scatter these ices towards the inner system, but giant planets within the snow line cannot bring this material from beyond the snow line to within the snow line. The authors find that the mean eccentricity for these giant planets within the snow line is 0.18, but the median is only 0.01. The big discrepancy between the mean and median suggests that this sample is heavily skewed by outliers. Additionally, there is a bias toward very short-period planets because orbits that were eccentric can become circular over time through tidal forces (see here). However, the mean and median eccentricity of giant planets beyond the snow line, like our Jupiter, are 0.29 and 0.23; these numbers being similar suggests that we are neither biased nor skewed within this sample.

The authors compared the eccentricities of the planets interior and exterior to the snow line using a Kolmogorov–Smirnov (K–S) test. In short, this statistical test is used to determine if two populations are drawn from the same distribution or from completely different distributions (see this earlier astrobite). For K–S test values near 1.0, it is most likely that the two groups are drawn from the same parent distribution, but for values near 0.0, it is most likely that we are truly seeing two different distributions — in this case, different distributions of eccentricities, which would imply different formation mechanisms. The results of this K–S test on the population of giant planets within and beyond the snow line yields a value very near 0.0, so they are truly different populations. However, when the authors cut out all planets with orbital periods less than 10 days (all within the snow line) and repeat the test, the K–S value is now 0.977, implying that they were from the same distribution. This is in line with earlier works that show that very close-in planets circularize their orbits, so when we exclude these very close-in giants, we recover that generally giant planets at all orbital separations are one population. With it established that giant planets (except for very close-in planets) form out of the same distribution of eccentricities regardless of orbital separation, the authors now ask a new question: does the eccentricity of a giant planet help or hurt the scattering of ices to the inner system?

To answer this, the authors run a suite of dynamical simulations. They initialize each simulation the same way: with a 1-solar-mass star and a 1-Jupiter-mass planet at a separation of 5.2 au (Jupiter’s true separation). Then they initialize the simulation with many thousands of “ice particles” at various locations within the system. Lastly, in one simulation, they give the Jupiter planet a small eccentricity of 0.05 and in another they set the eccentricity at 0.23. They then let the simulations run for 10,000 years and at the end, they count up all the ice particles that were scattered by the planet’s gravity into the inner system, measuring both how many cross within the snow line and how many cross within 1 au, Earth’s orbital separation. The results can be seen in Figure 1. They find that the high-eccentricity planet is much more efficient than the low-eccentricity Jupiter at scattering these ice particles into the inner system, at a rate of double for getting ices within the snow line and at eight times higher for getting ices within 1 au.

This has big implications for the formation of small, Earth-like planets and specifically for forming these planets with lots of water. It suggests that systems with higher-eccentricity Jupiter-like planets may be more efficient at forming small, watery planets. However, the authors note a few caveats to this finding. First, they acknowledge the bias in detecting more eccentric Jupiter-like planets. This in itself may have biased their choice of using 0.23 for the high-eccentricity case simulation. Second, they state that a higher scattering rate does not directly convey a higher delivery rate of these ices to small planets, it merely makes the collision events that do deliver these ices to be more probable. Lastly, they note that there are many other theories for how Earth received its water that are not necessarily tied to the delivery of ices from the outer solar system.

In all, this study sheds light on the population of Jupiter-like planets across the galaxy and shows that higher-eccentricity Jupiters are better at scattering ices to the inner system. This could point the way for further follow-up surveys to target systems with eccentric Jupiter-like planets in our search for small watery planets.

Original astrobite edited by Skylar Grayson.

About the author, Jack Lubin:

Jack received his PhD in astrophysics from UC Irvine and is now a postdoc at UCLA. His research focuses on exoplanet detection and characterization, primarily using the radial velocity method. He enjoys communicating science and encourages everyone to be an observer of the world around them.