Identifying the sculptor: what dynamical processes lead to observed planet multiplicity

Title: AGES OF “SINGLES” VERSUS “MULTIS”: PREDICTIONS FOR DYNAMICAL SCULPTING OVER GYR IN THE KEPLER SAMPLE

Authors: C. Lam and S. Ballard

Authors’ Institution:  University of Florida

Status: ArXiv [open access]

A puzzle from the Kepler Mission’s data set still remains with us even over a decade after the satellite was retired. Exoplanet scientists were surprised by how many single planet systems Kepler found, as opposed to systems with multiple transiting planets. This spurred the phrase, the “Kepler Dichotomy” to capture the sense that there seemed to be two kinds of planetary systems: singles and multis. Since then, many studies have been produced trying to explain the Kepler Dichotomy. Is it an observation bias? Is the average mutual inclination in planetary systems larger than expected so that we only see one of many planets as transiting from our Earthly perspective? Do planetary systems form through one of two mechanisms, where one produces single planet systems and the other produces multi planet systems? These are questions that have been explored, but not fully answered, to date.

Today’s Astrobite covers a new paper that takes a fresh look at trying to explain our observations of the Kepler sample. The paper looks at the problem through the lens of planetary system “sculpting”, or how the evolution and dynamics of a system over time can change the system’s architecture. For planetary systems that have multiple planets where each planet has a nearly circular orbit (low eccentricity) and low inclination (therefore all orbit in nearly the same planet) are said to be “dynamically cold”. This means that the planets in these systems essentially do not interact with each other over time. They each go about their orbits undisturbed, and therefore the system is very stable over time. The orbits of each planet today are likely nearly the same as the orbits of each planet millions to billions of years ago. On the other hand, “dynamically hot” systems have planets with varying eccentricities and inclinations, so their orbits are more often disturbed by encounters with neighboring planets and this can lead to an unstable system, where planets can collide or be completely ejected. But less catastrophically, a dynamically hot system stirs up the inclinations of the planets within the system and so it is less likely that all the planets in the system will be seen as transiting planets from Earth’s perspective. This can cause us to miss planets that are truly there, perhaps even causing us to find many single planet systems. Could this be an answer to the Kepler Dichotomy?

Today’s paper explores how a planetary system may go from dynamically cold in the past to dynamically hot today and how that might affect the system we observe today. The authors first look at the ratio of multi-planet systems to single planet systems as a function of host star age. If older host stars have more single planet systems, it could be evidence for systems becoming dynamically hot over time. They run this test twice, once for metal rich stars and again for metal poor stars, see Figure 1. Similarly they run the test for different stellar types. However, in all cases they find no statistically significant trend of planetary multiplicity and stellar age.

Figure 1 (Figure 3 in the paper): When splitting the population of systems between old and young stars, the fraction of planets in the young star bin vs the old star bin is nearly the same. And this is true no matter where you draw the line between young and old. It is similarly true if you also track the metallicity of the host stars.

Next the authors create a detailed and novel suite of simulations to test if or how a system might become dynamically hot over time. They create multi-planet systems and state the following rules: 1. some systems start as “intact”, that is to say all the planets are transiting from our perspective, 2. the dynamical temperature must move from cold to hot over time, 3.the probability of staying intact changes over time but always trends downward, and 4. the dynamical temperature can stop changing at some point. These 4 parameters fully describe their possible “sculptor”, or the mechanism that creates the planetary systems we see today. Then within these simulated systems, they compute how likely it is to see 1 or more planets as transiting from our perspective over time. The goal is to closely reproduce our current sample of real planetary systems from the Kepler mission. If a certain set of parameters, i.e. a “sculptor”, yields simulated results close to real results, that could be evidence for the simulated sculpting mechanism actually playing out in nature.

Overall, the team finds that while they cannot state for certain which sculptors are consistent with the real data, they can say which ones are inconsistent. This is a powerful designation on its own. They also find that our ability to determine which sculptor(s) do match the observed data hinges critically on how precisely we can measure the age of host stars. Right now, our precision on stellar ages is simply not good enough. The authors demonstrate this through artificially increasing the precision on the stellar ages in their sample and finding that under these conditions, they can find a point in their simulations where the sculptors deviate enough to differentiate them, and one seems to match real data better than the other, see Figure 2.

This paper presents a really novel and exciting set of simulations! This is an open and active area of research and this paper hints that more conclusive findings can be just over the horizon if we can measure stellar ages more precisely. 

Figure 2 (Figure 10 in the paper): When stellar age measurements are not precise enough, it is not possible to differentiate between the possible mechanisms responsible for creating our current observed number of planets (top). But if stellar ages were more precise (bottom) then it would be possible to explain a difference.

Astrobite edited by Tori Bonidie

Featured image credit: Lam and Ballard 2024

About 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.

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