The Journey of Lonely Planets: How Planet-Planet Scattering Creates Free-Floating Worlds

Planet-planet scattering is a dynamic process that occurs in many planetary systems, shaping their structure and stability. When planets interact gravitationally, they can experience strong scattering events that lead to collisions, mergers, or even ejections from their host star’s gravitational influence. This study by Hareesh Gautham Bhaskar and Hagai B. Perets focuses on how such scattering processes contribute to the creation of free-floating planets (FFPs) — planets that drift through space without orbiting a star. By simulating various planetary systems over billions of years, the authors provide a detailed view of the properties and evolution of these rogue planets.

Simulating Planetary Chaos

To explore the outcomes of planet-planet scattering, the authors conducted N-body simulations of planetary systems with 3 to 10 planets. Each system began with planets on nearly circular orbits around a star like our Sun. Key parameters, like the number of planets, their distances from the star, and their masses, were varied to assess how these factors influence ejection rates and other planetary behaviors. For a planet to be considered “ejected,” it had to reach 100,000 astronomical units (AU) from its star.

Key Findings: The Path to Freedom

The study revealed that 40% to 80% of planets are ejected from their systems, with most ejections occurring within the first 100 million years. Interestingly, all planets in a system are equally likely to be ejected, regardless of their starting position. Ejected planets typically leave their systems with speeds ranging from 2 to 6 kilometers per second. Inner planets tend to have higher ejection speeds due to their proximity to the star and stronger gravitational interactions.

Another finding is the role of planetary collisions. Larger planets or systems with closely spaced planets experience more collisions, reducing the number of ejected planets. Collisions often result in the formation of more massive planets that remain bound to the star. Increasing the initial spacing between planets delays ejections but does not significantly change the total number of planets ejected.

What Shapes the Fate of Ejected Planets?

The simulations also explored how factors like planet size and ejection distance influence the properties of FFPs. Larger planets with bigger radii are less likely to be ejected because they collide more frequently. On the other hand, the distance at which a planet is declared “ejected” (100 AU vs. 100,000 AU) does not significantly change the number of FFPs, but it affects how quickly planets are classified as ejected.

Comparing Theory to Observations

The study’s results align well with observations of free-floating planets detected through microlensing and imaging. The authors estimate that for each star, 5 to 10 planets must form to explain the observed number of FFPs. This suggests that while planet-planet scattering cannot explain all FFPs, it plays a significant role in producing the observed population, especially for low-mass rogue planets.

Bound Planets: Survivors of the Chaos

Not all planets are ejected; some survive as bound companions to their star. These planets often end up on highly eccentric (elongated) orbits, with some experiencing significant tilts compared to their original orbital plane. Systems with more initial planets tend to produce survivors with wider orbits and higher eccentricities. Interestingly, planets on the widest orbits are often the result of scattering events rather than direct ejections, highlighting the diversity of planetary outcomes.

Conclusion: A Cosmic Ballet

This study sheds light on the chaotic dance of planets and the dramatic events that lead to the creation of free-floating worlds. By simulating billions of years of planetary evolution, the authors provide a theoretical foundation to understand the origins of these lonely planets. As future telescopes like the Nancy Grace Roman Space Telescope and the Rubin Observatory discover more FFPs, studies like this will help scientists piece together their mysterious journeys through the galaxy.

Source: Bhaskar