Planet Formation and Exoplanets: A Royal Society Journey from Our Solar System to the Galaxy

Overview

In this Royal Society lecture, Rosanna Merrow explains how our view of planets has expanded from the eight planets in our solar system to thousands of exoplanets discovered around other stars. She describes the observational tools that reveal planet forming discs, the diversity of planetary systems, and the core ideas behind how planets form, including core accretion and disc instabilities. The talk emphasizes the synergy between observations and simulations, and it ends with reflections on whether our solar system is typical and what the next decades may bring in the search for Earth like worlds.

Introduction and scope

The talk begins by highlighting a thirty year shift in planetary science. The speaker notes that thirty years ago we knew only the planets in our solar system. Since then, observations and simulations have revolutionized our understanding, revealing thousands of extrasolar planets and providing a window into how planets form across diverse environments. The presenter, Rosanna Merrow, has worked on planet formation for nearly two decades and guides the audience through the current landscape, from observations to high performance computing that enables simulations of planet formation.

The scale of the cosmos and where planets live

To put planets in context, the speaker presents a cosmic perspective. The Earth is shown as a tiny part of the Milky Way, which itself is one of hundreds of billions of galaxies in the observable universe. The Hubble Extreme Deep Field is presented as the deepest image of the sky, illustrating the vast number of galaxies and, by extension, the potential abundance of planets. The talk emphasizes that while planets are small on cosmic scales, they are central to understanding cosmic structure and history. The audience is reminded that there are eight planets in our solar system, and this number has evolved with observations of extrasolar worlds.

Technologies and methods driving discovery

The talk surveys the instruments and computational resources that enable the study of planets beyond the solar system. Space based telescopes such as the Hubble Space Telescope and the Spitzer Space Telescope, along with ground based observatories, provide diverse data. High performance computing facilities are used to analyse observations and run simulations that illuminate the physics of planet formation. The combination of observations and simulations is essential for forming a coherent picture of how planets form and evolve.

Diversity of planetary systems

A central theme is that planetary systems are highly diverse. The TRAPPIST 1 system showcases seven Earth sized planets packed in an extremely compact configuration, with planets orbiting much closer to their star than Mercury does to the Sun. In contrast, the HR 8799 system contains four planets with masses up to ten times that of Jupiter, some located well beyond Neptune’s orbit. This diversity presents a substantial challenge to formation theories, as there is a wide range of architectures and dynamical histories to explain.

Planet formation environments

The environment of planet formation is explored through the view of star forming regions like the Orion Nebula. Through both visible and infrared observations, the disc around young stars is revealed and these discs extend to hundreds of astronomical units, far larger than the planetary orbits in our solar system. Observational images from the Atacama Large Millimeter/submillimeter Array show disc structures such as rings and gaps, which may indicate forming planets. The environmental context includes molecular clouds, disc formation, and the interactions within star forming clusters that can influence the formation and early evolution of planets and their atmospheres.

Planet formation mechanisms

The speaker explains two primary formation pathways. The core accretion model describes how dust grains collide and stick, gradually building a solid core that can attract gas to form a gas giant if the core becomes massive enough. A second possibility occurs when discs are very young and strongly gravitationally unstable, leading to the rapid formation of bound gas clumps that can contract into giant planets. Observations of discs with clear spiral structures and substructures provide evidence for these processes. The talk emphasizes that real discs are dynamic and may host multiple formation channels, contributing to the diversity of planetary systems observed.

Observations, simulations, and theory in concert

Merrow highlights the power of combining observations with computer simulations. Hydrodynamical simulations of gas and dust in discs create predictions that can be tested with telescope data. Observations of moving planets within discs help validate theories about planet-disc interactions and migration. The synergy between observation and simulation is essential for reconstructing the evolutionary path from a protoplanetary disc to a mature planetary system.

What the next decades may reveal

The talk closes with reflections on whether Earth like planets will be directly imaged, what planets are made of, and whether our solar system is typical. The field continues to evolve as new instruments and computing resources come online, promising to refine our understanding of planet formation and the prevalence of habitable worlds. The closing note asserts the universe invites discovery and that continued exploration will likely bring surprises that redefine our expectations about planetary systems and their origins.

Audience questions

Following the talk, questions touch on topics such as planetary spin, habitable conditions, and the nature of life. The presenter addresses how disc geometry and angular momentum influence planet spins, habitability criteria, and the complexity of life in potentially habitable environments. These interactions illustrate how the field moves from broad conceptual questions to concrete, testable hypotheses about planets, their formation, and their potential to host life.