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Building planetary systems: linking architectures with formation

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Exoplanet systems more complex than previously thought

Exoplanets – planets beyond our solar system – were first discovered in the 1990s; while most orbit stars, some are free-floating. Drawing on new observations, BuildingPlanS sheds light on the variety of these planetary systems’ orbital configurations.

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While thousands of exoplanets have been identified, most have been in a relatively small region of our galaxy. With improved detection techniques, discoveries could rise tenfold within a decade, revealing more about their orbital configurations or ‘planetary architectures’. For Richard Alexander from the BuildingPlanS project, “Almost any planetary architecture you can think of exists somewhere, most look nothing like our solar system!” But what determines these architectures? BuildingPlanS, funded by the European Research Council, investigated how exoplanet systems form and how that influences present-day architectures. “We benefited from new astronomical observations which told us that there’s more going on in these systems than previously thought. For example, planet-forming disks were thought to be smooth and featureless, but are actually highly structured, with rings, gaps or spirals,” says Alexander, professor of Theoretical Astrophysics at Leicester University, the project host.

Reverse engineering planetary architectures

Planets form in gas-rich disks around young stars, but after formation the interaction between the planets and these ‘parent’ disks causes the planets to migrate to different orbits. “While we can observe the beginning of planet formation – protoplanetary disks – and the end products, most of what happens in-between remains unobservable. This is due to the time scales involved and the fact that the intermediate materials – basically big rocks – don’t emit enough light for us to see,” explains Alexander. BuildingPlanS built computational models to simulate this intermediary process. Most were hydrodynamic models of planets and gas/dust disks, using both public simulation codes, particularly PHANTOM, and the team’s own software. The data inputs came from observatories such as ALMA, the ESO VLT and the Hubble telescope. The team set initial disk parameters – such as mass, spatial extent and temperature – and the orbits and properties of the stars and planets, then simulated the planetary system’s evolution. They also ‘post-processed’ using radiative transfer calculations to see how it would appear to telescopes. BuildingPlanS benefited from new data coming from ALMA which indicated types of exoplanets not thought to exist. This led to a key early result about the Elias 24 planetary systems. The team showed that its observed disk structure is likely due to a giant planet in the outer disk, beyond Neptune. Other results related to misaligned disks – where disks aren’t confined to a single plane. “At the beginning of the project most people regarded these as novel, but our work shows that they’re actually relatively common,” remarks Alexander. An offshoot was work on the HD143006 system. The team’s models suggests it is a single planet orbiting two stars. In this case, the orbit of the stars around one another is not aligned with that of the planet. If confirmed, this would be the first known example of a so-called ‘misaligned circumbinary planet’.

A brave new world

The project’s findings have led Alexander to come up with two overarching takeaways to inform future endeavours. Firstly, that dust is very important for the early evolution of planetary systems. As Alexander puts it, “It isn’t just the building blocks of planets, it actually directly influences the evolution of the gas disk.” Secondly, that the environment of planetary systems is also vital, meaning that researchers might miss crucial effects if they treat systems in isolation to simplify studies. Looking forward, Alexander says: “I think the biggest leaps will now come from new observations, especially from the James Webb Space Telescope, which our team is involved with, thanks in part to BuildingPlanS.”


BuildingPlanS, planet, gas, disk, ALMA, observatories, solar system, stars, orbits, simulation

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