Investigating the role of porous pebbles as vehicles for gas transport from the protoplanetary disk onto the surface of an accreting planet

Abstract

The pebble accretion scenario has been proposed as an alternative to the classical theory of planet formation via planetesimal core accretion. In this alternative scenario, once the rocky cores of planets exceed a certain threshold, the mass evolution of planetary embryos are thought to be driven through the accretion of small aggregates of dust grains, which form in the protoplanetary disk, called pebbles. This theory solves several problems within the classical scenario; primarily, that planets forming via core accretion must accrete mass on a time scale much longer than is feasible, while planets forming via pebble accretion gain masses comparable to those within the solar system, crucially on a timescale falling within the lifetime of the protoplanetary disk. Secondly, while the core accretion scenario accurately describes the masses of some planets within the solar system, the rate of mass accretion drops sharply with stellar distance, necessitating untenable assumptions about the column density of the protoplanetary disk in order to explain the masses of the farther planets such as Uranus and Neptune.
In this project we reproduce a 1D model of planetary formation via pebble accretion. We assume in this model that the pebbles are silicates and investigate their sublimation and survival-rates as they move through the protoplanetary envelope. By making use of a dimensionless filling factor, describing the solid volume fraction of the pebbles, we assume that the corresponding fraction of the volume of these pebbles is taken up by stellar gases within the pores of the mineral. We then use the sublimation temperature of (forsterite) silicates in order to predict the amounts of gas from these inclusions that are deposited into the magma ocean on the surface of the planet. Our goal is to investigate whether this mechanism may be a viable candidate for explaining the discrepancies between the isotopic compositions of four representative noble gas isotopes found within the sun and within the earth’s mantle. The results point to the model overestimating the delivery of the light 22Ne isotope, while under-delivering the three heavier 36Ar, 84Kr and 130Xe isotopes compared to survey measurements of the terrestrial surface and atmospheric inventories. Interpreting the results of the project as a proof-of-concept for the model of gas accretion into the protoplanetary core, the over-delivery of 22Ne shows that even by this simplified model there will likely be a meaningful contribution of gaseous material to the core material within the pebble accretion scenario.

Popular Abstract

What are planets made out of? How do they form? What makes Mars a small, rocky, planet and Jupiter a large gaseous planet? Questions like these have been fascinating us since before we formed ways of writing them down. Sumerians studied planets movements across the skies in hopes to interpret messages from the gods, Plato imagined the planets marked the boundaries between different ’spheres’ of existence itself, and Sir Isaac Newton used their paths across the skies to infer the universal nature of gravity. To this day, the open question of how planets form continues to be a topic of ongoing discussion. While we now have a better understanding of the processes which play a part in planetary formation, there are still many question-marks which need straightening out. Our current best understanding of how planets are formed is centered around what is known as the core accretion scenario, where large asteroids called ’planetesimals’ zip around a large disk of ice, dust, and gas which forms around a young star known as the ’protoplanetary disk’ (PPD). As these planetesimals move around, they crash into one another and collect dust until they are massive enough to bind an ’envelope’ of gas, akin to a more diffuse atmosphere, around them. This envelope functions as a drag-net around the protoplanet, and as it moves through the PPD the friction between the dust in the medium and the envelope may help it accrete even more mass. The research question we are trying to answer with this project is whether these dust grain-coagulates, or ’pebbles’ as they are more commonly known, may themselves contain small pockets of gas which could then be dragged down into the forming planet as it accretes mass. If this is so, then pebble accretion may be the mechanism behind a previously unexplained phenomenon; why we see such similarities between the isotopic signatures of noble gases, found here on Earth, and within the Sun. If these gases were transported into the rocky core of the planet via more massive meteorites, then the concentrations would favor the heavier isotopes. We also know that the envelopes around protoplanets exchange some gas with the PPD, but we still have yet to explain the mechanism by which these gases from within the PPD are transported into the rocky cores of forming protoplanets.

• author: Conradzon, Emrik
• supervisor: Anders Johansen
• organization: Department of Physics; Astrophysics
• course: FYSK04 20261
• year: 2026
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• keywords: Planet Formation, Pebble Accretion, Geochemistry, Astrophysics
• report number: 2026-EXA257
• other publication id: 2026-EXA257
• language: English
• id: 9248742
• date added to LUP: 2026-08-21 09:58:49
• date last changed: 2026-08-21 09:58:49