Forming wide-orbit planets via pebble accretion

Abstract

The possibility that detected substructures in protoplanetary disks, such as gaps, rings and cavities, are created by distant protoplanets has prompted an inquiry into the mechanism responsible for their formation. These wide-orbit young bodies are likely to form relatively rapidly in the outer regions of circumstellar disks. Pebble accretion is considered a promising candidate to explain their formation, as distant reservoirs of pebbles (mm-cm sized particles) have been confirmed, and the mechanism can enhance the growth rate of forming planets. However, the inwards migration of planets and the slower growth in the outer regions still challenge wide-orbit planet formation via pebble accretion. Therefore, the disk must harbour the appropriate characteristics for the fast growth of the body.

With the aim of constraining such features, we utilised existing models, which describe the growth and migration of distant protoplanets until the dispersal of the disk. We also presented a novel analytical model to explain the depletion of pebbles over the evolving disk. This model facilitates the simulation of more realistic formation scenarios when pebbles drift fast across the disk generating a strong but short-lasting pebble flux.

We found that a Moon-sized protoplanetary embryo formed at very early stages and located beyond 50 AU can grow up to become the core of a gas giant (and therefore open a gap) at 20-50 AU within less than 1 Myr. The outermost cores form when there is a strong but short-lasting pebble flux with a Stokes number of St \gtrsim 0.03. A metallicity of Z_0~0.01-0.02 and low turbulence of \alpha_t ~10^(-4) also enhances the formation of distant cores. As these cores form very early, they undergo a fast inwards migration while they accrete gas, and by the end of the disk lifetime (3 Myr), they become giant planets orbiting at <10 AU. In view of these results, we proposed a new mechanism for forming wide-orbit gas giants, and we showed that it might be possible to form gas giants at 10-50 AU at the end of the disk lifetime. According to our model, these planets are rare, in agreement with the low occurrence of gas giants in the outer regions from direct imaging surveys.

Overall, protoplanets migrate several AU before they become gas giants. That suggests that giant planets in our Solar System, such as Jupiter, might have started forming from protoplanetary embryos at distant locations. If the giant planets accrete most of their solid material in the outermost regions, this can have a significant impact on their ultimate composition.

Popular Abstract

Planets form alongside stars within protoplanetary disks composed of gas and solids. This material rotates around the central star and can convert into planets. The tiniest solid particles, called dust, can grow up to form rocky/icy planets or the core of giant planets. In the latter case, once the core forms, it attracts gas and becomes a gas giant.

The initial dust grows into pebbles, which are mm-cm size solid particles. The gas rotates slower than solids, but dust easily couples to the slower rotation. However, when dust grows up to the size of pebbles, the friction exerted by the gas decelerates them. Consequently, pebbles might fall towards the star. In this project, we studied how a Moon-size body, called the embryo, can attract these drifting pebbles. When the embryo effectively attracts pebbles, it can grow to over ten times the size of Earth, which is similar to the typical size of the core of gas giants like Jupiter and Saturn. We refer to this process as pebble accretion.

While the embryo grows via pebble accretion, it also gets closer to the star because of its gravitational interaction with the gas. Therefore, it is challenging to keep planets in the outer regions. Our understanding of the mechanism responsible for the observed wide-orbit exoplanets remains elusive. In addition, from observations of protoplanetary disks, we know that some disks have substructures, such as gaps in the outer regions, which might be formed by massive and distant planets. Moreover, some of these planets would have to form very quickly since the gaps are also observed within young disks. Our main goal has been to figure out if it is possible to form planets with these characteristics via pebble accretion.

The key finding of our work is that pebble accretion could explain the fast formation of cores in the outer regions under certain conditions. We need the embryos to form early in the outermost regions and the gas to decelerate pebbles efficiently. When the pebble mass reservoir is large, the distance away from the star at which cores can form increases. We also found that if the turbulent motions of the gas are weak, it is easier to form cores in the outer regions. The core can form far away from the star at the early stages, and therefore, it can still attract gas and become a gas giant. However, as there might be still plenty of gas remaining, the large amount of gas decelerates the core, and thus, it migrates inwards for a long distance. Drawing an analogy with the Solar System, the cores could form beyond Neptune's orbit but eventually evolve into gas giants orbiting between Jupiter and Saturn.

• author: Gurrutxaga, Nerea
• supervisor: Anders Johansen; Michiel Lambrechts
• organization: Astrophysics; Department of Physics
• course: ASTM32 20231
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Planet Formation, Pebble Accretion, Protoplanetary Disk, Wide-Orbit Planet
• report number: 2023-EXA212
• language: English
• id: 9130998
• date added to LUP: 2023-06-28 11:28:38
• date last changed: 2023-08-30 14:59:19