Dr. Antranik A. Sefilian - Planet Formation

PLANET FORMATION

The Impact of Cold Jupiters on Inner Planet Formation

Cold Jupiters are large planets located more than ~1 astronomical unit (AU; the Earth-Sun distance) away from their host stars, often exceeding ~0.3 times the mass of Jupiter. Traditionally, it is thought that the formation of a cold Jupiter in protoplanetary disks (which can happen relatively quickly) would block the flow of pebbles and planetesimals (the building blocks of planets) interior to its orbit, thereby hindering the formation of inner rocky planets. However, observations seem to suggest that, compared to field stars, cold Jupiters are more prevalent around stars hosting super-Earths (planets with a few Earth masses) at short orbital distances (interior to ~1 AU). These intriguing observations suggest that there might be more to the story; namely, that the presence of a cold Jupiter itself might somehow facilitate the formation of inner rocky planets.

In Best, Sefilian & Petrovich (2024), a study led by a PhD student under my co-supervision, we investigated the long-term orbital evolution of planetesimals embedded in a gaseous protoplanetary disk interior to a cold Jupiter (see Fig. 1 for a model schematic). In doing so, we considered the oft-neglected gravitational effects of the protoplanetary disk, alongside gas drag and the viscous evolution of the disk.

Figure 1: A schematic diagram illustrating the model system considered in Best et al. (2024).

fiducial_Best_et_al_2024.mp4

An animation showing the time evolution of planetesimal eccentricities (top panel) and apsidal angles (relative to the planet's; bottom panel) as a function of semimajor axis, together with the surface density of the planetesimals and the dissipating gaseous protoplanetary disk (middle panel; black and blue curves, respectively). Animation taken from Best et al. (2024).

Our findings reveal a fascinating process, which can be summarized as follows (see also the animation on the left). The cold Jupiter, being massive enough, carves a gap around its orbit. As a result, its orbit undergoes prograde apsidal precession due to the protoplanetary disk's gravity. On the other hand, the planetesimal orbits precess at a rate and direction set by the combined gravitational effects of the planet (prograde) and the protoplanetary disk (retrograde). Since the protoplanetary disk depletes over time (due to viscous evolution), the precessional frequencies change over time, leading to instances when the precession rates of the planetesimals and the planet match at some orbital radius. This commensurability, known as a secular resonance, drives the planetesimal eccentricities to relatively large values. These eccentricities are damped by the action of the gas drag, while forcing planetesimals to migrate inward, pushing them through the inner system as the resonance sweeps inward due to disk dissipation.

Consequently, a large percentage of the solid material available interior to the cold Jupiter's orbit is transported inwards. These planetesimals become concentrated into relatively narrow (size-segregated) rings with low orbital eccentricities and nearly aligned orbits, enhancing the chances for planet formation. We further demonstrated that this process is effective across various disk and planet parameters, particularly those similar to the Jupiters commonly detected in observational surveys. Finally, it is important to note that this process would not have occurred without accounting for the gravitational effects of the protoplanetary disk.

In summary, our study provides insights into the complex dynamics of planet formation, demonstrating how the presence of a cold Jupiter can facilitate the formation of inner planetary systems, including super-Earths.

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