[Paper Review] On the formation of planetary systems in photoevaporating transition discs

Caroline Terquem|arXiv (Cornell University)|Sep 20, 2016

Astrophysics and Star Formation Studies40 references4 citations

TL;DR

This study investigates planetary system formation in photoevaporating transition discs, focusing on 0.1 M⊕ and 1 M⊕ cores migrating in discs with X-ray-driven gap opening at ~2 au. It finds that low-mass cores form non-resonant, Earth-mass planets at 0.5–4 au, while massive cores may remain beyond the gap, producing systems resembling our Solar System; low mm flux transition discs likely host habitable-zone Earth-mass planets rather than hot super-Earths.

ABSTRACT

In protoplanetary discs, planetary cores must be at least 0.1 earth mass at 1 au for migration to be significant; this mass rises to 1 earth mass at 5 au. Planet formation models indicate that these cores form on million year timescales. We report here a study of the evolution of 0.1 earth mass and 1 earth mass cores, migrating from about 2 and 5 au respectively, in million year old photoevaporating discs. In such a disc, a gap opens up at around 2 au after a few million years. The inner region subsequently accrete onto the star on a smaller timescale. We find that, typically, the smallest cores form systems of non-resonant planets beyond 0.5 au with masses up to about 1.5 earth mass. In low mass discs, the same cores may evolve in situ. More massive cores form systems of a few earth masses planets. They migrate within the inner edge of the disc gap only in the most massive discs. Delivery of material to the inner parts of the disc ceases with opening of the gap. Interestingly, when the heavy cores do not migrate significantly, the type of systems that are produced resembles our solar system. This study suggests that low mm flux transition discs may not form systems of planets on short orbits but may instead harbour earth mass planets in the habitable zone.

Motivation & Objective

• To understand how planetary systems form in low mm flux transition discs undergoing X-ray photoevaporation.
• To investigate the migration and final architecture of 0.1 M⊕ and 1 M⊕ cores in evolving discs with gap opening.
• To determine whether such discs can form systems resembling the Solar system or instead produce tightly packed hot super-Earths.
• To assess the role of disc mass and photoevaporation in shaping planetary system architecture.
• To evaluate the potential for habitable-zone Earth-mass planets in low mm flux transition discs.

Proposed method

• Numerical simulations of planetary core migration in photoevaporating discs with X-ray-driven gap opening at ~2 au.
• Initial conditions include 0.1 M⊕ cores at 1–5 au and 1 M⊕ cores beyond 5 au, forming after ~1 Myr.
• Disc evolution includes viscous accretion and photoevaporation, with surface density decreasing over time.
• Core migration is modeled using type-I migration, with gap opening halting inward flow after ~3 Myr.
• Simulations track core interactions, collisions, and final orbital configurations post-disc dispersal.
• Outcomes are compared to observed systems, particularly Kepler-like hot super-Earths and the Solar system.

Experimental results

Research questions

• RQ1 Can 0.1 M⊕ cores form non-resonant planetary systems beyond 0.5 au in low mm flux transition discs?
• RQ2 Under what disc conditions do 1 M⊕ cores migrate into the inner disc or remain beyond the gap?
• RQ3 Why do low mm flux transition discs fail to produce hot super-Earths despite high core formation rates?
• RQ4 To what extent can the architecture of our Solar system be reproduced in photoevaporating discs?
• RQ5 Can low mm flux transition discs host Earth-mass planets in the habitable zone?

Key findings

• 0.1 M⊕ cores typically form systems of non-resonant planets with masses up to 1.5 M⊕ at 0.5–4 au, without significant inward migration.
• In low-mass discs, these cores may evolve in situ rather than migrating inward.
• 1 M⊕ cores form systems of a few Earth-mass planets, but only migrate into the inner disc in the most massive discs.
• When massive cores do not migrate significantly, the resulting system resembles the Solar system in architecture and mass distribution.
• Low mm flux transition discs are unlikely to form giant planets before gap opening, as material delivery ceases after gap formation.
• These discs are predicted to host Earth-mass planets in the habitable zone, consistent with the observed lack of short-period planets in such systems.