Astronomers Found Possible Evidence Of Planet Formation Through Gravitational Instability (Planetary Science)

Cadman and colleagues in their paper, presented their recent observations of the protoplanetary disc surrounding AB Aurigae. They have revealed the possible presence of two giant planets of masses 4–13 MJup (4–13 times that of the mass of the Jupiter) in the process of forming. Both planets observed at 𝑎 ≈ 30 AU and may have formed through gravitational disc instability (GI) in the natal AB Aurigae disc. Their study recently appeared on Journal ArXiv.

There are two widely held theories for how giant gas planets can form: core accretion and disk instability. Core accretion occurs from the collision and coagulation of solid particles into gradually larger bodies until a massive enough planetary embryo is formed (10-20 Earth masses) to accrete a gaseous envelope. On the other hand, gravitational instability mechanism occurs when the solar nebula breaks up through its own self-gravity into clumps of gas and dust, termed giant gaseous protoplanets (GGPPs), which then contract and collapse to form giant planets. Gravitational instability appears to be capable of forming giant planets with modest cores of ice and rock faster than the core accretion mechanism can.

Ab Aurigae is a 2.4 M, Herbig Ae/Be star, at a distance d ~ 162.9 pc. Various authors find an age for the star-disc system to be somewhere between 1–4 Myr. The young measured age of 1−4Myr for this system allowed Cadman and colleagues to place strict time constraints on the formation histories of the observed planets. Hence they may be able to make a crucial distinction between formation through core accretion (CA) or the gravitational instability (GI), as CA formation timescales are typically Myrs whilst formation through GI will occur within the first ≈ 104−105 yrs of disc evolution.

They focused their analysis on the 4−13 MJup planet observed at R ≈ 30AU. They found that CA formation timescales for such a massive planet typically exceed the system’s age. The planet’s high mass and wide orbit may instead be indicative of formation through GI. They used smoothed particle hydrodynamic simulations to determine the system’s critical disc mass for fragmentation, finding Md,crit=0.3M. Viscous evolution models of the disc’s mass history indicated that it was likely massive enough to exceed Md,crit in the recent past, thus it is possible that a young AB Aurigae disc may have fragmented to form multiple giant gaseous protoplanets.

They also done calculations of the Jeans mass in an AB Aurigae-like disc and found that fragments may initially form with masses 1.6−13.3MJup, consistent with the planets which have been observed. They therefore proposed that the inferred planets in the disc surrounding AB Aurigae may be evidence of planet formation through GI.

Featured image: SPH models of an AB Aurigae-like disc. Each disc is set up with 𝑀∗ = 2.4 M, 𝑅out = 400 AU, 𝑁 = 1 × 106
and Σ ∝ 𝑅¯1, 𝑐s ∝ 𝑅^ –0.25. They vary the disc-to-star mass ratios within the range 𝑞 = 0.08 – 0.15 (𝑀d = 0.2 – 0.35 M). They found the critical disc-to-star mass ratio for fragmentation in an AB Aurigae-like disc to be 𝑞crit = 0.125 (𝑀d,crit = 0.3 M). © Cadman et al.

Reference: James Cadman, Ken Rice, Cassandra Hall, “AB Aurigae: Possible evidence of planet formation through the gravitational instability”, ArXiv, pp. 1-12, 2021.

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