On the origin of wide-orbit ALMA planets: giant protoplanets disrupted by their cores

Abstract

Recent ALMA observations may indicate a surprising abundance of sub-Jovian planets on very wide orbits in protoplanetary discs that are only a few million years old. These planets are too young and distant to have been formed via the Core Accretion (CA) scenario, and are much less massive than the gas clumps born in the classical Gravitational Instability (GI) theory. It was recently suggested that such planets may form by the partial destruction of GI protoplanets: energy output due to the growth of a massive core may unbind all or most of the surrounding pre-collapse protoplanet.Here we present the first 3D global disc simulations that simultaneously resolve grain dynamics in the disc and within the protoplanet. We confirm that massive GI protoplanets may self-destruct at arbitrarily large separations from the host star provided that solid cores of mass ∼10-20 M ⊕ are able to grow inside them during their precollapse phase. In addition, we find that the heating force recently analysed by Masset & Velasco Romero (2017) perturbs these cores away from the centre of their gaseous protoplanets. This leads to very complicated dust dynamics in the protoplanet centre, potentially resulting in the formation of multiple cores, planetary satellites, and other debris such as planetesimals within the same protoplanet. A unique prediction of this planet formation scenario is the presence of sub-Jovian planets at wide orbits in Class 0/I protoplanetary discs.

The role of density perturbation on planet formation by pebble accretion

“…One major limitation of this approach is that it mainly produces giant planets, although tidal downsizing may follow, resulting in much smaller planets (Nayakshin 2010). Another limitation of the gravitational collapse is that it occurs preferably in the outer disc locations where the gas is dynamically cold enough to allow this process (Boss 1997;Boley 2009;Armitage 2010;Humphries & Nayakshin 2019). Thus it is difficult to explain the existence of the less massive planets in the inner disc parts based on the theory of gravitational collapse.…”

An effective method in investigating structures of polytropic protoplanets formed via gravitational instability

“…Upon satisfaction of the threshold of the dust-to-gas ratio, the pebble clumps can collapse straight into planetesimals. It is seen that the growth of the core via the accretion of pebbles is much quicker; however, the mechanism is not so effective [16] , [17] and may necessitate more pebbles than the observations indicate [18] . Furthermore, the formation of terrestrial planets is relatively fast [19] .…”

“…Furthermore, the formation of terrestrial planets is relatively fast [19] . With the problems in the CA mechanism, the GI paradigm, an alternative to the CA mechanism of planetary formation, has been reformulated with fragmentation from massive protoplanetary discs (PPDs) [11] , [18] , [20] , [21] , [22] , [23] . But, as in [20] , this paradigm has also received a lot of criticism.…”

“…Some noteworthy problems of the scenario are (i) PPDs are unable to form planetary embryos at the present location of Jupiter [24] , conflicting with the earlier results by Boss [9] ; (ii) sedimentation of dust is so a slow process that it is failed to yield observed solid cores in giant embryos of mass [25] ; (iii) OB stars (hot, massive stars of spectral types O or early-type B) are too rare to explain the abundance of Jupiter like planets observed in exosolar planetary systems [20] and (iv) it is believed that the formation of Earth-sized terrestrial planets is rarely possible by the GI [4] , [20] . However, many authors have tried to solve the problems found in the GI scenario [11] , [18] , [20] , [21] , [22] , [26] , [27] , [28] , [29] . The tidal downsizing mechanism, a revised form of the GI paradigm with planet migration inward and tidal disruptions of GI fragments in the inner regions of the disc [27] , could account for all observational facts relevant to the process of planetary evolution [22] .…”

“…The tidal downsizing mechanism, a revised form of the GI paradigm with planet migration inward and tidal disruptions of GI fragments in the inner regions of the disc [27], could account for all observational facts relevant to the process of planetary evolution [22]. With the theoretical work as well as simulations, Humphries and Nayakshin [18,19] exhibited that giant planets assembled by the GI can be very metal-rich as required by the solar and exoplanetary data and that these planets can migrate inward and explain the closer-in data as well [29]. Recently, Atacama Large Millimeter/submillimeter Array (ALMA) has been used widely to investigate the PPDs, which makes available information on the planetary systems orbiting stars other than the Sun during all stages of evolution [23,30].…”