Jupiter's Composition Suggests its Core Assembled Exterior to the N₂ Snowline

Abstract

Jupiter's atmosphere is enriched in C, N, S, P, Ar, Kr and Xe with respect to solar abundances by a factor of ∼3. Gas Giant envelopes are mainly enriched through the dissolution of solids in the atmosphere, and this constant enrichment factor is puzzling since several of the above elements are not expected to have been in the solid phase in Jupiter's feeding zone; most seriously, Ar and the main carrier of N, N 2 , only condense at the very low temperatures, 21-26 K, associated with the outer solar nebula. We propose that a plausible solution to the enigma of Jupiter's uniform enrichment pattern is that Jupiter's core formed exterior to the N 2 and Ar snowlines, beyond 30 au, resulting in a Solar composition core in all volatiles heavier than Ne. During envelope accretion and planetesimal bombardment, some of the core mixed in with the envelope causing the observed enrichment pattern. We show that this scenario naturally produces the observed atmosphere composition, even with substantial pollution from N-poor pebble and planetesimal accretion in Jupiter's final feeding zone. We note that giant core formation at large nebular radii is consistent with recent models of gas giant core formation through pebble accretion, which requires the core to form exterior to Jupiter's current location to counter rapid inward migration during the core and envelope formation process. If this scenario is common, gas giant core formation may account for many of the gaps observed in protoplanetary disks between 10s and 100 au.

Retrieving scattering clouds and disequilibrium chemistry in the atmosphere of HR 8799e

“…Because HR 8799e is the innermost planet of the HR 8799 system, this could indicate that all HR 8799 planets formed outside of the CO iceline. Similar formation distances, relative to the icelines, have been theorized for Jupiter in the Solar System (Öberg & Wordsworth 2019;Bosman et al 2019). Using sophisticated gas-grain chemical modeling for the protoplanetary disk we find that the planet could also have formed more closely to the star, but outside the CO 2 iceline.…”

“…Finally, we note that the nitrogen content may be a better way of constraining a planets formation location in the disk (Öberg & Wordsworth 2019;Bosman et al 2019), where a large N-content corresponds to a formation in the outer parts of the disk. However, this would require to study planets cooler than HR 8799e, for which most of its accreted nitrogen is in the form of N 2 , and therefore invisible due to the low N 2 opacity.…”

“…To constrain HR 8799e's formation history, we make the assumption that the disk from which the HR 8799 planets formed had a composition as specified in Table 1 of Öberg & Wordsworth (2019). The authors of the paper present this as a model for the young solar nebula.…”

“…Using the planetary mass of 4.81 +8.78 −3.33 M J , the metallicity of 0.48 +0.25 −0.29 and the C/O of 0.60 +0.07 −0.08 , inferred from our spectral retrieval with petitRADTRANS, we find that HR 8799e is likely heavily enriched in ices, accreted in forms of pebbles and planetesimals, and most likely formed outside of the CO iceline. This conclusion was obtained from fitting the O/H and C/H content of HR 8799e, derived from our spectral retrieval, with an abundance model of a planet that forms in a disk as defined in Öberg & Wordsworth (2019). For this we treated the planets mass, accreted solid mass, and the accretion locations of the solids and gas as free parameters.…”

“…If HR 8799e truly formed outside the CO iceline, this also allows to put constraints on the planet's possible migration. In Öberg & Wordsworth (2019), the CO iceline is situated at ∼20 au for the young solar nebula. In the disk around HR 8799, the same temperature due to the irradiation of the star would have been reached at ∼45 au (Marois et al 2010), neglecting the evolution of the stellar luminosity at young ages.…”