TNO colours as a test for a past, close stellar flyby to the solar system

Trans-Neptunian Objects (TNOs) are remnants of the planetesimal population formed during the planet formation stage \citep{Gladman:2021}. Unlike the planets, most TNOs move on inclined eccentric orbits. Different models are proposed to explain these dynamics. However, another constraint comes from the TNOs' chemical composition, which may provide additional valuable insights into the Solar System's early history. Both dynamics and composition have to be explained simultaneously using the same model. This puts tight constraints on any early solar system model. We show that a stellar flyby can stand this rigorous test of explaining the TNOs dynamics and colour distribution simultaneously.

 

Recently, we looked at a stellar flyby as an alternative to the planet instability model to explain the TNOs dynamics (Pfalzner et al. 2004). While migration of the giant planets during the early stages of Solar System evolution could have induced substantial scattering of planetesimals producing TNOs on inclined, eccentric orbits, this process cannot account for the small number of distant TNOs (p > 60 au) outside the planets' reach and retrograde TNOs. The alternative scenario of the close flyby of another star delivers all these TNO features simultaneously. We found that a 0.8 M⊙ star passing at a distance of p = 110 au, inclined by i = 70°, gives a near-perfect match. This flyby also reproduces the cold, hot, Sena-like, retrograde TNO populations. The next step is to test whether the same flyby can also account for the TNO's colour distribution.

 

Detailed compositional data mainly exists for the largest TNOs. The smaller TNOs are often too faint for spectroscopic observations (e.g. Emery et al. 2024}. As a result, their surface composition is typically analysed using broadband photometry. This type of observation shows that the colour distribution of TNOs ranges from grey to very red {e.g. Barucci et al. 2020}. At greater distances from the Sun, temperatures drop significantly, which could have strongly influenced local chemistry during planetesimal formation. Therefore, one may expect that the colours of TNOs varied with their distance from the Sun. However, observations do not show such a straightforward correlation between TNO colour and heliocentric distance {Jewitt:2001}. Early studies already showed that TNOs with low inclination and low eccentricity are predominantly very red. In contrast, TNOs with higher inclinations (i.e., i > 5 degrees) and higher eccentricities, typical of the hot Kuiper Belt, exhibit a mix of red and grey colors. Recent studies, including the Outer Solar System Survey (OSSOS) (Schwamb et al. 2019) and the Dark Energy Survey (DES) (Bernardinelli:2023), along with observations from the James Webb Space Telescope (JWST) (Pinilla:2024) have confirmed these findings.

 

We simulate the effect of a stellar flyby on a disc represented by massless test particles, which initially orbit the Sun on Keplerian trajectories. We assume that before the flyby, the model disc extended to 150 au and exhibited a colour gradient due to its TNO composition. We follow the trajectories of the test particles during the flyby and investigate their final properties using the REBOUND code.

 

We find that the flyby explains the observed colour structures found in the OSSOS and DES survey. The complex colour distribution directly results from this transport in the spiral arms. It successfully links the colours to the dynamics of the TNOs. In particular, the flyby naturally produces the increased dominance of grey vs. very red TNOs for higher inclinations. This results in the scarcity of very red TNOs for inclinations >21°. It also leads to the observed lack of very red TNO among very eccentric (e > 0.42) TNOs.

 

The lack of very red objects among the irregular moons can be explained as a direct result of originating in the outer regions ($r >$ 60 au) of the disc (Pfalzner & Govind 2004). We find now that they may originate from the same reservoir as the high-inclination TNOs.

 

The combined reproduction of the TNO dynamics and colours significantly strengthens the argument for a stellar flyby being responsible for the intricate structure of the solar system beyond Neptune. Up-coming instruments, in particular LSST, hold the promise of detecting many thousands of new TNOs.

 

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