Does Kuiper Belt In Our Solar System Still Produce Gas? (Planetary Science)

Summary:

⦿ Gas is now discovered commonly in exoplanetary systems with planetesimal belts.

⦿ Now, Karl and colleagues for the first time verified whether Kuiper Belt in our Solar System still produce gas.

⦿ They found that it is still producing carbon monoxide gas at the rate of 2 × 10¯8 M/Myr.

⦿ According to them, current instruments unable to validate it through observations. But future in-situ instruments having larger effective areas and pixel sizes could be able to detect it.


The past decade was very prolific in terms of detecting gas (mostly CO, C and O) around main sequence stars, therefore changing the paradigm of evolved planetary systems that were thought to be devoid of gas after 10 Myr. Indeed, most bright exoplanetesimal belts show the presence of gas, as demonstrated recently with ALMA, and it could be that all these belts have gas at some level (even if undetectable with current instruments). These belts, similar to our Kuiper belt, are made of large bodies colliding with each other and creating dust that can then be observed around extrasolar stars through its emission in the infrared above that of the star, which can be resolved at high resolution (showing, e.g., gaps and asymmetries that may be related to the presence of planets).

Now, Kral and colleagues by using sublimation model verified whether gas is also expected in the Kuiper belt (KB) in our Solar System. They predicted that gas is still produced in the KB right now at a rate of 2 × 10¯8 M/Myr for carbon monoxide (CO). Once released, the gas is quickly pushed out by the Solar wind. Therefore, they predicted a gas wind in our Solar System starting at the KB location and extending far beyond with a current total CO mass of ∼ 2 × 10¯12 M (i.e. 20 times the CO quantity that was lost by the Hale-Bopp comet during its sole 1997 passage) and CO density in the belt of 3 × 10¯7 cm¯3. They also predicted the existence of a slightly more massive atomic gas wind made of carbon and oxygen (neutral and ionized) with a mass of ∼ 10¯11 M.

Figure 1: CO gas production rate in the Kuiper belt M˙_CO as a function of time (t = 0 is when the gas release starts, i.e., probably after a few Myr to 10 Myr and the end of the lines on the right is today) predicted by their sublimation model. The solid line is for the Kuiper belt assuming it starts with a low mass similar to the current KB mass and the dashed line is for a more massive belt similar to the archetype β Pic belt. © Karl et al.

The question is, if KB really produce gas, why we haven’t been able to validate it through observations? Well, according to Kral and colleagues, we haven’t detected gas from KB yet, because the release is more diffuse as the emission comes from many Kuiper Belt Objects (KBOs) and it would be difficult to observe because of the lack of spatial contrast compared to extrasolar systems where most emission comes from a few beams. They predicted that Planck and ALMA (in a total power array mode) do not have enough sensitivity to detect the CO rotational lines of the diffuse wind.

The gas accumulated in the midplane of the KB along the line of sight to a background star would create some absorption in the UV on the star spectrum that could be identified as gas in our Solar System. However, they found that only future instruments may be able to detect this faint absorption. Furthermore, they suggested that the most promising technique would be to use in-situ missions similar to New Horizons to detect emission of resonance line scattering of carbon and/or oxygen excited by the Sun’s UV light. However, the effective area of the future potential instrument and its pixel size would need to be much larger to reach the low column density level predicted for atoms in the KB.

The presence of current gas predicted by their model in our Solar System would not impact the dynamics of bodies (dust or planetesimals) evolving around the KB. However, they note that in the past, when the Kuiper belt was much younger and heavier, the release of CO would have been orders of magnitude larger and the gas dynamics would have also been much different (e.g., in the fluid/hydrodynamic regime), potentially leading to some interesting effects, such as delivering some CO mass from the KB to planetary atmospheres as proposed recently for extrasolar systems.


Reference: Quentin Kral, J. E. Pringle, Aurélie Guilbert-Lepoutre, Luca Matrà, Julianne I. Moses, Emmanuel Lellouch, Mark C. Wyatt, Nicolas Biver, Dominique Bockelée-Morvan, Amy Bonsor, Franck Le Petit, “A molecular wind blows out of the Kuiper belt”, ArXiv, pp. 1-40, 2021. https://arxiv.org/abs/2104.01035


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