Discovery of Two Rings Outside Roche Limit of Quaoar

Some bodies in the solar system have rings, such as Saturn.Initially, the rings consisted of four giant planets, namelyJupiter・Saturn・Uranus・NeptuneSince it was known only inHowever, in 2014 an asteroidCharikloStarting with the discovery of the ring inKironandHaumea,andquaoarBut rings have been found (※1) 。

*1…However, the existence of the ring of Chiron is controversial, and there is also a negative opinion that it does not exist. This makes Kwaor’s ring the seventh or eighth ring discovered in the solar system.

The Kwaoar ring was just published in a paper in February 2023, and was revealed in an analysis of observational data obtained from 2018 to 2021.

The rings of small celestial bodies are extremely thin and faint, so they cannot be observed directly with a telescope.star eclipse” must be observed. When an asteroid crosses in front of a star, the star appears to disappear temporarily from the Earth because it is hidden by the asteroid. This is a phenomenon called star eclipse. When a celestial body without a ring crosses, the fixed star disappears only once, but in the case of a celestial body with a ring, the ring crosses before and after the body crosses, so the fixed star repeats disappearing and appearing several times. Become. By precisely measuring the timing and duration of the disappearance of a fixed star, we can obtain information such as the distance from the ring to the celestial body, the width of the ring, and the number of rings.

However, star eclipses are not a rare phenomenon. Since there is usually no way to know the existence of rings in advance, whether or not a ring is found on an object depends on chance.

An international research team, including the researchers who reported the first discovery of the Kwaoar ring, analyzed observational data from the eclipse that occurred on August 9, 2022. Since the existence of a ring had already been suggested, this observation was a rare opportunity to make an assumption in advance about the existence of the ring as well as the quaor itself.

During this eclipse, it was predicted that Kwaoar would cross in front of the star cataloged as “Gaia DR3 4098214367441486592”, which could be observed in Hawaii and North America. Did. resulting in,Detailed shape measurement of quaoar and discovery of new ringswas successful.

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【▲ Figure 2: The shape of the main body of Kwaoar estimated from the eclipse. It turns out that it is likely not a simple sphere or rugby ball shape, but a more complex shape called a triaxial inequal ellipsoid (Credit: CL Pereira, et.al.)]

First, the shape of the main body of the quaoar was clarified in detail. The radius at its longest point was estimated to be about 579.5 km, but its shape is not a simple sphere or rugby ball shape (spheroid), but a more complex one called a “triaxial inequal ellipsoid”. It was deduced to be an ellipsoid with different radii measured along the three axes, a sort of “hot water bottle” shape. This is a big difference, considering that it was previously estimated to be rugby ball shaped with an average radius of about 555.0 km.

【▲ Figure 3: Brightness change of Gaia DR3 4098214367441486592. The brightness changes as the quaoar body and the ring cross the front. A second ring (Q2R) was discovered in this observation. In addition, since the change in brightness due to the first ring (Q1R) is left-right asymmetrical, it was inferred that the dark part is biased to one side (Credit: CL Pereira, et.al.)]

In addition, the ring (Q1R), whose discovery was announced in February 2023, has a fairly accurate size and detailed structure determined by this observation. The radius of the ring is 4057±6 km, and the ring has dark and thin parts, suggesting that only part of the ring is darkened in an arc.

Furthermore, this timeNewly discovered second ring (Q2R). The radius of the second ring is 2520 ± 20 km, and it is found to be more inner than the first ring and narrower than the first ring. In addition, this observation also revealed that unlike the first ring, there is no clear structure like an arc.

The two rings of quaoar are both “Roche limitis outside the . This is a feature that distinguishes it from other celestial rings. The Roche limit is a value that limits the size of a satellite that orbits a celestial body. Objects orbiting a star receive tidal forces from the star. The strength of the tidal force increases as the diameter of the orbiting celestial body increases and as it approaches the main star. Therefore, an object that is too close to the main star and has a certain size cannot exist because it will be shattered by the tidal force.This limiting distance is called the Roche limit. (※2) 。

*2…Giant planets have satellites that orbit inside the Roche limit. Whether or not a celestial body actually breaks up at the Roche limit depends on the size and density of the celestial body, so it is thought that small, low-density satellites can exist even inside the Roche limit.

All the known rings other than Quoaar lie inside or at the boundary of the Roche limit, so the rings were thought to be aggregates of matter that could not become satellites due to tidal forces. In other words, outside the Roche limit, matter gathers to form satellites, so rings do not exist. However, the two rings found at Kwaoar are far outside the Roche limit, which contradicts this theory.

[Fig. 4: The two rings discovered this time are both outside the Roche limit, and are thought to be in an orbital resonance relationship with Quoaar’s rotation period or Waywot’s orbital period (Credit: CL Pereira , et.al. / Japanese characters have been revised by the author)]

According to conventional theory, the Kwaor ring, which lies far outside the Roche limit, is estimated to become a single mass, or satellite, in less than 100 years. In order for the rings of Quoaar to remain as they are without forming satellites, there must be some physical phenomenon that prevents them from clumping together.

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The research team believes that the rotation period of Quoaar itself and the existence of the satellite Waywot are the keys. The orbital period of the first ring is three times that of Quoaar and one-sixth that of Waywot. The orbital period of the second ring is 1.4 times the rotational period of quaor, which can be expressed as an integer ratio of 5:7.

In this way, the relationship in which the orbital period of a ring or celestial body has an integer ratio to other orbital elements is called “track resonance” is called. The orbital period that is related to orbital resonance is stable, but the other orbital periods are unstable, so there are restrictions on the orbital period.

In the case of a quaoar ring, it is thought that the materials that make up the ring are disturbed by orbital resonance, which prevents them from forming a single mass, and at the same time, prevents them from dissipating and annihilating the ring. . In addition, the arc-like structure seen in the first ring can also be explained by considering that orbital resonance causes matter to be concentrated in a biased location.

In addition, the discovery of rings outside the Roche limit has forced a review of the theory of ring formation itself. The discovery of a second ring in Kwaoar suggests that rings outside the Roche limit may not be uncommon. Further observations of the properties of quaoar rings may significantly rewrite theories about the properties and formation of rings, and may even shed light on the mysteries surrounding rings that have already been discovered.

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Text: Riri Ayae