The possibility of a ninth planet beyond Neptune has haunted astronomers who have been trying for decades to explain the gravitational perturbations of distant objects in the solar system. As they run out of places to look, simulations suggest that it may never have been Planet 9 at all, but a star that passed close to the Sun.
The mystery of the TNOs. Giant planets are known to have played an important role in the redistribution of small celestial bodies during the early stages of the solar system. Some objects were captured by giant planets, becoming irregular moons, and others were pushed to the outskirts of the solar system: asteroids and comets are now known as trans-Neptunian objects (TNOs).
The first TNO to be discovered was 2008 KV42. Sixteen years have passed, and astronomers still haven’t found a Planet 9 to explain its highly inclined, retrograde orbit, lending weight to the hypothesis that the ninth planet might not exist and that the strange orbits of TNOs could be due to other factors, such as a stellar encounter.
An alternative to Planet 9. An international team of researchers now suggests that instead of a ninth planet, a star passing close to the Sun may have pushed objects beyond Neptune toward the giant planets, explaining the origin of their irregular moons and certain features of trans-Neptunian objects.
Two studies, one published in Nature Astronomy and another in The Astrophysical Journal Lettersoffer a simple and coherent explanation for the origin of the irregular moons of giant planets and the orbital perturbations of TNOs. A passing star, and not Planet 9, could be the piece of the solar system puzzle we have been looking for for years.
More than 3,000 simulations. The researchers modeled the evolution of the solar system with thousands of simulations, varying the mass and distance of different stars that might have passed close to the solar system. A star of about 0.8 solar masses that would have passed at a distance of 110 AU from the Sun at an inclination of 70 degrees fits with the current orbits of TNOs and irregular moons.
A close star flyby is not unlikely. The solar system likely formed in a dense stellar environment, where close interactions between stars were more common, making it plausible that a stellar encounter influenced the current configuration of the outer solar system.
Possible influence on terrestrial life. According to this model, about 7.2% of trans-Neptunian objects were injected by the star’s passage into the giant planet region. Many of these objects had retrograde orbits (in the opposite direction to the Sun’s rotation), which would explain why there are more retrograde irregular moons than prograde ones around Jupiter and Saturn.
Although many TNOs were eventually ejected from the solar system, a significant fraction remained in regions where they were captured by the giant planets. These objects may have transported volatiles and prebiotic materials to the rocky planets, potentially contributing to the emergence of life.
Explains why there are no very red moons. Another piece that fits with this hypothesis is the color distribution of irregular moons and trans-Neptunian objects, particularly the absence of very red objects among the irregular moons of Jupiter, Saturn, Uranus and Neptune.
TNOs beyond 60 AU, where the injected objects originate, lack the reddish hues seen elsewhere, such as irregular moons. This is further evidence that a close encounter with a star could have altered the orbits of trans-Neptunian objects, sending them into the inner solar system, where they were captured by the giant planets to become irregular moons.
What’s next. While the unusual orbits of trans-Neptunian objects and irregular moons could be explained by the passing of a star and without the need for a Planet 9, further study will be needed to explore how this scenario fits with other features of the solar system; for example, Trojan asteroids.
The arrival of more powerful telescopes, such as the Vera Rubin Observatory, will make it possible to detect more irregular moons and TNOs to test this hypothesis.
Image | Pfalzner et al.
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