Scientists Have Been Searching for Planet Nine for Years. More Than 3,000 New Simulations Suggest That It’s Not a Planet and Never Was

There’s been a long-standing debate about the existence of a ninth planet beyond Neptune in our solar system. Astronomers have been puzzled by the gravitational disturbances observed in some distant solar system objects. As they run out of places to look, simulations now suggest that there might have never been a Planet Nine, but rather a star passing close to the Sun.

The mystery of TNOs. Experts believe giant planets played a crucial role in redistributing small celestial bodies during the early stages of the solar system. When giant planets captured some astronomical objects, they turned them into irregular moons, while others were pushed to the outskirts of the solar system. These objects, now known as trans-Neptunian objects (TNOs), include asteroids and comets.

A team at the Canada-France Ecliptic Plan Survey discovered the first TNO, named 2008 KV42, in 2008. Astronomers have not yet found a Planet Nine to explain its retrograde, highly inclined orbit. This lends 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 Nine. An international team of researchers has proposed an alternative explanation for the irregular moons of the giant planets and the orbital behavior of TNOs. Instead of the hypothetical ninth planet, they suggest that a star passing close to the Sun may have influenced these phenomena by pushing objects beyond Neptune toward the giant planets.

Two studies, one published in Nature Astronomy and the other in The Astrophysical Journal Letters, offer a straightforward explanation for the origin of the irregular moons of the giant planets and the orbital perturbations of TNOs. According to the researchers, a passing star may provide the missing piece in understanding the solar system’s dynamics rather than Planet Nine.

Thousands of simulations. The researchers conducted more than 3,000 simulations to model the evolution of the solar system, varying the mass and distance of different stars that could have passed close to it. They found that a star of approximately 0.8 solar masses passing at a distance of 110 astronomical units (AU) from the Sun at an inclination of 70 degrees aligns with the current orbits of TNOs and irregular moons.

The close passage of a star isn’t unlikely, given that the solar system likely formed in a dense stellar environment where close interactions between stars were more common. This makes it plausible that a stellar encounter influenced the current configuration of the outer solar system.

Possible influence on terrestrial life. The model suggests that about 7.2% of the trans-Neptunian objects were affected by the passage of a star near the region of the giant planets. Many of these objects had retrograde orbits, moving in the opposite direction of the Sun’s rotation. This could explain why there are more retrograde irregular moons than prograde ones around Jupiter and Saturn.

While some of the TNOs were ejected from the solar system, a significant number 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.

Absence of very red moons. The color distribution of irregular moons and TNOs, particularly the lack of very red objects among the irregular moons of Jupiter, Saturn, Uranus, and Neptune, also supports this hypothesis.

TNOs beyond 60 AU, where the affected objects originated, don’t exhibit the reddish hues observed elsewhere, like in irregular moons. These observations further support the idea that a close encounter with a star could have changed the orbits of the trans-Neptunian objects, sending them into the inner solar system where they were captured by the giant planets to become irregular moons.

What comes next? The passage of a star could explain the unusual orbits of trans-Neptunian objects and irregular moons, eliminating the need for a Planet Nine. However, further studies will be necessary to investigate how this scenario aligns with other solar system features, like Trojan asteroids.

The upcoming launch of more powerful telescopes, starting with the Vera C. Rubin Observatory in Chile, will enable the detection of more irregular moons and TNOs. Going forward, they’ll allow scientists to further test this hypothesis.

Image | Pfalzner et al.

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