A passing star may be responsible for more than three-quarters of Earth’s moons. solar system as star explorers hurl large rocky objects into our cosmic neighborhood, a new study suggests.
This new model challenges existing ideas about how the solar system formed as it is today.
The giant planets of the solar system are famous for their many moons. Saturn currently leads by 146 months at last count, with Jupiter second place with 95. Many of these moons resemble Earth’s moon in many ways. For example, they orbit their parent planets in the same direction as the planets rotate. In addition, these moons, called regular moons, follow nearly circular paths that share the plane of the planets’ equator.
But some satellites are much stranger. Take for example PhoebeOne of Saturn’s strangest moons. It has a tilted, oval-shaped orbit that moves in the opposite direction to Saturn’s rotation — a motion described as retrograde. In fact, satellites like Phoebe, also called irregular moons, are three times more common than regular moons in the solar system.
Scientists have previously linked the moon’s irregular motion to the movement of the sun. Neptune throughout the solar system, according to William Bottkea planetary scientist at the Southwest Research Institute in Boulder, Colorado. Most astronomers consider the pivotal step in the evolution of the solar system to be Neptune’s outward migration through the precursor of the Kuiper Belt. Today, the belt extends between 30 and 50 times the distance between Earth and the sun, but in the early solar system, the proto-Kuiper Belt was much closer to the sun.
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This makes the rocky objects in the Kuiper Belt unstable, causing many of them to be flung near the giant planets. From there, objects with certain orbits can be “captured” by the giant planets, Bottke told Live Science in an email.
However, this scenario cannot explain certain aspects of the irregular moon. For example, only a few are very red. However, a new study published on September 4 in Astrophysics Journal Lettersexplained this oddity by offering an alternative theory: that a passing star “kicked” the moon into place.
Susanne PalatzenThe study’s first author and a professor of astronomy at the Jülich Supercomputing Center in Germany, told Live Science in an email that he was inspired to explore this possibility after a different experience. Study suggests that a star flying through the solar system threw a Kuiper Belt Object (KBO) close to the planet.
So Pfalzner and his colleagues modeled a star moving through the young solar system. About four-fifths the mass of today’s Sun, the stellar visitor was modeled as passing at a distance of about 110 Earth-sun distances. The researchers calculated how the gravity of the Sun and the visiting star altered the trajectories of thousands of KBOs. Then, the team studied how the KBOs’ orbits evolved over a billion years.
The researchers’ simulations show that if the visiting star swoops past at a 70-degree angle to the ecliptic plane — the plane in which Earth orbits the sun — it ejects about 7% of the KBOs along elongated, oval-shaped orbits that take them close to the giant planets. Many of them — especially those whose new trajectories take them close to Jupiter or Saturn — have retrograde orbits, and a few are very red — both trends demonstrated by today’s irregular moons.
The researchers found that over a billion years, the passing star kicked nearly 85% of the KBOs ejected from the solar system. The KBOs that weren’t ejected formed irregular moons, the researchers explained.
The results of the simulations were “very surprising,” Pfalzner said. One advantage of the model is that it is simpler than older models, since it can explain how irregular moons form and how KBOs behave. Plus, the star trajectories are fairly common — about 140 million stars in Milky Way likely to have experienced such a flyby.
Not everyone agrees with the study’s conclusions, however. Bottke, who was not part of the study, notes that “such close passes, even from an era when our solar system was in a star cluster, seem extremely unlikely from a probability standpoint. A stellar encounter close enough to capture an irregular satellite around a giant planet would also likely perturb the orbit of the giant planet—enough that we would see these effects in its current orbit.”
