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The Sun Left Home in a Hurry

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A photograph of a dense knot of stars against a collection of more spread-out stars.
An open star cluster named NGC 2002, as viewed by the Hubble Space Telescope. The Sun may have been born in a similar cluster. [NASA, ESA and G. Gilmore (University of Cambridge); Processing: Gladys Kober (NASA/Catholic University of America)]

By simulating how the orbits of distant solar system objects were altered by close encounters with other stars early in the Sun’s life, astronomers have placed tight constraints on how long our home star stuck around its siblings after birth.

Born in Batches

Though our Sun currently travels on a solitary trajectory through the galaxy, its earliest childhood was not spent so lonely. Instead, the Sun was likely born as part of a litter of many other stars all collapsing out of the same cloud of precursor gas and dust. As a consequence, its early adolescence was spent in the company of dozens of other young stars, all zipping along on their own paths, destined to drift apart but initially packed close together.

A photograph of three bright stars within a circular cavity of gas.

The Hubble Space Telescope’s view of a collection of young stars still embedded within their natal nebula. [NASA, ESA, G. Duchene (Universite de Grenoble I); Image Processing: Gladys Kober (NASA/Catholic University of America)]

Despite their kinship, these young stars were not kind to one another when they passed nearby. When two stars grow close, the intense gravity of the encounter can severely disrupt their proto-planetary systems, scattering the objects orbiting farthest from their stars and potentially even ejecting some objects altogether. These early years likely left scars on the edges of our solar system that persist even today, billions of years after the early tussles.

Recent research led by Amir Siraj, Princeton University, leverages these scars or their apparent absence to ask the question: given the structure we observe in the outer solar system today, what limits can we place on the number of stars born near the Sun and the amount of time the Sun spent in its birth cluster?

Distance is Power

Several authors have asked this question over the past several decades, but Siraj and collaborators added a new twist: instead of studying either the giant planets or the cold classical Kuiper Belt, they instead focused exclusively on the “distant sednoids.” This rarefied collection of only nine known objects includes only the most distant minor planets in our solar system: the sednoids never come within 40 au of the Sun, and they spend much of their orbits beyond 400 au. Interestingly, however, all of them orbit on planes that are fairly aligned with that of the planets, and none ever strays farther than 20° from the ecliptic.

A black background with the sun at center surrounded by several large ovals, each labeled with the name of a minor planet.

An illustration of the orbits for some of the distant sednoids considered in this study. Click to enlarge. [NAOJ]

Through a suite of numerical simulations, Siraj and collaborators demonstrate that this relatively tight distribution of inclinations implies that the Sun couldn’t have been too roughed up on its way out of the cluster. By simulating many different close flybys and their influence on the distant sednoids, the researchers constrained the product of the number of stars in the Sun’s birth cluster and the time the Sun spent there to be less than or equal to 5 billion years per cubic parsec. Assuming a typical cluster density of 100 stars per cubic parsec, this suggests that the Sun cleared out of the densest and most dangerous part of the cluster within just 50 million years.

The authors stress that this conclusion leans on the assumption that the distant sednoids arrived on their extreme orbits essentially immediately, though in fact astronomers aren’t sure exactly how and when these objects ended up on the outskirts of the solar system. If the sednoids were in fact implanted onto their orbits early on, this limit on how long it took the Sun to leave its siblings is by far the strongest to date. With the Vera C. Rubin Observatory poised to discover thousands of new distant solar system objects, it’s likely that the bound will grow even more stringent in the next few years.

Citation

“Limits on Stellar Flybys in the Solar Birth Cluster,” Amir Siraj et al 2025 ApJL 993 L4. doi:10.3847/2041-8213/ae1025