Ancient passing star may have altered outer solar system

A solar-mass star could have scattered outer material in the early Sun's protoplanetary disk.
By Laurel Kornfeld | Aug 10, 2018
The low masses and eccentric orbits of small planets and Kuiper Belt Objects (KBOs) in the outer solar system may be the result of the influence of a star that came close to the solar system billions of years ago.

A research team led by Suzanne Pfalzner of the Max Planck Institute for Radio Astronomy in Bonn, Germany, noted that unlike planets in the inner and middle solar system, which circle the Sun in orbits that are largely circular with only minor orbital inclinations, those in its third zone beyond Neptune have highly inclined and elliptical orbits.

Because the Sun formed from a collapsing cloud of gas and dust, which produced a flattened disk, one would expect all solar system planets to orbit in the same plane, which the distant dwarf planets and KBOs beyond Neptune do not do.

Distant dwarf planet Sedna has an extreme orbital inclination and eccentricity that could not have been produced by the scattering of neighboring planets.

Additionally, the cumulative masses of all these distant dwarf planets and KBOs plummets by close to three orders of magnitude starting in the same location beyond Neptune where objects' orbits become eccentric.

While scientists have been studying the effects a passing star could have had on exoplanetary systems, until now, they did not apply these principles to our own solar system, Pfalzner said.

If a passing star came close to the Sun in its early days, that star could have disrupted remaining material in its protoplanetary disk, hurling it into eccentric and inclined orbits. Computer simulations indicated conditions in today's outer solar system could have been produced by a passing star with approximately the same mass as the Sun or one slightly less massive.

That star likely approached at about three times the distance of Neptune.

Other unusual features of the solar system, such as the mass ratio between Uranus and Neptune, and the presence of two separate types of KBOs, can also be explained by interaction with an ancient passing star.

Such an interaction would be unlikely to occur now, but these were probably more frequent billions of years ago because the Sun, like many other stars, is believed to have formed in a stellar cluster, whose individual stars eventually traveled away from one another.

Even though the idea of an ancient stellar flyby seems to explain the arrangement of objects in the outer solar system, Pfalzner emphasized it is still just a theory.

Findings of the study have been published in The Astrophysical Journal.

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