Stealing From the Solar System: The Effects of a Stellar Fly-by

Could a ‘rogue’ star passing nearby have disturbed outer parts of the early solar system? Beyond Neptune things become somewhat different.

The outer reaches of our solar system harbor a number of mysterious features. Astrobites reports on whether a single stellar fly-by could help explain them all.

A star is born from the gravitational collapse of a cloud of gas and dust. Yet not all of the material ends up in the star, and instead forms a flat protoplanetary disk that surrounds the new star. Over time, the materials in this disk coalesce to form planets, moons, asteroids, and most other objects you might expect to find near a typical star.

Since protoplanetary disks are flat, the expectation is that all of the planets and objects orbiting a star that formed out of a protoplanetary disk should orbit on a single plane. So when we find stars with planets that orbit at multiple different inclinations, this raises questions.

A recent astrobite discussed such a case, where an exoplanet was observed orbiting on a completely different plane than the other exoplanets in that same system. But we needn’t look that far to find deviations like this — our very own solar system exhibits several features that don’t line up, so to speak.

Inclinations and Eccentricities and Truncations, Oh My!

For the first 30 AU around the Sun (until right around where Neptune orbits) things are relatively “well-behaved”: most planets’ orbital inclinations only differ from each other by 1–2 degrees, and no planet has an inclination of more than 8 degrees. But beyond Neptune, in the outer solar system, orbital inclinations are considerably higher.

Pluto, recently demoted from planet to dwarf planet, is one example; its orbital inclination is more than 17 degrees. The same trend exists for orbital eccentricities, which tend to be significantly larger for objects beyond 30 AU compared to those inside the 30 AU cutoff.

A similar pattern also exists in our solar system’s surface density profile, which can be obtained by smoothing out the cumulative mass of solar system objects (planets, moons, asteroids, etc.) to approximate what the Sun’s protoplanetary disk might have looked like. The surface density profile gradually declines until ~30–35 AU, where it drops abruptly by a factor of nearly 1,000 (a phenomenon often referred to as disk truncation). Coincidence? Perhaps not.

Astronomers seek to develop theories that can help explain these peculiar features in our solar system. One proposed explanation claims that the existence of a yet undiscovered faraway planet (sometimes called Planet 9) could cause these effects on the outer solar system. A second possible explanation, involving a supernova going off near the solar system in its early days, was covered in another recent astrobite.

Today’s paper offers yet another explanation — what if a star flew by our Sun early on, stealing a bunch of the outer material from the Sun’s protoplanetary disk with it, and throwing what was left into inclined and eccentric orbits?

Figure - Three simulations from today’s paper, showing the trajectory of the perturbing star in black. Going from left to right, the perturber masses are 0.5, 1, and 5 solar masses. The top row of panels depicts the average positions of the particles that remain after the fly-by, colored by how eccentric their orbits have become (note the black circle around 30 AU, the radius within which almost all particles are blue, meaning they all have very low eccentricities). The bottom row of panels indicates the initial positions (before the fly-by) of the different eccentricity populations shown in the top row, with the grey regions signifying particles that became unbound due to the perturber [Pfalzner et a. 2018].

Continued here.