Passing Stars May Have Kept a Distant Alien World Tethered to Its Sun

We tend to think of our Solar System as a static, constant environment. The implications of the protoplanetary disc and the demarcation line between our inward rocky planets, the outer gas giants, and the farthest “ice giants” all combine to create a nifty little model in which the heaviest bands of material with the most rock coalesced in the inner solar system, while large amounts of gas were diffused by the solar wind and blown into the outer reaches of the solar system. But one of the most profound findings of the past few decades has been the dynamism inherent in these systems.

Planets don’t just form statically and then remain in the same orbits for billions of years. Planets can migrate dramatically through a star system, thanks to interactions with each other and with their own stars. In one case, gravitational interaction with a passing binary may have actually saved a planet from being flung away from its host star.

HD 106906 b is an unusual world. It’s 11x the mass of Jupiter and located 738 AU from its host star. Neptune, for reference, is just 30.1 AU from the Sun on average. Its orbit is tilted out of the plane of the ecliptic, by 21 degrees. These are all extremely unusual data points, and they indicated that something had a major impact on the planet’s formation. Even Planet 9, the hypothetical super-Earth-sized disruptor that may be whizzing about in the depths of our own solar system, is only estimated to sit 400-800 AU from Sol, and it isn’t thought to be anywhere near 11x Jupiter mass if it exists at all. (11x the mass of Jupiter would be roughly 1 percent the mass of the Sun).

Scientists have been curious about HD 106906 b since we found it, and have worked out that a close binary passage about three million years ago may have impacted HD 106906 b’s position around its host star in a way that kept the planet from exiting the system altogether. Current thinking is that HD 10906 b was lobbed into an eccentric orbit after a close encounter with its own stars, until a passing circumbinary object nudged it back towards its primary, on an exceedingly unusual orbit.

These types of gravitational interactions may have been common in our own solar system. Triton, the largest moon of Neptune, contains over 99 percent of the mass in the Neptunian moon system and is extremely similar to Pluto in size and bulk composition. It orbits retrograde, which means it couldn’t have formed around Neptune in the first place. One line of thought is that Triton had a binary pair when it approached Neptune, but gravitational interactions led to the destruction or ejection of the binary, leaving Triton behind. There are models of our early solar system formation that predict a fifth ice giant (ejected due to gravitational interactions with Jupiter and Saturn). There’s another theory, known as the Grand Tack hypothesis, that posits that Jupiter’s inward migration to 1.5 AU choked the material available to form Mars before the gas giant migrated outwards again after Saturn was captured in orbital resonance.

Our own solar system may have been impacted by the type of stellar near-misses that characterize HD 106906, though none so dramatically. 70,000 years ago, Scholz’s Star passed within 0.82 light years of the Sun. This may explain the orbits of certain small objects in the solar system. They appear to have originated from an area of space that corresponds with the area Scholz’s Star would’ve disrupted as it moved past our own system.

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