Planet Nine or Planet Nein? The quest to understand the weird outer Solar System

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One Big Planet Nine, Or A Swarm Of Small Icy Worlds?

For Forbes:

The outermost Solar System is a confusing and complicated place. Once you get Neptune, the comets, Kuiper belt objects, and other small icy worlds orbit the Sun in odd patterns. A few of those, including the very distant world known as Sedna, have orbits that make very little sense compared with other Solar System inhabitants. For that reason, some astronomers think there’s a Planet Nine hiding far beyond Pluto’s orbit: a giant world roughly 10 times the mass of Earth.

But a new study by University of Colorado researchers proposed an alternative explanation. Astronomer Ann-Marie Madigan and her student Jacob Fleisig realized they could reproduce the strange orbits of icy worlds just by the way they interact with each other: no Planet Nine necessary. The idea is they sometimes swarm (in a broad sense) during their orbits, and when multiple Moon-sized bodies are in the same general region, it’s enough to kick other worlds like Sedna into their wild trajectories. It’s an eminently sensible explanation, and since two years of hunting for Planet Nine haven’t turned up anything, the hypothesis is definitely worth pondering more. However, we haven’t seen enough of these small worlds yet either, so the race is now on to see which explanation is correct.

[Read the rest at Forbes…]


Artist’s conception of the Kuiper belt. [Credit: Don Dixon]

When we talk about big advances in planetary science, we often are thinking about Mars rovers or the discovery of exoplanets. However, one area where we’ve learned a lot over the last few decades is the Kuiper belt: a region beyond the orbit of Neptune inhabited by small bodies of ice and rock. Before 1992, Pluto was the most distant known Solar System object, but between then and now, astronomers have discovered a wealth of Kuiper belt objects (KBOs).

A new paper (coauthored by Mike Brown of Pluto-killing infamy) describes a puzzle arising from a survey of many KBOs: some of them don’t fit in with the standard model of planet formation:

A new study of large scale surveys of KBOs revealed that those with nearly circular orbits lying roughly in the same plane as the orbits of the major planets don’t fit the Nice model, while those with irregular orbits do. It’s a puzzling anomaly, one with no immediate resolution, but it hints that we need to refine our Solar System formation models. [Read more…]

Some planet-like Kuiper belt objects don’t play “Nice”