Hey! Don’t forget that this week’s “supermoon” brings with it a lunar eclipse on the evening of the 17^th/18^th, where just a thin sliver of the Moon will fall into the dark part of Earth’s shadow. Should be pretty cool to watch. That link to my Scientific American article is me debunking the idea of a supermoon, because it’s silly. However, I also make a point that you should always take a look at the Moon when you can. Because it’s beautiful.

New research just published as a pair of journal papers (paper 1, paper 2) may explain a couple of mysteries in the outer solar system. One is, why are so many trans-Neptunian objects (TNOs) — large, icy bodies orbiting the Sun well past Neptune — on highly elliptical orbits, some tilted hugely with respect to he inner planets? We expect everything in the solar system to orbit the same direction (by convention counterclockwise as seen from the north; that is, looking down on Earth’s north pole), but some don’t. Some even orbit backwards!

The other mystery is, why do the gas giants like Jupiter and Saturn have so many small, icy moons that orbit backwards? A lot are known, and there could be hundreds more. Many seem to have similar characteristics as TNOs, but what’s the connection? 

The news research ties them together by a pretty cool scenario: the passing of a star very close to the solar system!

Theoretically, if another star in the galaxy gets close enough to the Sun, its gravity can disturb the TNOs, dropping them in toward the planets. This has been known for some time, but what happens to those bodies once they get closer?

The team of scientists created computer models of a star passing the Sun to see how its gravity would affect TNOs and see if they could answer this mystery. They ran the model over and over again, changing the parameters a little bit each time (like incoming direction, star mass, and so on) and found that one about 0.8 times the mass of the Sun passing just 16.5 billion km out (110 times the Earth-Sun distance and four times farther out than Neptune) works best — its gravity would have scattered the TNOs nearby, dropping them toward the Sun. Some would get put on highly elliptical orbits, matching in general what we see today. And that explains the first mystery!

It also would’ve dropped so many to the inner solar system that the gravity of the giant planets took over. Most (85%) would get ejected from the solar system when a planet’s gravity slingshot them away, but many could be captured by the planets. That part is tricky, and not detailed in the paper, but we know that many TNOs are binary. If the pair encounter a planet, the gravitational dance can eject one of the TNOs while capturing the other, leaving a new moon behind that could orbit the planet backwards relative to moons that formed along with the planet long before. 

By the way, this could have rained down giant comets on Earth, too. There are some ideas about that, like the Late Heavy Bombardment, though astronomers are still arguing over it. 

Encounters with other stars are relatively rare because space is pretty big, but the authors note that other research indicates that over 4.55 billion years (the age of the solar system) there’s a 20 – 30% chance of such a close pass [link to journal paper].

I didn’t realize it was that high! Nearly a coin flip, while I would’ve thought it was much lower: there’s an physics exercise done at grad school to calculate how often a star should pass close to the Sun, and the answer you get is a loooong time. Longer than the age of the galaxy… but that assumes stars are evenly distributed throughout the galaxy, which isn’t really the case. Plus they don’t all orbit along with the Sun, making collisions more common (like cars in a racetrack all moving in the same direction versus one coming in from the side, say). In the linked paper, they assume the Sun was born in a big star cluster, which makes encounters inevitable right after the Sun formed, but they find that close passes happening even a billion years later aren’t as uncommon as once thought. It’s possible the encounter that affected the TNOs (assuming it actually happened) could have occurred when the solar system was very young, and such events were more common as well.

All in all, very tidy. I’ll have to keep my eyes open for follow-up work, including observations of TNOs that can support or refute this idea. Having a cool idea that explains stuff is great, but we always want lots more evidence before accepting it as real.

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