Sabine Hossenfelder’s Lost Planets Credit: NASA, ESA, CSA, STScI We may have lost two planets. And no, I don’t mean astrophysicists have another disagreement about what we are allowed to call a planet. I mean the result of a recent study which found that the Solar System used to have six giant planets, not four, but we lost two of them billions of years ago. That’s relatable; I mean, who hasn’t lost a planet or two. But seriously, what the hell happened there? I have a brief summary. The Solar System was not born as the neat diagram you find in school books with the planets all in place. It formed from a huge cloud of gas and dust. The cloud contracted under the pull of its own gravity and began to spin. The centre became so dense and hot that nuclear fusion ignited it; that became our Sun. The leftover material formed a flat disk around the young Sun. Inside that disk, tiny grains began to stick together and slowly built larger and larger objects which became the planets and their moons. So far, so clear. The devil is in the details. And it’s the details that the authors of the new paper have now put together and compared with evidence. What we do know is that in the early Solar System, it was so hot close to the Sun that only rock could clump and form planets. That is why the inner planets, Mercury, Venus, Earth, and Mars, are small and rocky. Farther out, it was cold enough for ice to also clump. That gave the young planets out there much more building material. Jupiter and Saturn grew quickly and became massive enough to pull in huge amounts of gas. Uranus and Neptune also grew large, but not large enough to collect as much gas before the disk disappeared. These four planets are called ‘gas giants’ and they’re fascinating. If Jupiter were hollow, about 1,000 Earths could fit inside. Saturn has such a low density that it’d float in water, if you could find a bathtub to fit it in. But here is the thing. If physicists try to reproduce the formation of our Solar System on a computer, it doesn’t work if there are only these planets. The way that they do this is to start with the initial distribution of matter and let it form a Solar System. Then they keep only those cases which result in something that looks similar to what we in fact observe. The thing is now that to match our observations, they need at least one, but better two large extra planets, otherwise the Solar System does not come out correctly. The idea is not entirely new. The biggest hint that something dramatic must have happened in the young Solar System is Uranus, because its axis is tilted a lot to the side and is not aligned with the overall axis of the Solar System. This does not naturally come about during the formation of the Solar System. It seems that something hit Uranus, hard. The new paper now looks at Uranus’s moons and finds that these fit the picture as well. The moons also orbit around the tilted axis. And these moons, especially one by the name of Miranda, have more ice than one would expect. The authors now say this all fits together like this: the early Solar System had not 4 but probably 6 of these ice giants. They were much closer together than the ice giants are now. If these giant planets are close together, this makes their orbits very unstable. Solar systems are naturally chaotic, and in a case like this, the chaos sets in very quickly. So 2 of the giant planets didn’t stay on regular orbits but instead began to wander around. One of them hit Uranus and tilted its axis. In the aftermath, Uranus’s moons get shaken out of their orbit and collide, which created multiple smaller moons which then put on extra ice. The two extra planets eventually ended up on a path that took them out of the Solar System entirely. This would have happened billions of years ago, so they would be far away from us now. They may still exist somewhere, but by now they would be wandering between the stars, in the cold and dark, alone, and probably not subscribed to my channel. I give this paper a 3 out of 10 on the bullshit meter. Not because of the paper but because of the general difficulty of the subject. We’re trying to patch together what happened in the distant past from what we see today, so all the evidence we can ever have is indirect. The problem is not so different from trying to understand the origin of life, but it’s more dominated by physics, so I think it’s more doable. I couldn’t think of a closing joke, so I asked ChatGPT, here’s what it came up with: “Six ice giants close together. This is not a Solar System, this is a physics department after the coffee machine breaks.”