Pluto is really far away. The spacecraft called New Horizons was launched back in 2006, and next month, it’ll finally get to Pluto. When it was launched, Pluto was officially a planet. Since then, Pluto has been re-classified as a dwarf planet because it’s in the Kuiper belt, a large group of rocky, icy bodies like the asteroid belt, only much farther away. But how did the Kuiper belt get out there in the first place? Why is it so far away? Can we figure out where Pluto came from?

It used to be that scientists assumed that the solar system formed looking pretty much like it does now. That view has changed radically in the last few years. But let’s start at the beginning of the solar system’s story. We’ve had the first few steps right for a while:

• The raw ingredients were made in a star that is now dead. A big star in our neighborhood burned hot and bright, but then it ran out of fuel, and it collapsed and blew up in a supernova. The star had already made the lighter elements that are abundant on earth: carbon, oxygen, silicon, nitrogen. This supernova explosion scattered heavy, rare elements like uranium and mercury into the mix. The result was a big cloud of dust and gas that would become our solar system.

• The gas cloud got lumpy –The cloud of gas and dust started very spread out, but little regions were more dense, and because gravity, attracted more stuff. Then they became larger, and attracted more stuff, and grew and grew from there. The Sun started to form at the center and various pre-planets formed throughout the cloud. This process of small balls of mass growing by attracting the gas and dust around them is called accretion.

We learn about solar system formation through two processes: pointing our telescopes other star systems and modeling the physics with computers. There are lots of stars in our galaxy, and some of them are still forming, so we have pictures of star systems in many stages of formation. But since the formation process takes a few million years, we’ve not seen any of them from beginning to end. The only way we can watch systems develop in time is through computer simulations. We start by programming in many particles of gas and dust and code in all the physics we know for their interactions. Then we let the program run and watch the system develop. If it produces something that looks like a real star system, it worked. If not, we did something wrong and we have to figure out what we’re missing.

For our solar system, we assumed that this process resulted in all the planets forming more or less at the same time and where they are now. But a few years ago, we started to suspect this assumption was wrong. A model called the Nice Model (neece – as in the city in France) was proposed, but it had some unknowns. Two papers this spring have really helped us fill in the holes in the theory, so I’ll tell the story including them.

This is what we think happened after those first few planets were forming through accretion:

• Many planets formed that aren’t around anymore. Jupiter, Saturn, Uranus and Neptune, the gas giants of our current outer solar system, were in that first generation, along with a lot of other pre-planets.
• Those gas giants got really big, and went stomping around the solar system like bulls in a china shop, smashing and swallowing some smaller planets and even elbowing at least one other giant planet so hard it flew right out of the solar system. [ A new paper in April explained this part! ]
• As the giants moved, they were wading through dust and gas and debris and small pre-planets, flinging some stuff ahead of them and leaving a wake behind them.
• Jupiter and Saturn came close to the Sun, wandering a long way from the orbit they now consider home– it seems Jupiter came to about where the Earth is now.
• Then Jupiter and Saturn locked into an orbital pattern – a resonance – that caused them to creep back out. As they moved outward, they pushed rocks, gas, and other debris ahead of them, forcing some out to the outer solar system. They left a wake of rocks behind. The technical term for all this moving in and moving out again is the Grand Tack.
• The stuff the big planets left behind eventually formed the inner planets (Mercury, Venus, Earth, Mars) and the asteroid belt. These are younger than the outer planets. The stuff the big planets pushed outward became the Kuiper (pronounced kai-pur) belt. One of the largest objects in that Kuiper belt is Pluto, the ex-planet. I mean dwarf planet.

Here’s an animation done by an astrophysicist at the University of Leicester for a paper. You can see the big concentrations of mass forming, and you can see them pushing, pulling, and flinging around the smaller stuff as they move in and out.  Watch how the orange/red planets manipulate the green dust and blue gas.

So that’s how we think Pluto got to be where it is, and why Pluto isn’t a planet anymore. Since we’ve found over 1300 other objects in the Kuiper belt and think there are thousands more, Pluto is just one member of a population, not a planet on its own.   There are several dwarf planets in the Kuiper belt as well as lots of smaller bodies, and the largest asteroid in the asteroid belt, Ceres, is also a dwarf planet.

All right, we have all this theoretical explanation. A lot of it lined up with our observations and simulations, but we had never actually seen any kind of Kuiper belt around any other star system. Is ours unique?

In May, a new paper came out revealing a star system forming with a Kuiper belt. Using the Gemini telescope in Chile, the authors have actually taken a picture of a distant and previously unkown Kuiper belt like our own. In the photo, a band of rocks and debris is blocking the light of the new star in the star system inside.

The image of the Kuiper-belt-like ring around the star HD 115600. The belt is the bright region stretching from upper-left to lower-right. The black disk in the center is added to the telescope taking the picture because the star there is so bright that it will overwhelm the disk if it’s not blocked out. Blocking out the star allows us to see the disk. The blue is the diffuse cloud of dust and gas that hasn’t formed into planets yet.

This alien Kuiper belt is about the same size as ours and has roughly the same chemical composition. It is probably shaped by large planets farther in, closer to the star, which I regret to report is called HD 115600. Gemini got a picture of the new star and its belt using an incredibly advanced optics technology that is very new, allowing for amazing pictures even though the telescope is on the ground instead of in space.

Getting an actual picture is a big deal. We’ve found lots of planets around other stars, but they were all by indirect techniques.  We can tell they’re there by how they affect their stars, but we don’t have an actual picture (distant planets are dim and hard to spot). This gets us one step closer to an actual picture of a planet outside our solar system. It also gets us one step further toward understanding where Pluto came from, as all eyes turn to it in July.

Thayne Currie, Carey M. Lisse, Marc J. Kuchner, Nikku Madhusudhan, Scott J. Kenyon, Christian Thalmann, Joseph Carson, & John H. Debes (2015). Direct Imaging and Spectroscopy of a Young Extrasolar Kuiper Belt in the
Nearest OB Association Astrophysical Journal Letters arXiv: 1505.06734v1
Konstantin Batygin, & Gregory Laughlin (2015). Jupiter’s Decisive Role in the Inner Solar System’s Early Evolution Proceedings of the National Academy of Sciences arXiv: 1503.06945v2