In 2015, the New Horizons mission introduced Pluto to the world, bringing with it almost as many questions as answers.

And in a paper published last week in the journal Icarus, astronomers from the Southwest. Research Institute tackled one of the most basic of those questions: how did Pluto form?

Turns out, it might be more like a giant super-comet than anything else. Before New Horizons, most astronomers thought Pluto formed like the full-fledged planets, from the protoplanetary disk — the collection of gas, dust, and ice that swirled around the Sun in the early solar system. But the Southwest researchers realized the data from the flyby supports a very different idea: that Pluto is made out of a bunch of comets crushed together.

Possibly a billion of them. The main evidence for this comes from Pluto’s heart, the region known as Tombaugh Regio. Specifically, the left half of it, which is basically one big nitrogen glacier.

Nitrogen drives many of the geologic processes on Pluto. It flows across the surface, might erupt from cryovolcanoes, and makes up the atmosphere. The thing is, all that nitrogen had to come from somewhere.

Now, it is possible that it came from ice in the protoplanetary disk, which would have had a bunch of nitrogen in it. But it could have also come from comets. We don’t know exactly how much nitrogen is in the typical comet, but we do know how much is in 67P, thanks to the Rosetta orbiter.

Based on that, the researchers came up with a broad estimate for the nitrogen we’d expect to find in comets in general. And when they crunched the numbers, they found that if a billion of those comets came together, they could form a world just like Pluto. At this point, it’s hard to tell which theory is more likely.

It mostly depends on how much nitrogen Pluto has lost to space over the years. If a lot of it has been lost over time, the idea that Pluto came from the protoplanetary disc is probably a better fit, because it could have formed a world with much more nitrogen than Pluto has now. But if Pluto hasn’t lost too much nitrogen, the giant comet model might work better.

We just don’t have enough data on Pluto yet to know how its nitrogen content has changed. It wouldn’t hurt to get nitrogen measurements for more comets, either. In the meantime, New Horizons isn’t done yet.

It’s headed for a flyby of another, much smaller object out there, called MU69, on. New Year’s day of 2019. What it discovers could tell us a lot more about what’s floating around in the space near Pluto, and might get us closer to understanding how it formed.

Speaking of the more remote ends of our solar system …. You might have heard of Planet

Nine: the unknown planet that might or might not exist far out in the solar system, orbiting between 200 and 1200 times the distance from Earth to the Sun. And in a paper posted online this week, researchers discovered even more evidence for it. Planet Nine is probably about 10 times the mass of Earth, a type of planet known as a super-Earth in between smaller rocky worlds like ours and gas giants. If it’s really out there, that is.

We’ve never actually seen Planet Nine, which is why it stays theoretical. But we have seen a handful of dwarf planets and other small bodies beyond Neptune that all have the same weird, oblong orbits and bizarre inclinations above the flat plane of the solar system. And many researchers don’t think that’s a coincidence.

Instead, they think these orbits come from the gravitational pull of an undiscovered planet, in what’s become known as the Planet Nine Hypothesis. The team that proposed the current version of the hypothesis in 2016 based their calculations on just six objects with these weird characteristics. But as astronomers have discovered more objects with similar qualities, the evidence for Planet Nine has grown.

Last week’s paper brings the number up to 14. The team discovered a new object beyond Neptune, called 2015 BP519, which they nicknamed Caju. And it has an even more extreme orbit than the others, following a path researchers had seen in computer models but never in real life.

It also happens to be big enough to possibly qualify as a new dwarf planet. It’s worth noting that this paper hasn’t been accepted for publication yet. The authors posted it to the preprint website arXiv, but it hasn’t been peer reviewed.

Still, assuming it passes the peer review process, the researchers have just added another big piece to the body of evidence for Planet Nine … at least for those who agree with the hypothesis. Which not everyone does! Some think it’s possible that these orbits are just a coincidence.

They could be within the range of what you’d expect, statistically. And there could be some bias toward discovering a higher proportion of objects with these weird orbits because of the methods we use to look for things beyond Neptune. One way to settle the debate would be to actually, you know, find it.

But first we have to figure out where it is, which researchers are trying to do based on the weird orbits we’ve found so far. From there, we’d have to spot it moving against the background of stars. But we’ll need a powerful telescope to find it, because even the closest point in its orbit is incredibly far away, and it’s dark out there.

Researchers are working on it, but unless we can narrow down the area where Planet Nine might be, there’s a lot of sky to cover. For now, we’ll just have to keep looking. Thanks for watching this episode of SciShow Space News!

If you’re interested in learning more about Planet Nine, you can check out our episode about the landmark 2016 research.