A few months ago, planetary astronomers made a big splash with the announcement that idiosyncrasies in the orbits of a few distant, Trans-Neptunium Objects (TNOs) were silent witness to something big lurking in the icy shadows of the far outer solar system. Some ventured to guess that one likely explanation was a large TNO, Pluto-sized or bigger, with a smaller possibility of a larger object. More recent research gives hope to to believe it may indeed exist and be on the larger side.
So we emailed Dr. Michael Brown, TNO hunter extraordinaire and author of How I Killed Pluto and Why It Had It Coming (who was kind enough to jump into comments right here on Daily Kos a few years ago to talk about his fascinating discoveries), to ask him about the possible nature of this hypothetical world. Here’s part of what he said:
Planet Nine is hypothesized to be about 10 times more massive than the Earth, making it a little smaller than Neptune. And while large infrared surveys from space have ruled out anything the size of Saturn or larger anywhere in the outer solar system, objects the size of Neptune could easily have escaped our detection so far.
Whoa! That is news, my friends. That is a freaking planet, a whole new world, on the larger side of a super-Earth, pushing into the heavier category of an ice-giant. But until it is within the frame of a telescope with a state-of-the-art spectrometer, we can only speculate on what such a planet, if it does indeed turn out to exist, might be like. The chart above gives some basic idea about the gross physical dimensions of such a planet. Beyond that, drop on down and let the speculation begin …
Let’s assume this planet is five to 10 times the mass of the Earth with an orbital period well in excess of 1000 years (actual estimated values surpass 10,000 years at the higher end, similar to Sedna) and highly eccentric, meaning the orbit is both tilted to the plane of Earth’s orbit and much more elliptical than our own. The chief characteristic of such a planet is cold—icy barely describes the space it lives in.
In the more temperate parts of the solar system, a planet at that mass, composed of about three-quarters hydrogen and one-quarter helium, would probably not last. Incident sunlight and solar wind would cause it to dissolve away to whatever flakes and nubs of ice and rock and metal it managed to squirrel away over the eons. This happens quickly on astronomical time scales. But way out past Pluto, such a world might be infinitely more stable. It could attract smaller, icy bodies, even some rock and metal, to form a small, almost pinpoint core. This kind of planet is like nothing we have ever seen up close: It is a gas dwarf.
But gas dwarfs are thought to top out at more like three or four terrestrial masses at most. If we take the experts at their word, this planet sounds more like a super-Earth—and probably larger. Exoplanet hunters have found evidence for plenty of super-earths circling other stars. They are working to extract as much info as possible from the tiny stellar wobbles and minute dips in starlight through which these objects are inferred. But we don’t have any official super-earths in our solar system to study directly. We know little about them.
Which brings us to an arrested ice-giant, a planet that didn’t quite make it to the masses of Uranus or Neptune but has similar characteristics. This planet is still at least 20 percent hydrogen-helium gas by volume, getting more and more dense as we plunge deeper into it, until the gas and impurities like water and ammonia slowly take on the properties of a liquid and then transform into exotic ices, hot ice, forced by pressure into alien, crystalline forms only achieved in high pressure chambers on Earth. From what Brown and others are saying, this sounds like the best possibility.
Then there are progressively denser components—water, methane, ammonia, carbon, rock, and metal—that give the possible Planet Nine a more Earth-like size and composition. Even in this scenario, it’s likely there would be a substantial atmosphere and significant internal heat. But there could be something passing as a solid surface underneath oceans of gas and liquid. This planet is a sort of cold, massive, very dark Venus.
Most of this depends on where the planet formed. If it formed here, in our solar system, modern accretion models would have it do so within the orbit of present day Saturn, at the dawn of planetary formation beginning more than 4.5 billion years ago. After growing for hundreds of millions of years, it might have been slung out by an ancient resonance between Jupiter and Saturn, a dynamic thought to have flung out Neptune and Uranus to their present positions. This scenario favors a smallish ice-giant.
If it formed somewhere else and got captured, a statistically unlikely scenario, or if it formed inside the orbit of Jupiter, all bets are off. It could be something truly surprising, a silicate planet not unlike Earth or Mars in basic chemical composition, or even a metal planet or carbon world. This is the realm of one of the weirdest denizens yet proposed: The mega-Earths, solid terrestrial planets larger than super-Earths, some with a much thinner atmosphere than an ice giant. A mega-Earth could be enormous but still have weather and other features we might recognize, right down to a defined liquid or solid surface glimpsed through whirling vortices and mega-storms.
It sure seems likely such a world would have moons. All the outer planets do, lots of them. Even diminutive KBOs like Pluto and others have them. It could even have a healthy set of rings. Which brings up an intriguing possibility: Those moons might be warmed above the harsh background of a few dozen degrees Kelvin, either by internal shear caused by tidal forces or the planet itself glowering in the infrared like an invisible sun. We know from our own moons in the outer solar system that a liquid-solid surface and subsurface can develop in extremely cold places. Speculation abounds that such an environment might even support some kinds of life.
While we have absolutely no evidence whatsoever anything like this has happened, anywhere, let alone on a world that no one can confirm even exists, it’s just too tempting to imagine what such creatures might be like if they developed intelligence. They might gaze inward with strange, alien eyes, boosted by stranger alien telescopes, perhaps with mirrors and lenses made of shiny ices. They might try to make sense of our radio signals and wonder what kind of weird, exotic complex life could possibly gain a toehold and survive on a hellish planet like Earth. A planet so hot that solid ice is heated into a gas and rains from the sky as a molten liquid, a planet orbiting smack dab in the middle of a radioactive maelstrom, mere millions of miles from a lethal, active star …
At which point we should mention, long-term, there are plenty of objects to be worried about in the solar system and elsewhere. A run-of-the-mill comet or asteroid could wreak terrible havoc on our home-world, or a nearby supernova could cook off our protective ozone. But Planet Nine as proposed is not in that class. It poses absolutely no danger to us and any media outlet that blasts out those kinds of headlines is behaving irresponsibly.
Planet Nine is just plain exciting! It would certainly be a BFD for planetary astronomy to discover a whole new, large planet that had been out there all along. But what’s really cool is it wouldn’t have to remain an inferred statistic or a tiny dot in a giant telescope. We could visit it by robotic proxy within your lifetime. It might take 20 years or more to get there—where exactly it is on that presumed eccentric orbit would determine how fast we could reach it. But the New Horizons mission to Pluto and beyond demonstrates that such an effort, although ambitious, is now arguably within our technological ability.
This is all assuming the thing is out there at all. We simply don’t know yet. But for now the race is on to find out if there really is a Planet Nine in deep outer space. And if there is, the first team to find it gets the astronomical bragging rights of the century.