Caltech astronomer Michael Brown lost a bet earlier this month, but he made that bet because he’s confident that the big news — finding our 9th planet — is on its way:
Brown, along with Konstantin Batygin, Fred Adams, and Juliette Becker, wrote “The Planet Nine Hypothesis”, which came out May 3 in Physics Reports. Why, exactly, do these and other astronomers think there’s one more planet out there? They make a pretty good case in this paper, I must say, and I’m going to try to summarize it for you here so you won’t have to read the whole thing. (It wouldn’t kill you if you did, though… 😏)
I’m avoiding the terms “Planet Nine” and “Planet X” myself here, because the above authors and Alan Stern (who leads NASA’s New Horizons mission) each are palpably opposed to one term but embrace the other, and they disagree on which, mostly for reasons surrounding the legacy of Clyde Tombaugh and the discovery of Pluto. Yeesh, these scientists. I don’t know either of them, but I like Mike Brown and Alan Stern, so I’m staying out of this one. Let’s make everybody happy. So...
I’m calling it “our 9th planet”.
Not the 9th planet, but our 9th planet, because deep down, we consider the other members of the Solar System to be members of our family. If there’s really one more planet out there, we need to find it, and we want to get to know it, and we want to visit it someday. We don’t want it to be all alone out there in the dark. Very few people have living memory of a new planet being found, so this discovery would be a really big deal to us.
Because we love all the members of our Solar System as family, we have been personifying them for a very long time:
People took it personally and reacted strongly when Pluto was demoted from a planet to a dwarf planet. How could they do that to Pluto, our friend? We love Pluto! It’s still part of our family. So, for example, the complete set of plush planets called Celestial Buddies, regardless of any demotion, still includes Pluto, and Pluto’s best friend Charon:
The notion that Pluto isn’t “really” a planet isn’t new at all, by the way. This clipping is from 1934, with that personification thing happening again. (The fact that the planets are standing on a cloud for some reason is also kind of amusing...)
Anyway, I’m actually OK with Pluto being a dwarf planet, because that way it’s not the smallest planet, but instead it gets to be the leader of the Kuiper Belt. And the Kuiper [kī′ pər] Belt, right now in Solar System astronomy, is positively where it’s at. Pluto is making new friends all the time out there, with fun names like Quaoar and Makemake, and there’s a lot for us humans to learn from them.
It’s the Kuiper Belt objects (KBOs), in fact, that may, at long last, lead us to our 9th planet.
Many of the KBOs we’ll be talking about have elliptical orbits where the Sun isn’t at the center, as you see below. It’s easiest to talk about these huge distances in AU, or “astronomical units”. One AU is the distance from the Sun to the Earth, or about 93 million miles.
Orbits can also have an inclination, like the Round Up ride. You start out with an inclination of 0 degrees at the beginning, but this one has tilted up to about 40 degrees:
When we look at the KBOs that we know about, all the relatively nearby ones have pretty normal orbits that we can explain based on the known planets. But the KBOs with longer orbits behave in ways we can’t explain. Once a gets past about 250 AU, some weird things happen. The orbits start lining up in strange ways.
Let’s consider the group of KBOs with a of more than 250 AU, and with perihelion farther out than Neptune’s orbit (30 AU), and with inclination less than 40 degrees. We know of 14 of those, and we found those 14 because they happen to be closer to the Sun in their orbits right now, making them easier to spot. Remember it can take these guys 10,000 years to complete a single orbit.
Here are those 14 orbits:
Purple orbits are the most stable, gray are sort of stable, and green are most subject to change. The arrows at the center point in the direction from the Sun to each perihelion. Notice a pattern here? All the stable orbits are lined up on the same side. With the influence of only the planets we know about (up to Neptune), there’s no particular reason for these KBO orbits to line up like that. In fact, the known planets should make these orbits precess, which means the perihelion directions should drift over time, until they all point in random directions. You can calculate the odds of this weird alignment happening at random, and it’s only about 4%.
But there’s another weird thing about these 14 orbits. If you think of them all as Round Up rides, the shafts sticking up out of the middle almost all point in similar directions, too, so that once again the chance of it happening at random is about 4%.
So we have two independent events, each of which has only a 4% chance of happening. 4% of 4% is less than 0.2%. That means there’s better than a 99.8% chance that SOMETHING IS UP OUT THERE. Some other gravitational influence almost certainly made them do this. And that gravitational influence has got to be pretty recent, or else the orbits would have drifted to randomness.
Now comes the cool part that you won’t see in news stories about this: The authors did an orbital simulation using well-known gravitational laws (and a lot of computing time!). They started with a bunch of random hypothetical KBOs, the Sun, and the known planets. But they also inserted several guesses, one at a time, at the size and orbit of our virtual 9th planet. In each case (including one with no 9th planet at all), they let everything go ahead and orbit the Sun for 4 billion years, a little less than the age of the Solar System.
Something startling happens in a few of these simulations when you get out beyond a = 250 AU:
THE ORBITS SUDDENLY LINE UP! Just like they do in real life. And they don’t line up like this unless you insert that new planet.
The blue traces on the graph are projected orbits of KBOs that would be observable to us without too much trouble; that is, the ones with perihelion distances of less than 100 AU. The 14 known KBOs we’ve been talking about are overlaid on the graph as the green, gray, and purple dots. The purple guys, again, have the stablest orbits. Along the bottom of the graph is the semi-major axis, and up the left side of the graph is the direction of the arrow pointing from the Sun to perihelion.
Two things jump out:
1) The solid blue on the left side means that the KBOs with smaller orbits (smaller a) have perihelion arrows pointing in all directions randomly, just like we observe in real life, and
2) the blue stripe along the center of the graph means that KBOs with longer orbits (a more than 250 AU) will tend to line up their perihelion directions, just like we observe in real life, if our 9th planet is really out there. And their perihelion directions will be opposite that of our 9th planet (180 degrees off).
The particular graph I’m showing is the one that best fit the known KBOs we have. That means that our 9th planet, if it’s out there, is most likely to be about 5 times as massive as the Earth and have an orbit with an a of around 500 AU and an inclination of about 20 degrees.
They did a similar simulation of the other weird alignment I mentioned, the inclination direction, and they found that a 9th planet, with the same properties they just found above, also can explain that.
Hmmm.
There is one more population of weirdos that our hypothetical 9th planet can explain. Some KBOs have seriously inclined orbits, like a Round Up ride gone haywire. Some are even so extreme they are turned upside down and orbit the Sun the “wrong” way. We can’t really explain these orbits with just the formation of the Solar System and the influence of the known planets.
These wackos aren’t so easy to show in 2-D, but our valiant authors plot them out for us anyway:
Simulations once again show that if you start with a random bunch of KBOs with orbits that aren’t very inclined, as you’d have in a young Solar System, the presence of our 9th planet, with the same properties as predicted above, is able to generate these kinds of crazy, highly inclined orbits. There are other explanations for how these orbits could have gotten this way, but none of them are very good, and none of them involve only the known planets.
The authors go on to do A LOT of parameter fitting and coordinate transformations and all kinds of craziness, but what they get out of it is some stricter limits. They conclude that our 9th planet can be no heavier than 10 Earth masses, its inclination can’t be much more than about 20 degrees, and its orbit has to be at least somewhat elliptical, or else it couldn’t make the perihelion directions of the KBOs line up.
So NOW it’s clear why finding more KBOs is important. If we can figure out their orbits, we get more data that helps us narrow down where our 9th planet might be. We have found a couple more not-so-near KBOs recently; you might remember the names FarOut and FarFarOut popping up within the last year, but we haven’t figured out their orbits just yet. After all, they’re ... well, far out, and so they take a while to advance in their orbits noticeably.
Okay, okay! So if it’s out there, when are we going to find it?!
There are several ongoing sky surveys, so to be honest, some young astronomer could be shouting “yahoo!” on a mountain in Hawaii as we speak. That’s where the Pan-STARRS sky survey is based. It’s already done plenty of surveying; here’s the whole sky visible from Hawaii:
The real version of that picture is so detailed it would be a mile and a half wide! Now compare that to the diagram below, which tells you the magnitude (faintness) of objects we can get down to with the Pan-STARRS telescope and still find 98% of what’s out there:
Notice the arc of the galactic plane (the Milky Way) on both pictures. If an object is hiding in the galactic plane, as you’d guess, it’s a little harder to find.
The best guess at our 9th planet’s magnitude, based on its distance and size, is somewhere between 19 and 22. The larger a magnitude number, the fainter the object. An increase in magnitude of +1 means a decrease in brightness of about 2.5 times. You can see things as dim as about magnitude 6 with the naked eye, so we’re in the range of about a million times dimmer than that. But, the huge Pan-STARRS telescope can handle it.
The Pan-STARRS data is in, but analyzing it takes a long time. There are other surveys going on as well, some that can get down to even dimmer objects. You can actually help process data from NASA’s WISE (Wide-field Infrared Survey Explorer) project. Just go to Backyard Worlds: Planet 9 to view actual data and mark anything that you see moving from frame to frame. If you’re very lucky, you could be the first human being to see our 9th planet.
Even in the worst-case scenario, the authors of “The Planet Nine Hypothesis” have this to say about the prospects of finding it:
A higher mass and thus more distant Planet Nine will require a dedicated survey along the predicted orbital path, but [even] with a lower limit to the brightness of 24th magnitude, such an object is readily observable by the current generation of telescopes with wide field cameras such as the Dark Energy Camera on the Blanco 4m telescope in Chile and the Hyper-Suprime Camera on the Subaru telescope in Hawaii. Finally, all but the very faintest possible Planet Nine will be observable with the Large Scale Synoptic Telescope (LSST), currently under construction in Chile and scheduled for operations in 2022. Therefore, Planet Nine – if it exists as described here – is likely to be discovered within a decade.
“Extraordinary claims require extraordinary evidence,” as Carl Sagan said. We do have a lot of good evidence, but we always have to be careful. People have proposed other planets in the past and been wrong. One timely example of that relates to the precession of the orbit of Mercury, which, prior to Einstein, no one could explain.
It’s timely because 100 years ago this Wednesday, May 29, there was a total eclipse of the Sun that proved Einstein’s theory of relativity to be correct, and one thing that this accomplished (among many) was to explain Mercury’s not-quite-right orbit. I hope you’ll go see my previous diary about the 100th anniversary of the 1919 eclipse.
Anyway, before Einstein, Urbain-Jean-Joseph Le Verrier had tried to explain Mercury’s orbit by proposing a planet called Vulcan, which would orbit the Sun in just 3 days. He did it because it made mathematical sense, and it had worked brilliantly for him before:
After painstakingly checking and re-checking his calculations, Le Verrier offered a groundbreaking hypothesis: some other object, unknown and unseen, was exerting a gravitational pull on Mercury’s orbit. “A planet, or if one prefers a group of smaller planets circling in the vicinity of Mercury’s orbit, would be capable of producing the anomalous perturbation felt by the latter planet,” he wrote. Le Verrier speculated that the sun’s glare had prevented the object from being positively identified in the past. Still, he argued that it should be easily spotted under the proper conditions.
The scientific community welcomed Le Verrier’s theory, and for good reason—he had a proven track record of finding new planets. Thirteen years earlier, the Frenchman had made a similar prediction while trying to account for a gravitational waver in the orbit of the planet Uranus. When astronomers scanned the heavens using his figures, they had found the previously unknown planet of Neptune. The discovery had vaulted Le Verrier to international scientific stardom, winning him admittance to the French Legion of Honor and a post as the head of the Paris Observatory. One fellow mathematician had since described his intellect as “almost superhuman.”
Maybe our 9th planet is the right explanation, or maybe there’s just something else about the Solar System we don’t yet understand.
But enough scientific restraint already — I want there to be a 9th planet out there! Don’t you?? If we find it, there will be a virtual party in this space, and a real one at my house! Come on, 9th planet!!
Yale astronomer Gregory Laughlin has a closing thought:
“Finding a ninth planet within our solar system would be both transformative and extraordinarily inspiring”, he said. “It would be this dramatic confirmation of the scientific method, which would be pretty refreshing in the current age where the truth is on trial.”
I leave you with an ode to our 9th planet, or perhaps it’s our 9th planet’s ode to us:
You can keep going down to the devil
Or you can stop here and live with me
Where the music plays
And the singer sings
And the lovers kiss
On the street
Far away
And hard to see