Has Planet Nine Been Discovered?

Images by Space Engine

Our solar system is home to a cornucopia of observable night sky phenomena: asteroids, comets, meteorites, and… more asteroids. But the title for the biggest and most influential space objects orbiting our sun has to go to our robust list of eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Honorable mention to the late Pluto, of course.

For all of human history, mankind has known about six of our eight planets. The other two, Uranus and Neptune, were only just discovered in the last 300 years.

But did you know there could still be an undiscovered planet lurking in the reaches of our solar system, hidden from mortal eyes for millennia?

Here’s the story.

All the way back in 1906, American astronomer Percival Lowell predicted that the orbits of Uranus and Neptune—our farthest planets—had been disturbed from their ideal positions at some point. This disruption could only be caused by another object of large size exerting its gravity upon them. Fueled by the implications of an astronomical breakthrough, Lowell created a program of research at the Lowell Observatory in Arizona which focused on locating this planetary interloper.

One of the workers at this facility was none other than Clyde Tombaugh, AKA the man who discovered Pluto. Tombaugh found our misunderstood dwarf planet almost by accident while searching for Planet Nine, and for a time Pluto was promoted as the fabled ninth planet of our solar system.

Of course, this wasn’t to last. In 2006 our catalogue was demoted back to eight planets for various reasons (see my other article, xxx). So long, Pluto.

However, there were doubts as to the merit of Pluto’s assumed status. Was this really the ninth planet the world of astronomy was searching for?

No, actually. It wasn’t. Why? Pluto’s mass couldn’t be calculated until long after its discovery, in 1978. One of the dwarf planet’s moons, Charon, was finally found and allowed astronomers to obtain the mass of Pluto. It turned out Pluto was far tinier than they anticipated, meaning it couldn’t have possibly had as much gravitational influence on Uranus and Neptune as was expected.

That means that even to this day, Planet Nine—Lowell’s predicted planet from 1906—is still at large.

Along with the modified orbits of our two farthest planets, another much greater oddity has recently emerged in the solar system which gives credence to a new planet:

The existence of Extreme Trans-Neptunian Objects (ETNOs).

An average Trans-Neptunian Object (TNOs) is an asteroid which orbits the sun beyond the orbit of Neptune. Pluto falls under the TNO category. ETNOs, as their name implies, orbit even greater distances than TNOs, with their closest approaches to the sun ranging from 40 to 60 AU. One AU is the distance from Earth to the sun, or about 93,000,000 miles. Compare the distances of ETNOs to Neptune, which hovers around 30 AU from the sun; they’re pretty far away!

Today over 20 ETNOs have been discovered, but three stand out from the rest. Known as the Sednoids, these three ETNOs consist of Sedna, 2012 VP113, and Leleākūhonua (quite a mouthful). These Sednoids all have perihelia well beyond the Kuiper Belt at the edge of the solar system, leaving them truly alone and highly suspect. The ETNO Sedna was discovered in 2003 and caused quite a stir in the astronomical community, boasting a perihelion of about 76 AU! Its furthest point from the sun takes it nearly 1,000 AU away. Another asteroid named 2006 SQ372 holds the record for further asteroid orbit from the sun, traveling over 1,400 AU away. Sedna, however, maintains the second highest perihelion of any asteroid in the solar system, beaten only by its fellow Sednoid 2012 VP113, nicknamed “Biden” after Joe Biden, the U.S. vice president at the time of its discovery in 2012.

Asteroid Sedna orbits much further away than Neptune. How did it get pushed out there?

ETNOs, however, are special in other various ways than TNOs. Their enormously high perihelia (closest points to the sun) are very unusual.

How does an object get moved to such an orbit naturally?

Let’s say, for example, you have an asteroid orbiting near Neptune. We’ll call it Watney. Its orbit is very similar to Neptune’s, being generally circular around the sun about 30 AU away.

One day, Watney has a run-in with Neptune. It swings around the ice giant and gets catapulted much further away from the sun, raising its aphelion to over 100 AU. This turns its circular orbit into an oval shape, or an ellipse, around the sun. However, Watney’s perihelion would remain at 30 AU, since that’s where it initially began.

We do not know of many objects with circular orbits beyond Neptune. Most things out there have highly erratic orbits in the Kuiper Belt. Therefore, it is common to see objects beyond Neptune with crazy elliptical orbits.

But what would cause an asteroid to have its perihelion raised beyond Neptune? We don’t know of anything massive enough out there to have a gravitational effect like that.

Ergo, hypothetical Planet Nine.

Adding to the complications is the clustering of these odd ETNOs, with perihelia all roughly packed together. The similarities between these orbits would be far more likely if something was out there moving them around.

The orbits of all discovered TNO and ETNO asteroids. Sedna’s orbit is marked red. Notice the clustering of their nearest points to the sun.

Now, there are other proposed explanations for ETNOs and their bizarre orbits. But an undiscovered ninth planet with a mass equal to about five to ten times Earth’s mass could likely be the answer we’re looking for.

If we’re so hopeful about Planet Nine’s influence, why hasn’t it been discovered yet?

Unsurprisingly, Planet Nine would orbit extremely far from the sun, be relatively small, and hardly receive any light from the sun. Astronomers have estimated its orbital path and still attempt to snap a picture of the elusive troublemaker to this day. Unfortunately, they haven’t had much luck so far. The WISE and other powerful telescopes continue searching for an echo the planet, and have been doing so for years. Until we have visual confirmation of a new planet, we cannot (and should not) come up with an official name and assume it exists. Thus, it is entirely hypothetical at the moment of this writing.

The hypothesized distance of Planet Nine, represented by the distant, brown orbit. You can just barely see the orbit of Neptune much closer to the sun!

Planet Nine is still fun to think about, though. There are still so many secrets hidden in our own solar system!

Do you think Planet Nine exists? What other evidence is out there that I didn’t include? What do you think it would be named if it was found? Let me know!