Categories: Guide to Space

How Do We Know There’s a Planet 9?

At this point, I think the astronomy textbook publishers should just give up. They’d like to tell you how many planets there are in the Solar System, they really would. But astronomers just can’t stop discovering new worlds, and messing up the numbers.

Things were simple when there were only 6 planets. The 5 visible with the unaided eye, and the Earth, of course. Then Uranus was discovered in 1781 by William Herschel, which made it 7. Then a bunch of asteroids, like Ceres, Vesta and Pallas pushed the number into the teens until astronomers realized these were probably a whole new class of objects. Back to 7.

Then Neptune in 1846 by Urbain Le Verrier and Johann Galle, which makes 8. Then Pluto in 1930 and we have our familiar 9.

But astronomy marches onward. Eris was discovered in 2005, which caused astronomers to create a whole new classification of dwarf planet, and ultimately downgrading Pluto. Back to 8.

It seriously looked like 8 was going to be the final number, and the textbook writers could return to their computers for one last update.

A predicted consequence of Planet Nine is that a second set of confined objects should also exist. These objects are forced into positions at right angles to Planet Nine and into orbits that are perpendicular to the plane of the solar system. Five known objects (blue) fit this prediction precisely.
Credit: Caltech/R. Hurt (IPAC) [Diagram was created using WorldWide Telescope.]
Astronomers, however, had other plans. In 2014, Chad Trujillo and Scott Shepard were studying the motions of large objects in the Kuiper Belt and realized that a large planet in the outer Solar System must be messing with orbits in the region.

This was confirmed and fine tuned by other astronomers, which drew the attention of Mike Brown and Konstantin Batygin. The name Mike Brown might be familiar to you. Perhaps the name, Mike “Pluto Killer” Brown? Mike and his team were the ones who originally discovered Eris, leading to the demotion of Pluto.

Brown and Batygin were looking to find flaws in the research of Trujillo and Shepard, and they painstakingly analyzed the movement of various Kuiper Belt Objects. They found that six different objects all seem to follow a very similar elliptical orbit that points back to the same region in space.

All these worlds are inclined at a plane of about 30-degrees from pretty much everything else in the Solar System. In the words of Mike Brown, the odds of these orbits all occurring like this are about 1 in 100.

Animated diagram showing the spacing of the Solar Systems planet’s, the unusually closely spaced orbits of six of the most distant KBOs, and the possible “Planet 9”. Credit: Caltech/nagualdesign

Instead of a random coincidence, Brown and Batygin think there’s a massive planet way out beyond the orbit of Pluto, about 200 times further than the distance from the Sun to the Earth. This planet would be Neptune-sized, roughly 10 times more massive than Earth.

But why haven’t they actually observed it yet? Based on their calculations, this planet should be bright enough to be visible in mid-range observatories, and definitely within the capabilities of the world’s largest telescopes, like Keck, Palomar, Gemini, and Hubble, of course.

The trick is to know precisely where to look. All of these telescopes can resolve incredibly faint objects, as long as they focus in one tiny spot. But which spot. The entire sky has a lot of tiny spots to look at.

Artist’s impression of Planet Nine, blocking out the Milky Way. The Sun is in the distance, with the orbit of Neptune shown as a ring. Credit: ESO/Tomruen/nagualdesign

Based on the calculations, it appears that Planet 9 is hiding in the plane of the Milky Way, camouflaged by the dense stars of the galaxy. But astronomers will be scanning the skies, and hope a survey will pick it up, anytime now.

But wait a second, does this mean that we’re all going to die? Because I read on the internet and saw some YouTube videos that this is the planet that’s going to crash into the Earth, or flip our poles, or something.

Nope, we’re safe. Like I just said, the best astronomers with the most powerful telescopes in the world and space haven’t been able to turn anything up. While the conspiracy theorists have been threatening up with certain death from Planet X for decades now – supposedly, it’ll arrive any day now.

But it won’t. Assuming it does exist, Planet 9 has been orbiting the Sun for billions of years, way way out beyond the orbit of Pluto. It’s not coming towards us, it’s not throwing objects at us, and it’s definitely not going to usher in the Age of Aquarius.

Once again, we get to watch science in the making. Astronomers are gathering evidence that Planet 9 exists based on its gravitational influence. And if we’re lucky, the actual planet will turn up in the next few years. Then we’ll have 9 planets in the Solar System again.

Fraser Cain

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

View Comments

  • I don't think we can confidently say that Planet 9 is of no danger, until it is officially found, and its motion analyzed. Multiple ancient cultures spoke of a planet-sized orbit on a long, highly elliptical orbit, which would disturb the inner solar system every time it passed through. There is also geological evidence that supports planet-wide catastrophes every few thousand years.

    There is also evidence that NASA had some awareness of this object as far back as a Neowise conference in the mid-1990's, as scientists shared results of an infrared survey of the space surrounding our solar system, and admitted there was probably a Jupiter-sized object lurking out there, but then said little else about it.

    I think sometimes in Universe Today's haste to poke fun at "conspiracy theorists", you end up making some rather unscientific leaps.

    • If Planet Nine regularly came anywhere near the inner Solar System we would see the obvious gravitational effects from previous passes. The ancients where 2000 years behind the times. At closet approach, Planet Nine would be a Neptune mass object 6 times further from the Sun than Neptune is.

  • No, we never were back to 8 planets because of the discovery of Eris. It is disappointing to see you give yet more attention to Mike Brown and his ridiculous branding of himself as the "plutokiller." He never "killed" planet Pluto; he is not even a member of the IAU, of whom only four percent most of whom are not planetary scientists, voted on the controversial demotion of Pluto. That decision was opposed in a formal petition by an equal number of (several hundred) professional astronomers.

    Why do you not mention that the number of planets in our solar system remains a matter of debate, with the leading Pluto scholar in the world, New Horizons Principal Investigator Alan Stern, and most of the New Horizons team, continuing to consider Pluto and all dwarf planets as a subclass of planets?

    This is why the term "Planet Nine" should not be used for this undiscovered object; instead, I respectfully request you refer to it by the conventional designation used for a theorized planet, which is "Planet X." The letter "X" here refers to the unknown, not to the Roman numeral 10.

    Brown deliberately referred to this object as "Planet Nine" to get in yet another jab at Pluto and supporters of its planethood. Stern accurately described this move as a "despicable" attack on the late Clyde Tombaugh and a diminishing of his discovery.

    For those who hold to the geophysical planet definition, according to which a planet is any non-self-luminous spheroidal body orbiting a star, free floating in space, or even orbiting another planet, this object, should it be actually be discovered, would be more like Planet 14. The geophysical definition sees dwarf planets as a subclass of planets and does not require an object to clear its neighborhood to be a planet. This means the number of planets in our solar system will continue to increase as more discoveries are made. Some of us actually find this exciting.

    Please seriously consider presenting a more objective discussion of this subject in the future.

    • I'm not unconvinced that if he WERE to refer to it as "Planet X" that you wouldn't just come back and whine about "X MEANZ 10 LAWL"

      Why do you insist on copy/pasting this response each and every time this site comes out with a Planet 9 article? Do you realize how ridiculous you sound? Do you think you're protecting Pluto's sensitive feelings by defending its honor so strongly? Because I can guarantee that the non-planet (that's right, I went there) couldn't care less about what it is called, because it is a floating mass of rock, ice, etc.

      The fact of the matter is that the word "planet" has two definitions: one that means "spherical object revolving about the sun" and another that means "non-dwarf-planet."

      If all you want to do is complain about the English language, here's a better topic: why do we insist on using the letter 'c' when it serves no purpose that cannot be filled by 'k' or 's'? Isn't that SO MUCH MORE infuriating?

    • Some people are just incapable of accepting that Pluto is now classified as a KBO. In the grand scheme of things, it's of no significance.

    • Or how about the attention you drain, droning on about semantics like it matters at all?

  • Seriously, Fraser?

    Planet 9 is Pluto and other "Dwarf Planets" (check with Dr Alan Stern about THE DEFINITION...he coined the term after all) ARE Planets too!

    Your "Planet 9" is in reality Planet X just as Pluto was before it was found!

  • Seriously People? I would think Pluto would like to be called the first dwarf planet. It's a distinction that rarely comes in astronomy to be called the first of something. I mean Eris can't even be called the first dwarf planet because Pluto was already in that spot. Which star is considered the first star? The one closest to us? Or the one that is the brightest? Or the one we didn't even realize was a star until we final started measuring the distance to stars and realized that they were related to the one we orbit about once a year.

    • Dwarf planets are planets too, just like dwarf stars are still stars, and dwarf galaxies are still galaxies. Alan Stern coined the term "dwarf planet" with the intention of designating a third class of planets in addition to terrestrials and jovians, not with the intention of designating non-planets. So Pluto is the first of the dwarf planet subcategory of planets.

      • Under this classification I would have thought Ceres would have been "the first of the dwarf planet subcategory of planets" having been discovered 130 years before Pluto.

        • I think the definition of dwarf planet should be further subcategorized into Icy and Stony. Naturally this is an arbitrary categorization based on our Earth-centric point of view (water ice is a perfectly good stone farther out in the solar system) but it's a start.

        • Welcome to the Jeff Boerst school of science where 'words are just words.' The beauty of Jeff's nomenclature is that you can measure anything according to your wishes. Just think of it! Win any argument about your theories.. Just tell them 'words are just words.!' My Meter is actually a measurement of time that's why your theories are wrong! Jeff recommends starting your own website instead of ever having a scientific opinion... Just words.. Got it?

  • Actually, Fraser, there is one detail that you are most probably wrong about: Planet 9 (Tobe Ra'ah being my choice of name) is very possibly the cause of the heavy bombardments. Once Tobe Ra'ah is actually observed and its orbit sufficiently charted, research should be done to correlate asteroid/comet/meteorite impacts to see if there is a discernible relationship between Tobe Ra'ah and impact frequencies.

    You might think you are serving society by poopooing any effect Tobe Ra'ah might have, but such an assertion is highly unlikely to be accurate, is certainly gravely unscientific (especially for a member of the science community), and is insulting to those of us who have common sense.

    • Yes.. The level of intellectual insult grows with these articles... Brown should stick to Sy Fi documentaries and leave classification of planetary bodies to a democratic consensus instead of being so churlish and disingenuous to Tombah and common sense in general. Eventually I will stop bothering reading much on the net if this Planet 9 crap keeps on..

    • And to tone down my rhetoric a little I will say that time with such writing would be better spent espousing critical thinking and the Baloney Detector Kit of the late Great Carl Sagan. Just way to many arguments from authority happening here with naming and classification of solar system/s of late...
      This Planet X (my name for it) could neatly explain why Homo Sapien Sapiens has existed in genetic form for 100,000 years but only can account for far less than 8,000 years of written history (more like 3,300 if we start with Cuneform)...

  • How about Plutwo as a name? Or rename Pluto as Goofy (still a dog) and just name the new real planet Pluto. It would make it easy for us older folk to remember and would turn the "Pluto is SO a planet!" crowd incandescent with fury! Sounds like fun. ;-)

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