Somewhere Out There Could Be a Giant New Planet in Our Solar System: So Where Is It?

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The possible ninth planet is thought to be quite substantial with a mass around ten times that of Earth and a radius that’s two to four times bigger than Earth’s. This characterizes it as a Neptune-like object.

What’s truly remarkable about Planet Nine, as it has been dubbed, is its very long orbit. It is estimated to take between 10,000 to 20,000 years to orbit our sun, on an elliptical orbit that stretches way beyond the Kuiper Belt.

The Kuiper Belt is a ring of icy objects (which includes Pluto) that circles the sun beyond the orbit of Neptune. Neptune orbits about 30 times further from the sun than the Earth and astronomers refer to Neptune’s distance from the sun as being 30 astronomical units (au) (where one au is the Earth-sun distance). Pluto follows an elliptical orbit that brings it as close as 29.7au from the sun, then out to almost 50au at its most distant point.

Planet Nine’s proposed elliptical orbit takes it from 200au at its closest to the sun (or perihelion) and between 500au to 1,200au at its furthest (aphelion). When it comes in close, it should be bright enough for high-spec backyard telescopes to pick it up.

But unfortunately, most of the time the planet will be much more distant and that represents a greater challenge. It will require the world’s largest telescopes, such as the 10m diameter Keck telescopes and Japan’s 8.2m Subaru telescope (both located on Mauna Kea in Hawaii) to have a hope of seeing it.

Ghostly Pull of Gravity

The planet has yet to be seen. So why is it thought to be out there? And how can we know so much about it? Planet Nine is the best fit to explain the orbits of six distant objects.


The six most distant known objects in the solar system with orbits exclusively beyond Neptune (magenta) all mysteriously line up in a single direction. Also, when viewed in three dimensions, they all tilt nearly identically away from the plane of the solar system. Batygin and Brown show that a planet with ten times the mass of the earth in a distant eccentric orbit anti-aligned with the other six objects (orange) is required to maintain this configuration. The diagram was created using WorldWide Telescope. Image Credit: Caltech/R. Hurt (IPAC)

What’s odd about these six objects is that they have peculiar but remarkably similar orbits. These objects have been nudged off kilter and yet they are all shepherded together in the same region of space.

The first of these objects to be discovered was Sedna. It was observed in 2003, as it approached perihelion. When its 11,400-year orbit was calculated, the discovery team realized that this object was orbiting in a kind of “no man’s land” (or more correctly “no person’s land”).

It was too distant to belong to the Kuiper Belt and not far enough away to be among the sphere of comets orbiting the sun in the Oort Cloud.

Sedna was also beyond the gravitational pull of Neptune, so something else, perhaps a large planet or possibly even a passing star (one of the sun’s many siblings perhaps), might have nudged it off course. What makes Planet Nine feasible is that it can explain the orbit of Sedna along with the other five objects.

At their closest approach to the sun, these six objects sit within the plane of the solar system. Planet Nine would have an orbit that is anti-aligned to the six objects and provides the gravitational tug needed to keep those planets in check.

And there’s more. What makes good science is when a proposed model explains something above and beyond its original intention. Simulations of Planet Nine predict that there should also be objects in the Kuiper Belt that have orbits perpendicularly inclined to the plane of the solar system.

Turns out, these objects exist. Five such objects have been known about since 2002, although their orbits have been unexplained until now.


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. This diagram was created using WorldWide Telescope. Image Credit: Caltech/R. Hurt (IPAC)

Haven’t We Seen This Before?

If Planet Nine does exist, it’s not the first time that a planet in our solar system has been discovered theoretically before being directly observed. In 1845, deviations in the orbit of Uranus, suggested there might be an eighth planet to the solar system and in 1846, Neptune was observed exactly where it was predicted to be.

There have also been predictions that haven’t stood the test of time. Back in the 1980s, scientists proposed that the sun might be a binary, with a dim undiscovered companion moving along on elongated orbit. Every 23 million years (or so), this star named Nemesis would pass through the solar system causing a deluge of comets to impact Earth and produce mass extinctions.

More recently, around the turn of the millennium, astronomers noticed an asymmetry in the distribution of new comets coming in from the Oort Cloud. In theory, comets should come evenly from all directions, but there was a slight excess distributed around a great circle on the sky. One of the explanations was that there could be a Jupiter-mass planet in the Oort cloud, known as Tyche.

In 2014, NASA’s Wide-Field Infrared Survey Explorer (WISE) examined the entire sky across infrared wavelengths. It was the perfect telescope to detect Nemesis or Tyche, but failed to find any evidence of either.

Will We Find Planet Nine?

Scientists are sceptical by nature. It’s exciting to have a model that predicts the existence of Planet Nine but this prediction must also be tested. Astronomers have begun searching through astronomical surveys, such as the WISE survey, the Catalina Sky Survey, and the Pan STARRS surveys in the hope of making a sighting.

So far, nothing has been seen. The conclusion, as described in a blog by astronomer, Mike Brown (who proposed Planet Nine along with colleague Konstantin Batygin) is that Planet Nine, if it exists, is likely in the hardest place to find.

It seems to currently be at its furthest point from the sun, at least 500au away; it’s probably fainter than 22nd magnitude (that’s 1,500 times fainter than Pluto); and very possibly it’s aligned with the plane of the Milky Way Galaxy (meaning that Planet Nine may currently be hidden against the background stars of our Galaxy).

Regardless, the hunt is on and there just may be a great discovery out there, waiting to happen.

The ConversationTanya Hill, Honorary Fellow of the University of Melbourne and Senior Curator (Astronomy), Museum Victoria and Jonti Horner, Vice Chancellor's Senior Research Fellow, University of Southern Queensland

This article was originally published on The Conversation. Read the original article.

Banner Image: Caltech/R Hurt (IPAC)

Discussion — 10 Responses

  • Quantium January 22, 2016 on 9:36 am

    Probably a stupid idea, but could one of the Voyagers or New Horizons be pointed at it?

    • horseshoe7 Quantium January 22, 2016 on 11:45 am

      1) Those probes have already passed any point where the could be re-targeted using the slingshot effect of a planet’s gravity.
      2) We really don’t have a clue where the planet is even if we could.
      3) It would take some 300 to 700 years to get there, even if we did send a new probe, using current chemical rocket technology.

      • Quantium horseshoe7 January 23, 2016 on 8:13 am

        That wasn’t the idea. Do you remember the “pale blue dot” picture of Earth taken by Voyager 1? I was thinking of something like this, only along an arc of space where planet 9 may be expected. As it is from a different position, P9 might not be obscured by light from the Milky Way as it would from here.

    • drogers000 Quantium January 22, 2016 on 2:20 pm

      I wondered the same thing myself…

    • Christopher Carr Quantium January 22, 2016 on 4:44 pm

      They don’t know where (or if) it is.

      • Quantium Christopher Carr January 22, 2016 on 11:55 pm

        I thought they had worked out where it might be, and the problem is that from Earth it would be seen against the plane of our galaxy and be a dim object against the bright band of stars known as The Milky Way. Pointing a camera at this position from a different viewpoint might produce a “pale grey dot”. Or maybe that is still asking too much of the cameras available on deep space probes.

        • Christopher Carr Quantium January 25, 2016 on 12:21 pm

          As I understand it, if the object exists (and they think it most likely does), they know its orbit — but not its present location.

  • Christopher Carr January 22, 2016 on 4:43 pm

    Would this be a gas giant, or a rocky super-earth?

  • Amber Lite January 25, 2016 on 9:00 am

    Is this not the infamous planet Nibiru? What we don’t know about the universe could probably fill the universe several times over. All these intellectuals and scientists working vigilantly day after day from all around the world and we still are so very uninformed about the universe around us. Why is that? It’s like cancer research – billions and billions are dollars are spent every year and the results are beyond lacklustre. They are down right pathetic, perhaps the way we view the universe is why it is still such a mystery to us. Maybe if we take off our human blinders and constraints as to what life looks like, or needs or the requirements for such to begin, then we can start looking at the universe differently and yielding more interesting and innovative results.

  • Mobick January 25, 2016 on 11:59 am

    Amber Lite: You make some valid points but the real problem really is funding. Unlike Cancer research, we don’t spend Billions and Billions of dollars on space research or funding NASA. They just don’t get that much money (relatively speaking), considering how important it is, we should all lobby our congressman/women to fund more to NASA. The advent of private space research industries is helping, but again, the Obama Administration cut funding when he got into office. The only president’s that were really into funding NASA, as much as possible, were JFK and Reagan. They really do need more money and that’s a place that I am happy to see my tax dollars go too. We spend more on food stamps than we do on NASA. That is sad. I do believe there is some incompetence on NASA for not knowing something that was recorded by ancient civilizations. Many ancient civilizations, including the Summarians, Egyptians and the Mayans, wrote about Planet 9 or Niruibu and it was all dismissed by academia. So, even our college professor’s are guilty of this incompetence for not wanting to believe the possibility of something’s existence, even though Ancient Civilizations wrote it in stone. Just a year ago, NASA released a statement saying there was no “planet 9” or Nirubu in the solar system and we know that as a fact ( a Soviet Astonomer had written a paper to prove it’s existence and it was simply ignored and categorized as “conspiracy”. We would know if there were a planet that large in our solar system”. That really is incompetence as they left out any possibility of its existence even though they had no empirical data to rule it out. The whole idea of of its existence was thrown into the conspiracy files. It is said, that art least 10% of all conspiracy theories have some foundation in fact. I believe that number is higher. To deviate here, just look at President Kennedy’s assassination. The Zipruter (sp) film shows the second bullet blowing his head backwards, not forwards, from where the shot allegedly came from (behind him) . The Laws of Physics themselves prove there was another shooter. When visiting the Book depository in Dallas, the answer is obvious. But, I just relate this to support some support for “conspiracy” theories, of which this idea (Planet 9) was posited. And posited a long time ago.