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Planet Nine

June 27, 2016

One of my accomplishments as a ten year old scientist was memorizing the names of the planets in order of their distance from the Sun. These are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Later, I learned that there were mnemonics that supposedly aided their recall, but I didn't think that any of those were that helpful. These were the days, of course, when Pluto was still considered to be a planet.

Just five planets were known to the ancients as members of the ensemble of classical planets that included, also, the Sun and the Moon. Excluding the Sun, these were the night sky vagabonds (planets, from the Greek, πλανης), all of which are visible to the naked eye. Telescopes are needed to see the rest.

Uranus was discovered by William Herschel in 1781. This planet was noted several time before that, but mistaken for a star. Neptune was discovered quite a few years after, not by an astronomer, but by the French mathematician, Urbain Le Verrier in 1846 using its gravitational affect on the other planets. Neptune had also been mistaken as a star by earlier telescopic observers.

Pluto was discovered at the Lowell Observatory in Flagstaff, Arizona, in 1930 by farm boy turned astronomer, Clyde Tombaugh. He discovered Pluto while doing a search for so-called Planet X predicted by Percival Lowell and William Pickering to exist through its influence on the orbit of Uranus. Pluto, however, is about the size of other Solar System objects, so it was reclassified as a dwarf planet by the International Astronomical Union in 2006.

An array of Trans-Neptunian objects

The sky is getting crowded near the orbit of Pluto. These Trans-Neptunian objects include Eris, Makemake, Haumea, Sedna, 2007 OR10, Quaoar, Orcus, and their moons. (Wikimedia Commons image by Lexicon.)

While the mathematics surrounding the discovery of Neptune and the possible existence of Planet X had a scientific basis in Newtonian gravitation, there's another mathematical formula for the orbits of known, and possibly undiscovered, planets that dates back to 1724. This formula, known as Bode's Law, gives the semimajor axes of the planets in astronomical units (AU) according to the simple formula,
AU = 0.2 + (0.3)(2k)

where k = -∞, 0, 1, 2, 3, etc., where k = 1 specifies the Earth. As can be seen by the graph, this formula gives good values for the planets through Uranus, but only when k = 3 is ignored. Quite to everyone's surprise, an object resembling a small planet was found at the k =3 orbit. That was Ceres, discovered by the Italian priest, mathematician, and astronomer, Giuseppe Piazzi, on New Year's Day, 1801.

Bode's law

Bode's Law.

There's good agreement of values up through Uranus. Neptune gives a large error, but Pluto fits well at the k=7 place.

Mercury has a k-value of negative infinity that effectively removes the exponential term.

(Created using Gnumeric and Inkscape.)

When the asteroid belt was discovered at Ceres' orbit, there was evidence that this orbit should have contained a planet, but it somehow failed to form. The asteroid belt was great fodder for 1950s science fiction, as the remnants of an exploded planet; possibly, a planet with an advanced technological civilization that destroyed itself.

When Pluto was demoted from planetary status, the number of planets dropped from nine to eight. Then, in 2014, Chadwick A. Trujillo, co-discoverer of Eris and an astronomer at the Gemini Observatory (Hilo, Hawaii), and Scott S. Sheppard of the Carnegie Institution for Science (Washington, DC), reported the discovery of the Trans-Neptunian object, 2012 VP113, whose perihelion is 80 AU.[1]

Trujillo and Sheppard noted regularities in the orbits of this object and the similar Trans-Neptunian object, Sedna, that might be the influence of a planet-sized object in the far reaches of the Solar System. This true ninth planet, called Planet Nine, would be in a highly elliptical orbit that places it far from astronomer's searching eyes most of the time (see figure).[1]

The orbit of Planet Nine and other Trans-Neptunian objects

The orbit of Planet Nine and other Trans-Neptunian objects. Note, especially, the small size of the orbit of Neptune in this illustration. (Wikimedia Commons image by nagualdesign.)

How such a large planet might have formed and relegated to such a highly elliptical orbit at a distance from the Sun of about 40 billion to 140 billion miles (400 - 1500 astronomical units) is still quite a mystery.[2-5] It could have formed in that orbit; or, it could have formed closer to the Sun and then ejected into such an orbit. A passing star might have pulled Planet Nine into the far reaches of the Solar System, but it's more likely that such a star would tug the planet completely away from us.[5]

A recent computer simulation shows only a 10% probability that a passing star could do this, and only when Planet Nine was quite distant from the Sun.[5] The conclusion from such simulations is that Planet Nine was more likely to have formed nearer the Sun and then nudged outwards by the other planets.[5] These same simulations indicate less than a 2% chance that Planet Nine is a captured exoplanet from a passing planetary system.

Figure caption

A Pale Blue Dot?

An artist's conception of Planet Nine.

(Caltech/R. Hurt image via Harvard-Smithsonian Center for Astrophysics.)

As an example of dueling theories quite common in the sciences, astronomers from Lund University (Lund, Sweden) and the Laboratoire d'Astrophysique de Bordeaux (Floirac, France) have argued in favor of the captured exoplanet origin of Planet Nine in a recent letter in the Monthly Notices of the Royal Astronomical Society.[6-7]

They consider the possibility that the Sun captured a planet from a nearby star in its early history, about 4.5 billion years ago. For this to have happened, the encounter between stars must have been more distant than ∼150 AU, or else the Sun's Kuiper belt would have been disturbed. Also, the planet of the other star must have been in an orbit greater than 100 AU.[6]

This is a plausible scenario, since stars are born in clusters, and they often pass close to each other. Says study co-author, Alexander Mustill of Lund University,
"Planet 9 may very well have been shoved by other planets, and when it ended up in an orbit that was too wide around its own star, our sun may have taken the opportunity to steal and capture Planet 9 from its original star. When the sun later departed from the stellar cluster in which it was born, Planet 9 was stuck in an orbit around the sun... There is still no image of Planet 9, not even a point of light. We don't know if it is made up of rock, ice, or gas. All we know is that its mass is probably around ten times the mass of earth."[7]


  1. Chadwick A. Trujillo and Scott S. Sheppard, "A Sedna-like body with a perihelion of 80 astronomical units," Nature, vol. 507, no. 7493 (March 27, 2014), pp. 471-474, doi:10.1038/nature13156.
  2. Gongjie Li and Fred C. Adams, "Interaction Cross Sections and Survival Rates for Proposed Solar System Member Planet Nine," arXiv, April 26, 2016.
  3. Scott J. Kenyona and Benjamin C. Bromley, "Making Planet Nine: Pebble Accretion at 250--750 AU in a Gravitationally Unstable Ring," arXiv, March 25, 2016.
  4. Benjamin C. Bromley and Scott J. Kenyon, "Making Planet Nine: A Scattered Giant in the Outer Solar System," arXiv, March 25, 2016.
  5. Christine Pulliam, "Planet Nine: A World That Shouldn't Exist," Harvard-Smithsonian Center for Astrophysics Press Release No. 2016-11, May 3, 2016.
  6. Alexander J. Mustill, Sean N. Raymond, and Melvyn B. Davies, "Is there an exoplanet in the Solar system?" MNRAS Letters, vol. 460, no. 1 (July 21, 2016), pp. L109-L113.
  7. Theft behind Planet 9 in our solar system, Lund University Press Release, May 31, 2016. Press Release in Swedish

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