Planet X: A New Hope?

Assuming you haven’t been living under a rock this past week, you certainly caught onto the excitement of the apparent discovery of a new planet, Planet X (which is officially called Planet Nine and informally Planet ‘Phattie’, but I digress).  If you, like me, still feel that Pluto was robbed of its planetary status, then you will also be a skeptic of this new discovery. Especially considering the intentional heartfelt image taken by the New Horizons probe last year:

pluto_color_beforeclosestapproach-crop-original-original
Clearly Pluto loves us. Why can’t it be a planet? Source: Slate

We first consider what a planet is. Ab initio I list three requirements that I would think planets should follow:

  • Are relatively large bodies (such as to support an inner core)
  • Are in some orbital (slightly elliptical?) pattern around the Sun
  • Should be so large as to sustain the orbit of other smaller astronomical bodies (moons?) around themselves.

Ok, now let’s look at what the actual definition is. The Greeks originally claimed that planets were ‘wandering stars’ – in fact, the word planet comes from planetai, meaning “wandering.” Remember, in those days the Earth was at the center of the universe (and our solar system), and therefore every celestial body that ‘wandered’ in the sky over the course of the year was a planet. So yes, even the Sun and Moon were initially considered planets!

Gradually the Sun was understood to be at the center of our Solar System and other ‘planets’ were discovered. Moons and satellites were eventually separated from planets. Fast forward to the turn of the millenium (~1995) and surprisingly the only concrete definition of a planet was:

  • Any “large” body that orbited a “star”

The main dispute was over the “slightly large” planets and the “slightly small” planets, particularly:

  • Not too “large” such as to support core fusion- consider the difference between planets and brown dwarfs; the latter of which fuses deuterium (Brown dwarfs)
  • Not too “clustered” such as to be surrounded by other celestial objects- such as a belt of objects, like an asteroid belt (Asteroid Belt)

This second argument was the key in dismissing Pluto as a planet – an impressive number of objects were observed in the Kuiper Belt, close to the far reaches of Pluto’s orbit. To solve this issue, the IAU in 2006 came up with this definition for a planet:

  • Orbits the Sun
  • Has sufficient mass for self-gravity (so as to overcome rigid body forces) and therefore maintain a nearly spherical shape
  • Has cleared the neighborhood of its orbit

This new definition instantly made Pluto and Ceres- an asteroid- ‘dwarf planets.’ Certainly the definition is still a work in progress as more and more objects are discovered around our solar system… like Planet X!

Here’s the shimmy on Planet X: we haven’t actually seen it yet. The original paper, incidentally written by the astronomer who ‘killed Pluto’, describes a unique idea for evidence of a out of range planet. By observing an abnormal dynamic orbital clustering in the Kuiper Belt scattered disk (see below), Brown proposes that this change could only be caused by the orbit of a planet (or eccentric perturber, as stated in the article) as the alternative would be that the Kuiper Belt contains a large internal gravity, which it doesn’t. The presence of a planet would explain the orbital clustering right at the perihelion- the idea mirrors our own planets orbiting the Sun.

fig21
Orbital clustering of six unique Kuiper Belt objects (KBO). There is a peculiar clustering at the perihelion (short end) of their orbits, in addition to the orbital overlap (only occurs 1% of the time). Source: aasNOVA

To account for this orbital clustering, Planet X is estimated to have a mass of 10 Earths with a orbital period of 15,000 years. These are decent conditions for a planet, but now the true adventure begins. With an orbital period of roughly 15,000 years, Planet X spends most of its time between 600 and 1200 AU- that’s 600 to 1200 times farther out from our Sun than the Earth is. Most telescopes and observatories will have trouble discovering this planet mainly because if it is indeed a planet, it will be incredibly dim. The best chance is to exhaustively make use of the Subaru observatory in Hawaii. Brown estimates that it will take close to 5 years to cover the area where the planet could be lurking.

The other problem is to explain how this ‘planet’ has left the ‘inner’ Solar System and entered the Kuiper Belt, while maintaining orbit of the Sun.  And well, to also ensure that the planet is not in the neighborhood of other celestial bodies nearby. Furthermore- this potential discovery would imply that many other ‘planets’ could also potentially exist out there. In the meantime, best of luck to the team in discovering Planet X! Oh… and give Pluto a second chance :).

p9_kbo_orbits_labeled-news-web
Another view of the KBO orbits, this time with the proposed orbit of Planet X included. Our Sun is the smaller light in the middle of Planet X’s orbit. Source: CalTech

Additional links:

DailyGalaxy: History of Planets

CalTech: Planet X

ScienceMag: Planet X

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