spaceSpace and Physics

If Planet 9 Exists, What Is It Made Of?


Robin Andrews

Science & Policy Writer

911 If Planet 9 Exists, What Is It Made Of?
The simulated structure of Planet Nine, with its curiously rocky innards. Esther Linder, Christoph Mordasini, Universität Bern

It’s understandably difficult to try and predict what a planet’s made of when you can’t even be sure it’s there. That’s the confounding challenge facing astronomers right now when it comes to the hypothetical Planet Nine, perhaps the newest member of our Solar System; nevertheless, a team from the University of Bern in Switzerland have given it their best shot.

As revealed in their study, published in the journal Astronomy & Astrophysics, Planet Nine would be a decidedly cold, icy place at the surface, but curiously hot and rocky the further down you go. The researchers think that its core is still cooling down, and excess heat is still escaping up to the surface.


“With our study candidate Planet Nine is now more than a simple point mass,” said Christoph Mordasini, a professor of astrophysics at the University of Bern and the study’s co-author, in a statement. “It takes shape, [it has] physical properties."

The potential ice giant, which could even be an exoplanet captured by our Sun in an act of celestial-scale kleptomania, was first proposed in January by Caltech Astronomer Mike “Pluto killer” Brown and his colleagues. By tracking the strange movements of objects in the distant Kuiper Belt, a vast region of comets far beyond Pluto, Brown concluded that something must be perturbing their paths through the cosmos.

This could only be due to an object with a mass of 10 Earths, orbiting the Sun on an elliptical path, at a distance of 200 to 700 times the Earth-Sun distance. For this new study, the researchers took this orbital and mass data and ran computer simulations designed to explain the evolution and composition of this mysterious, distant world.

An artist’s impression of Planet Nine. Neptune’s orbit is shown as a small ellipse around the Sun, top right. Tomruen, nagualdesign/Wikimedia Commons; CC BY-SA 4.0


The most likely simulation, according to the study, pointed to a planet like Uranus or Neptune, but not quite as massive.

If you sliced through this frigid world, you’d cut first through an atmosphere of hydrogen and helium, before reaching a far thicker gas layer made of the same elements. At this point, it’s literally freezing, with surface temperatures clocking in at -226°C (-375°F). That’s just 47 degrees above absolute zero, the coldest possible temperature in the universe.

Next comes a water ice layer at a toasty temperature of 1,827°C (3,320°F). It may seem contradictory that water ice could be so hot, but the gaseous atmosphere  –or “envelope” – on Planet Nine is likely to be around 1.4 times the mass of the Earth. This would exert an enormous pressure on the water ice beneath it, which would allow it to retain its solid ice form.

Just below the ice lies a mantle region composed of silicates, somewhat similar to that of several other rocky planets, including our own. Beneath the mantle lies the core, thought to be iron – again, just like our own – at a temperature of around 3,427°C (6,200°F). Overall, the solid parts of the planet come in at around 8.6 Earth masses, meaning that the entire world is about 10 Earth masses, with a radius 3.7 times greater than our own world’s.


Planet Nine, then, is Neptune-like in its upper layers and Earth-like in its deeper layers. “In its whole appearance, I would rather say it is a smaller version of Uranus and Neptune,” Esther Linder, a Ph.D. candidate at the University of Bern and co-author of the study, told IFLScience.

Of course, only time will tell if this estranged planetary cousin is actually there at all.


spaceSpace and Physics
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  • structure,

  • evidence,

  • composition,

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  • appearance,

  • Planet Nine,

  • ice giant