spaceSpace and Physics

Planet Nine May Not Be The Only Hidden Planet In Our Solar System


Robin Andrews

Science & Policy Writer

An artist's rendering of Planet Nine. Neptune's orbit can be seen as a small ellipse around the Sun in the top right corner of the image. Tomruen, Nagualdesign/Wikimedia Commons; CC BY-SA 4.0

The scientific community was abuzz earlier this year when it was revealed that, despite Pluto’s demotion, the Solar System may have a hidden ninth planet after all, hanging out in the far reaches of the distant, frigid Kuiper Belt. Based on the perturbed orbits of massive rocky and icy bodies drifting way beyond the orbit of Neptune, it has yet to be directly imaged, but evidence for its existence, according to some astronomers, is fairly strong.

Now, a new piece of research uploaded to the arXiv pre-print server suggests something even more remarkable is happening beyond our current line of sight. If Planet Nine is actually there, it’s unlikely it could have survived in our Solar System on such a stable orbit for 4.5 billion years without some planetary accomplices. In short, there may be a Planet Ten and Eleven, and possibly more, in the shadows, whose orbits are stabilizing each other’s.


“Planet Nine, if it exists, moves in an elongated orbit that may be vulnerable to long-term perturbations. In this context, a lone Planet Nine may not be able to survive in its present orbit for the age of the Solar System,” the researchers write in their study. “A planet within a planetary group has better chances to be long-term stable. Therefore, if Planet Nine exists, it is probably not alone.”

content-1465922132-planet-nine-animationThe team came to this striking conclusion using the same types of numerical models that were first used by astronomers to conclude that our Solar System does indeed contain a concealed planet. Without these extra buffering planets, close encounters with other stars may otherwise pull it away from our own Sun.

This new study, led by astronomer Carlos de la Fuente Marcos of the Complutense University of Madrid, also concludes that several large objects near Planet Nine – including Sedna, a dwarf planet that’s only slightly smaller than Pluto – will be able to remain in their own orbits for many millions of years to come.

However, a few others – such as 2004 VN112, another dwarf planet – are already in such eccentric, elliptical orbits that their interaction with Planet Nine will eventually cause them to be ejected from the Solar System and into the deep, dark reaches of space.


Gif in text: Planet Nine's hypothetical orbit compared to known orbital paths within the Solar System. nagualdesign/Wikimedia Commons; CC BY 3.0

The interactions of massive objects, whether they are moons, planets, asteroids, stars or even black holes, are highly complex, and cause a variety of effects. Sometimes, if they’re too close to each other, they can collide and destroy each other, which explains where, among other things, Saturn’s rings came from.

Conversely, as highlighted in this mathematical study, planets on a very eccentric and unstable orbit can be perturbed by other nearby, similarly sized planets. Planet Nine’s hypothetical trajectory around the Sun is somewhat eccentric, and the team’s models suggest that without being pulled around a bit by other similarly sized worlds nearby, it would have left the Solar System by now.

The orbital parameters of Planet Nine in 3D. caltech via YouTube


Speaking of interactions between massive objects, one recent study suggested that if Planet Nine is real, it could have been captured by our own Sun from another star system as our local star left its stellar nursery at the start of its life. This may explain why its orbit is fairly eccentric in the first place.

Like another study that claims Planet Nine is likely to be part-Neptune, part-Earth in terms of its geology, this is all just speculation – albeit rigorously scientific in nature – until Planet Nine is directly detected. The hunt is on, as they say.


spaceSpace and Physics
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  • exoplanets,

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  • dwarf planets,

  • orbits,

  • hypothetical,

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  • Planet Nine,

  • extra planets