Could a super-Earth-sized planet exist beyond the outer solar system?


Yesterday's announcement of newly-discovered dwarf planet 2012 VP113, far outside the orbit of Pluto, came with a very interesting addition: the strange orbit of this dwarf planet, along with the equally strange orbit of fellow dwarf-planet Sedna, hints at the possible existence of a 'super-Earth' planet lurking somewhere beyond the outer solar system.

Recently, after an extensive survey of the space around our solar system, astronomers working with NASA's Wide-Field Infrared Survey Explorer (WISE) announced that they found no evidence of the so-called Planet X. This hypothetical planet was first proposed as a way to explain irregularities in the orbit of Uranus, since estimates of Neptune's mass at the time couldn't account for them. Astronomers thought that another planet, roughly the size of Saturn or Jupiter, could exist far out beyond Neptune, which would solve the problem. Although updated calculations of Neptune's mass eliminated the need for Planet X to explain Uranus' orbit, this didn't mean that we'd heard the end of it, though. These days, Planet X is used to spread doomsday 'prophecies' that say it has caused mass extinctions here on Earth, by sending a deadly swarm of comets inbound from the Oort cloud — the vast 'shell' of icy comets that's predicted to surround our solar system.

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Despite these portents of doom, WISE found nothing to indicate that there was any planet the size of Saturn out to a distance of 10,000 astronomical units from the sun, and no planet the size of Jupiter or larger out to a distance of 26,000 astronomical units (out to roughly a quarter of the distance through the Oort cloud).

However, although we're now confident that there isn't a planet the size of the dreaded Planet X, that doesn't necessarily mean that a smaller planet couldn't exist out there, such as a super-Earth — a planet larger than Earth, but smaller than Neptune.

Now, there's nothing solid to support this idea from the astronomers' findings. This is just speculation based on the orbits of these two dwarf planets, 2012 VP113 and Sedna (as seen in the image below), and some computer simulations to test how these orbits could develop.

It's assumed that both Sedna and 2012 VP113 started out with roughly circular orbits, like most of the objects in the Oort cloud are assumed to have, and then something gave them a boost to put them on these highly elliptical orbits. They may have interacted with each other at some point (their orbits do come fairly close to one another at one point). A passing star or rogue planet may have disturbed them into this orbit, or they may be captured objects from other star systems that were nearby when our solar system formed. Or, maybe, just possibly, it could have been due to something from our own solar system.

"A rogue planet could have been ejected from our solar system and perturbed their orbits," Scott Sheppard, an astronomer at the Carnegie Institution for Science who was one half of the pair that found 2012 VP113, told National Geographic. "Definitely, it could still be out there."

So, if this planet were to exist, how big would it be, where would it be located and why wouldn't we have seen it yet?

According to the study, published today in Nature, the astronomers used a super-Earth-sized planet — between 2-15 times the mass of Earth — in their simulations, and put it around 250 astronomical units from the sun (or 250 times farther away from the sun than Earth is). The WISE survey was able to look for planets the size of Saturn and Jupiter, but seeing something smaller than that would be very difficult.

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Does this mean that all the fears over Planet X could be justified? No, not really, even though it's likely that those spreading these fears will be using it as confirmation of their predictions.

The idea that this planet may exist is only one possibility of several for how the orbits of Sedna and 2012 VP113 got this way. Even if it does exist, a super-Earth planet would just be too small to cause a calamitous stream of comets from the Oort cloud, simply because the objects out there are spread out so far apart from one another. If that could happen, Earth and Mars would be having a huge effect on objects in the asteroid belt and all the centaurs in the outer solar system as well. Gravity does extend out to infinity, but its strong effects only reach out to a fairly short distance away (astronomically-speaking). As for any possibility that this could be Nibiru, the planet that doomsday 'prophecies' say is supposed to destroy us by a collision or near-collision someday, it's far more likely that an ejected super-Earth planet would just keep on going, rather than swinging around for another pass.

Still, the idea that there could be another planet out there is intriguing. Finding it would be difficult, but if it did exist, it would be very interesting to see exactly where it came from in our solar system, and how it got to where it is now. Hopefully, we'll see a followup study about this, to see if there really is something else out there.

(Images courtesy: NASA, Scott S Sheppard/Carnegie Institution for Science)

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