Given the huge number of exoplanets discovered in recent years, the discovery of two new planets would come as no surprise—except that these two, discussed in a new study, may be part of our Solar System.

The presence of the closer of the two planets had already been suggested in a previous work. The new study provides more evidence for its existence and adds a second planet. Both studies are based on observations of objects far beyond Neptune’s orbit, called extreme trans-Neptunian objects (ETNOs). These ETNOs display shared patterns in their orbits, which suggests they’re all being influenced gravitationally by heavier objects, much further away from the Sun.

While this conclusion is based on a small sample (13 bodies), the authors confirm that their results are statistically significant and that at least two planets, orbiting far beyond Pluto’s orbit, are the most likely explanation for the observations.

Extreme trans-Neptunian objects

Following the discovery last year of the ETNO planetoid 2012 VP 113 , there's been scientific interest in the existence of another large planet outside Pluto’s orbit. VP 113 has the most distant perihelion (the point of its closest approach to the Sun in its orbit) of any known minor planet. Intriguingly, VP’s perihelion occurs as it crosses through the other planets’ orbital plane, at an angle of about zero degrees—a feature which seems to be shared by other asteroids and comets beyond Neptune’s orbit. All 13 featured in the study reach their perihelion right as they pass through the Solar System’s orbital plane.

This pattern defies predictions for how ETNOs should behave. Their perihelia should be randomly distributed rather than converging at the value of zero degrees. They also have wildly varying distances, with an average distance from the Sun (or semi-major axis) of up to 525 astronomical units (au)—that value is expected to have around 150. Their orbital inclinations are predicted to roughly match the planets’, but instead they differ by about 20 degrees.

These orbital characteristics led the researchers to hypothesize the presence of additional planets, which could be perturbing the ETNOs’ orbits through gravitational interactions (technically, through what's called the Kozai mechanism). "This excess of objects with unexpected orbital parameters makes us believe that some invisible forces are altering the distribution of the orbital elements of the EETNOs, and we consider that the most probable explanation is that other unknown planets exist beyond Neptune and Pluto," said lead author Carlos de la Fuente Marcos, of the Complutense University of Madrid.

The Kozai mechanism is a process in which a larger body causes a smaller body’s orbital characteristics to oscillate around a specific point—in this case, the ETNOs’ perihelia oscillate around zero degrees. As a reference, the researchers considered an example of the Kozai mechanism in action, which occurred in more familiar territory: Jupiter’s effect on the comet 96P/Machholz1.

Meet the new planets!

To get a sense of the distance of these worlds, first consider that light, the fastest thing in the Universe, takes a full eight minutes to get from the Sun to the Earth. Earth is located one au away from the Sun. Jupiter, meanwhile, is about five au from the Sun, while Neptune is about 30 and Pluto, at 39, is far out enough that the Sun would look like an abnormally bright star from its frozen surface.

With that in mind, consider that the closer of the new planets would be about 200 au away, while the further one—the one initially suggested in the prior study—has a semi-major axis of about 250au. At those distances, it’s no surprise that the planets (should they actually exist) haven’t been observed until now. They would be very hard to detect, being so dim and small in the sky.

And unlike Pluto, these would be proper planets, big enough to be termed super-Earths. The farther one could have a mass of up to 10 times that of our planet. And the term "super"-Earths is apt in a way: the comic book planet Krypton, whose stronger gravity gave Superman the ability to leap tall buildings in a single bound on Earth, is only five times more massive than Earth. (Not that the comparison means anything in a real-life physical sense, of course, but this is a heavy planet).

The other, closer planet would also be more massive than the Earth.

As if that weren't enough, the two planets might not be all—there could be others as well, waiting to be discovered. “The exact number is uncertain, given that the data that we have is limited, but our calculations suggest that there are at least two planets, and probably more, within the confines of our solar system,” said de la Fuente Marcos.

Leaping hurdles (in a single bound)

The authors acknowledge, however, that their data comes up against two problems. The first is a conflict with models of stellar system formation, which don’t predict the formation of these super-Earths so far beyond Pluto’s orbit. But this may have been addressed by another recent discovery. The ALMA telescope’s observation of the star system HL Tauri revealed a stellar system in the process of forming. These images show that planets can be present several hundred au from their star, even as the system is still forming, contrary to current models.

Although the authors’ conclusions are statistically significant, they recognize that their small sample size remains a hurdle. However, in the coming months, more work will be done to obtain a larger sample.

If these hurdles can be leapt—however many bounds it takes—we might eventually get used to reciting names the two super-Earths alongside the other eight planets. “If it is confirmed, our results may be truly revolutionary for astronomy,” said de la Fuente Marcos.

MNRAS, 2015. DOI: 10.1093/mnrasl/slu084 (About DOIs)