There could be at least two unknown planets – members of our own solar system – hidden well beyond Pluto. That’s according to new calculations by scientists in Spain and the UK. Their work has been published as two articles in the journal Monthly Notices of the Royal Astronomical Society Letters – one in September, 2014, which you’ll find here – and the other in January, 2015, which you’ll find here.

Since Clyde Tombaugh discovered Pluto in 1930, astronomers have been speculating about yet-more-distant objects. But, so far, no large planets beyond Pluto have been found.

The new calculations – from the Complutense University of Madrid (UCM, Spain) and the University of Cambridge (UK) – are based on the orbital behavior of known objects at our solar system’s fringes. The most accepted theory of our solar system establishes that the orbits of extreme trans-Neptunian objects should be distributed randomly. By an observational bias, their paths must fulfill a series of characteristics. For example, according to the established theory, objects beyond Pluto must have a semi-major axis – the axis which defines a planet’s farthest point from the sun – with a value close to 150 AU (or 150 times the distance between the Earth and the sun; by contrast, Pluto’s orbit has a semi-major axis of 39 AU). Plus, according to the theory, their orbits must be inclined to the plane of the solar system by almost 0°.

Yet this isn’t what astronomers observe a dozen of known small bodies beyond Pluto. The values of the semi-major axis are between 150 AU and 525 AU. The average inclination of their orbits is around 20°.

In other words, solar system theory doesn’t match what is observed. When that happens, astronomers scratch their heads and wonder why. These astronomers believe the reason is that there are unknown large planets out on the fringes of the solar system, waiting to be discovered.

Carlos de la Fuente Marcos, astrophysicist at the UCM and co-author of the study, said in a press release:

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 ETNO and we consider that the most probable explanation is that other unknown planets exist beyond Neptune and Pluto. 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.

To carry out the study, the researchers analyzed the effects of the so-called Kozai mechanism. In celestial mechanics, this mechanism describes the way that the gravity of a large body can exert an influence on the orbit of another body that is smaller and farther away object. As a reference, they looked at the way this mechanism works in the case of a short-period comet called 96P/Machholz1, which is under the influence of Jupiter.

The authors say that their data comes up against two problems.

First, their proposal goes against the predictions of current models on the formation of the solar system, which state that there are no other planets moving in circular orbits beyond Neptune. The astronomers say the recent discovery by the ALMA radio telescope of a planet-forming disk more than 100 astronomical units from the star HL Tauri, which is younger than the sun and more massive, suggests that planets can form several hundred astronomical units away from the center of the system.

Second, the team recognizes that their analysis is based on a very small sample of known extreme trans-Neptunian objects. They looked at the orbits of only 13 objects in all in their study. But, they point out, in the coming months more results are going to be published; we should know more extreme trans-Neptunian objects soon.

That will make the study sample potentially larger … and you can bet these researchers will be looking at the orbits of any newly discovered, small objects in the outer solar system.

Bottom line: Since Pluto was discovered in 1930, astronomers have been speculating about possible large planets beyond it. But no additional large planets have been found at the edge of our solar system. New calculations by astronomers in Spain and the UK suggest that not one, but two unknown planets might exist in our solar system, beyond Pluto’s orbit.

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