Uranus and Neptune may not be the “ice giants” we thought: a new study suspects that the two planets are rocky giants.
Our idea about the interiors of the Solar System’s most distant planets may be wrong. A group of researchers from the University of Zurich and the Swiss NCCR PlanetS center has developed new models that question the traditional classification of Uranus and Neptune as “ice giants”: inside them, the scholars explain, there could be much more rock and much less water than has always been thought.
The study, published on Astronomy & Astrophysicsdoes not claim to define the composition of the two planets with certainty, but shows that it is not at all mandatory to imagine them as worlds dominated by ice. A conclusion that also agrees with what we know about other distant objects, such as Pluto, which is surprisingly rich in rocky material.
Planets yet to be understood. Two extreme and still little-known planets Uranus and Neptune are the furthest from the Sun: Uranus orbits about 2.9 billion km from Earth (on average). It has a diameter of approximately 50,700 km, while Neptune is located even further away, 4.3 billion km from our planet, and has a diameter of approximately 49,200 km.
Both are traditionally labeled “ice giants”, in contrast to the gas giants Jupiter and Saturn. But this distinction, according to the Swiss researchers, would be too simplistic.
“The classification of ice giants is overly simplified,” explains Luca Morf, doctoral student at the University of Zurich and first author of the study. “Uranus and Neptune remain very little understood planets.”
A new model. So far, researchers have used two different approaches: physical models, which are accurate but lead to too many questions to answer; empirical models, simple but not complete and too distant from reality. The new method developed by the Swiss team combines the strengths of both for the first time, creating an “agnostic” model, i.e. without preconceptions about the internal composition.
How does it work? The researchers started from a random internal density, without assuming that there is ice. Then they calculated the gravitational field compatible with the observational data. From this information they derived a possible composition. The entire process, however, was repeated thousands of times until the model coincided best with the real measurements.
The result? A whole new range of possibilities. «It’s an idea that we had already proposed 15 years ago», says Ravit Helled, professor at the University of Zurich and coordinator of the project.
«Now we finally have the numerical tools to demonstrate that Uranus and Neptune could be rich in water… but also in rocks». The researchers, however, did not go so far as to define the possible internal structure with the diameters of the possible rocky and/or icy levels, because the parameters are still too labile.
“Crazy” magnetic fields. One of the most enigmatic characteristics of the two planets is the magnetic field, completely different from that of Earth. On Earth the magnetic poles are two and well defined; on Uranus and Neptune, however, the internal dynamo produces irregular, inclined fields with multiple poles.
The new models suggest the presence of layers of “ionic water” – water compressed at extreme pressures and temperatures, capable of conducting electricity – that would generate magnetic dynamos in unusual areas. Furthermore, according to the researchers, Uranus’ magnetic field originates deeper than that of Neptune.
new missions. Although the results are promising, there remain margins of uncertainty. “We still don’t know well the behavior of materials in the extreme conditions of the heart of a planet,” explains Morf. “This could influence the results, and for this we will have to improve the models.”
One thing, however, is clear: the observations available today are not sufficient to say whether Uranus and Neptune are truly ice giants or, in part, rocky giants. To find out, new probes would be needed, as already happened for Jupiter with the Juno mission or for Saturn with the Cassini mission and future dedicated missions. “We need missions to Uranus and Neptune that can reveal their true nature,” concludes Helled.
