Uranus and Neptune are the two furthest planets in the Solar System and have been visited only once by human spacecraft \u2013 by Voyager 2 over 30 years ago \u2013 so there is a lot about them that we do not know. One thing we thought we knew, however, was what type of planet they are. Now, a new study wants to challenge something quite crucial about how we classify them: these worlds, it argues, are not ice giants.<\/p>\n
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The four “rocky” planets of the Solar System \u2013 Earth, Mars, Venus, and Mercury \u2013 are small terrestrial planets made of solid rock and metal. The four giant planets \u2013 Jupiter, Saturn, Uranus, and Neptune \u2013 are divided into two categories because, though large, they are not the same. The first two are “gas giants” because they are overwhelmingly composed of hydrogen and helium, over 90 percent by mass. The latter two are known as “ice giants<\/a>“. Hydrogen and helium comprise less than 20 percent of the planets’ masses. Uranus and Neptune, instead, are rich in numerous molecules like water and ammonia that were present in solid ice when the planets formed billions of years ago, as models tell us.<\/p>\n For decades, our understanding of Uranus and Neptune’s interiors has been based on what we can glean from their surface features, the behaviors of their moons, their magnetic fields, and other indirect means. Which has occasionally led us astray<\/a>.\u00a0<\/p>\n If I was one of these billionaires\u2026 just floating around with all my money, I would fund two missions: I’d fund an orbiter to Uranus and an orbiter to Neptune.<\/p>\n Prof Brian Cox<\/p>\n This new work suggests a different way of looking at these outer planets. Instead of modeling the interior of these two worlds based on the potentially flawed information we have, they created random models and then compared those to observational data, building a catalog of models that fit. They looked at both water-dominated and rock-dominated scenarios and concluded that while the planets have this mixture of molecules, a rockier internal structure makes more sense with the current observations.\u00a0<\/p>\n \u201cOverall, our findings challenge the conventional classification of Uranus and Neptune as \u2019ice giants\u2019 and underscore the need for improved observational data or formation constraints to break compositional degeneracy,\u201d the authors write in their paper.<\/p>\n So maybe Uranus and Neptune are not ice giants; perhaps we have the Solar System’s first “rocky giants”. Or another suitable name that doesn\u2019t get them confused with the rocky planets of the Solar System. With a better understanding of what goes on inside these planets, we can explain their peculiarities, such as their magnetic fields, which in the case of Uranus is truly bizarre<\/a>.<\/p>\n While the new paper is intriguing, the authors stress the need for dedicated missions<\/a> to Uranus and Neptune to constrain their properties better and even get more accurate color pictures<\/a>.\u00a0<\/p>\n They’re not the only ones who want to revisit these outer worlds. “Really, the case for a big mission, an orbiter to both Uranus and Neptune, I think, is so overwhelming! If I was one of these billionaires\u2026 just floating around with all my money, I would fund two missions: I’d fund an orbiter to Uranus and an orbiter to Neptune,” Professor Brian Cox told IFLScience<\/a> last year.<\/p>\n \u201cWith the potential for future dedicated missions to Uranus and Neptune, our method also provides a flexible and unbiased tool for interpreting forthcoming data,” the authors write. “Ultimately, the interiors of Uranus and Neptune remain enigmatic, not because they are beyond reach, but because the data required to resolve their secrets are still out of grasp. Until then, only a plurality of models, not a singular one, can capture the full extent of possibilities for their hidden depths.”<\/p>\n