The Milky Way swarms with planets with masses between those of terrestrial Earth and gas giants such as Jupiter. Scientists expect that many of these planets contain a lot of ice. Despite their ubiquity (as well as their solar system counterparts, the ice giants Uranus and Neptune), the structure and evolution of icy worlds remain poorly understood.
Modeling the internal structures of icy worlds is difficult because of their unique mass regime and insufficient observational data. Until now, planetary scientists have resorted to a simplified layered description. In that model, an icy shell surrounds a rocky mantle, which in turn encloses an iron core. Sometimes—but not always—a gassy envelope blankets the planet.
However, at least a portion of rocks and ice in a newly formed planet is likely to be mixed. In a new study published in the Astrophysical Journal, scientists calculated how a mixed structure like that would evolve. Their modeling suggested that ice and rocks in the planetary interior stay mixed for billions of years after formation. “We cannot continue to think in terms of layers; otherwise, we get to the wrong conclusions,” said Allona Vazan, an assistant professor at the Astrophysics Research Center of the Open University of Israel who led the study.
An ice-rock mixture affects models of planetary evolution and alters interpretation of the relationship between the mass and the radius of a planet. The temperature and the chemical composition of an atmosphere are affected by the state of the mixture as well. “I think [the study] is important and a great step towards more realistic models of smaller giant planets,” said Jonathan Fortney, director of the Other Worlds Laboratory at the University of California, Santa Cruz, who was not involved in the study.
The new results predict signatures that may help identify ice-rich worlds among the growing exoplanet population. But the study also illustrates how difficult it is to understand the exotic conditions in the bellies of intermediate-mass planets.
What Is Going On in There?
Planets form from a gaseous protoplanetary disk surrounding a newborn star. According to the standard model, pebbles or larger solid objects called planetesimals (or both) first build up a core made of heavy elements. A core of sufficient mass starts accumulating gas from the surrounding disk to form an atmosphere. The newly formed planet could contain a significant fraction of ice if it forms beyond the ice line, a distance at which it is cold enough for compounds like water to condense into ice grains.
Models suggest that ices and rocks in newborn planets are mixed to begin with, but it is unclear to what extent. Even more perplexing: No one knows how the mixture behaves. “The concept of ice is misleading” to begin with, said Ravit Helled, an associate professor at the University of Zurich in Switzerland who was not involved in the study. The water ice inside a planet is very different from the ice that people are familiar with at home, Helled explained. Despite high temperatures inside a planet, high pressures force the water into a crystalline form that moves, mixes, and melts in ways that scientists are still trying to fully understand. “It is water at high pressure and temperature, and it can be very different from ice that we imagine in our head.”
Recent laboratory experiments looked at how ice and rock mix under high pressures (about 100,000 times the atmospheric pressure at Earth’s surface), but those pressures are still much lower than the conditions deep inside an intermediate-mass planet. Both Vazan and Helled hope that the new study will spur additional research into how mixtures of different rocks and ices behave at extremely high pressures.
More detailed scrutiny of possibly ice-rich Neptune and Uranus is critical for the field. The two outer planets have been visited by only one probe—NASA’s Voyager 2 spacecraft flew by in 1986 and 1989, respectively. The lack of in situ observations reflects a poor understanding of these two planets and ice-rich planets in general.
“We have Uranus and Neptune, and we don’t even know what is the rock-to-water ratio in these planets,” said Helled.
—Jure Japelj (@JureJapelj), Science Writer