The number of confirmed exoplanets has exploded in the past decade. Today, NASA has an inventory of over 6,000 planets beyond our solar system, each classified by their size and density. The range is vast\u2014from gas giants larger than Jupiter to rocky terrestrial planets smaller than Earth. The most abundant type are planets sitting somewhere between Earth and Neptune size, called sub-Neptunes. There is nothing comparable in our solar system.<\/p>\n
\u201cWe don\u2019t exactly know the real nature of sub-Neptunes, but we can model their composition by knowing their density,\u201d Shim says. Astronomers have done this for years. Using gravitational observations, they can calculate a planet\u2019s density and then use that value to extrapolate its composition based on the most common elements in the universe.<\/p>\n
Two general types of sub-Neptunes have emerged from these models: dry, hydrogen-rich worlds and wet, water-rich worlds, Shim says. The dry worlds can \u201chave a thick atmosphere of hydrogen, and the interior is essentially magma,\u201d he says. Theoretically, the gas should react with the molten rock. As someone with a background in chemistry, Shim wanted to know how that reaction would actually proceed.<\/p>\n
The difficulty with these types of experiments, Shim says, is getting reaction materials, specifically hydrogen gas, to a temperature and pressure similar to those on the surface of a sub-Neptune exoplanet. During the team\u2019s initial attempts to compress hydrogen gas, crushed olivine\u2014a silicate mineral\u2014and iron between diamond anvils, \u201cwe realized that hydrogen can diffuse into diamond anvils, and then it breaks the anvil material,\u201d Shim says.<\/p>\n
After a few years of development (and a few broken diamonds), the group successfully combined the diamond anvils with a high-powered pulsed laser to achieve pressures of up to 40 GPa and temperatures of above 3,000 K. Under these conditions, the hydrogen and silicates melt together. The researchers used X-ray diffraction and Raman spectroscopy to characterize the materials in the resulting reaction product. From these data, they were able to determine the specific redox reactions occurring between the olivine, iron, and hydrogen. Finally, using mass balance calculations, they quantified the amount of liquid water produced.<\/p>\n
\n A gray-scale scanning electron microscope image that has a light-colored blob of material within a cavity of small crystals.<\/p>\n
Using scanning electron microscopy, researchers in Dan Shim\u2019s group identified iron-rich alloys (center, white) that formed from the reaction between hydrogen and silicate. Water also formed through this reaction.<\/p>\n
Credit:
\n Harrison Horn\/Arizona State University<\/p>\n
The results were a surprise. \u201cThe theoretical prediction that was made before our study underestimates water content by a factor of 2000\u20135000,\u201d Shim says. \u201cWhat the experiment shows is that, in fact, the reaction produces enough water for a water world.\u201d<\/p>\n
Shim is not the only scientist probing the geochemistry of exoplanets with high-temperature, high-pressure laboratory experiments. At Carnegie Institution for Science, planetary geochemist Anat Shahar<\/a> and her group also used diamond anvils and a laser to probe the chemistry of molten silicate glass and hydrogen gas at high temperatures and pressures. They also found evidence for water formation<\/a> (Nature 2025, DOI: 10.1038\/s41586-025-09816-z). Both researchers were surprised to see each other\u2019s work published in Nature the same week; the research was done independently.<\/p>\n Unlike Shim\u2019s study, \u201cwe start with no iron metal in our experiment,\u201d Shahar says. Instead, her team uses an iron-containing silicate glass called pyrolite. The synthetic material is an analog of Earth\u2019s primitive mantle. In an exoplanetary context, Shahar says, it\u2019s similar to what a magma ocean would likely be composed of after the planet\u2019s metallic core has formed.<\/p>\n Upon reaching the extraordinary temperatures and pressures characteristic of sub-Neptunes, Shahar\u2019s team, led by researcher Francesca Miozzi, saw indirect evidence for the formation of liquid water during the experiment: iron-rich blobs of material and a large, empty cavity that likely used to contain water. \u201cThe liquid water in our experiment comes from the reduction of the iron [oxide] that\u2019s in the silicates to then form iron metal,\u201d Shahar says.<\/p>\n When researchers in Anat Shahar\u2019s group combined molten silicate glass with hydrogen gas under high temperatures and high pressures and observed the resulting material using scanning electron microscopy, they found iron-rich blobs and empty cavities. The scientists attribute these products to the reduction of iron oxide to iron metal, which would produce the liquid water that likely filled the cavity during the experiment.<\/p>\n Credit: Shahar\u2019s team also used mass spectrometry to quantify the amount of hydrogen bound up in the final material. \u201cWe can\u2019t know exactly how the hydrogen was bonded; we don\u2019t know if it was H2, we don\u2019t know if it was OH\u201d because of limitations of the mass spectrometry technique, she says. But the results do show how the solubility of gaseous hydrogen into molten rock changes with temperature and pressure. \u201cWe found that it was mostly a function of temperature\u201d rather than pressure, Shahar adds.<\/p>\n Shahar thinks the two papers complement each other well. \u201cWhat\u2019s cool is we come to basically the same conclusion,\u201d she says. \u201cThey have direct evidence for water. We have direct evidence for hydrogen solubility, and we have indirect evidence for water,\u201d Shahar adds. \u201cIt all makes sense together.\u201d<\/p>\n The studies both suggest that a lot of water could be created through the geochemistry occurring on the surface of hot exoplanets with thick atmospheres, says planetary scientist Quentin Williams<\/a> of the University of California, Santa Cruz. He was not involved in either study but authored the Nature commentary published alongside Shim\u2019s work<\/a>.<\/p>\n \u201cThat\u2019s a big deal in terms of not only the water content but also what you\u2019re doing to the hydrogen envelope and the magma ocean as well,\u201d Williams says. Essentially, the results open a new avenue for understanding planetary evolution.<\/p>\n Scientists have assumed that wet planets collected water from their surroundings as they grew. In our own solar system, the most water-rich worlds\u2014Uranus, Neptune, and the icy moons around Jupiter and Saturn\u2014exist beyond the snow line, where temperatures drop low enough for gaseous water to freeze into ice grains and accrete onto budding planets.<\/p>\n But more observations revealed that many water-rich sub-Neptunes orbit their stars well within the snow line. Based on traditional models, such planets would need to grow in an area beyond the snow line and then migrate into their current orbits. But if an exoplanet can internally generate water, \u201cyou don\u2019t have to move [it] in from someplace where ice condensed in the solar system,\u201d Williams says. In effect, a dry world can become wet all on its own.<\/p>\n![\"A](\"https:\/\/www.europesays.com\/ie\/wp-content\/uploads\/2025\/11\/how-an-exoplanet-gets-wet---236800.jpg\")
\n A gray-scale scanning electron microscope image showing a sample that has lighter and darker regions. The light regions are labeled as iron-rich blebs, and the darker region is labeled as a residual cavity.<\/p>\n
\n Francesca Miozzi\/Carnegie Institution for Science<\/p>\n
![\"Fionna](\"https:\/\/www.europesays.com\/ie\/wp-content\/uploads\/2025\/11\/2025-fionna.jpg\")
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