Water worlds, brewed under pressure

The presence of water is the first question asked of any planet that might be habitable, and for a long time the answer was assumed to follow a single story: water arrived, frozen in the icy building blocks of the planet’s formation, and melted or released later into oceans and atmospheres. A paper in the 30 October issue of Nature reports evidence for a different route, and the route is a chemical one. Some exoplanets, the researchers find, may brew their own water during formation, through reactions between the rocks of the young planet and the hydrogen of its surroundings, under the crushing pressures of planetary interiors.

The chemistry at the centre of the work is an old idea given new evidence. Under the right conditions, the oxides in silicate rock can react with hydrogen to produce water, and a planet growing inside a hydrogen-rich environment, as planets do in the gas discs that surround young stars, is exposed to exactly that possibility. The question has been whether the conditions inside a forming planet, the temperature, the pressure, the available hydrogen, actually favour the reaction. The paper’s contribution is to show that they can, and that the process could produce significant water where the traditional icy-inheritance route predicts little.

The implications rearrange the map of wet worlds. If water can be synthesised during formation, then planets that the old model would have judged dry, rocky worlds with no inheritance of ice, could turn out to be water-rich, and the family of potentially habitable planets widens accordingly. The findings also complicate the interpretation of the exoplanets already observed: the water detected in the atmospheres of distant worlds may record not only what fell onto them but what they made themselves.

The significance extends to the earliest stages of planetary history, the period the paper studies, when a planet’s composition is set and its subsequent fate, its oceans, its atmosphere, its habitability, is largely determined. Understanding that period, and the chemistry that runs within it, is the field’s route from observation, the spectra of planets we cannot visit, to mechanism, the account of how those planets actually became what we see.

The work is a reminder of how young the science of exoplanets still is, and how much of its reasoning runs on inference. The water worlds that this research proposes were not observed directly; they were argued for, from the physics of formation and the chemistry of rock and hydrogen, and the argument, if it holds, adds a whole route to wetness that the textbooks had not included. The planet that brewed its own ocean, from its own rock, under its own weight, is a new figure in the planetary bestiary, and the paper makes a careful, quantitative case that the figure is real. The case will now be tested, as the field tests such claims, by the observations that can discriminate between the routes, the inventories of water on the worlds that form in different environments, and the models will be revised accordingly. That is how the science grows, one proposed mechanism, one discriminating observation, one revision at a time, and the paper has added its mechanism to the ledger.