Unlocking the Secrets of Exoplanets: Water May Be Hidden in Their Iron Cores

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Unlocking the Secrets of Exoplanets: Water May Be Hidden in Their Iron Cores

Are exoplanets, those distant worlds orbiting other stars, more watery than we thought? A recent study led by Caroline Dorn, a professor of exoplanets at ETH Zurich in Switzerland, suggests that up to 95% of an exoplanet’s water could be trapped deep inside its iron core, making them potentially even more habitable than previously imagined. The findings, published in the journal Nature Astronomy, challenge the conventional view that most of the water on exoplanets is located on their surfaces or in their atmospheres.

To understand how water can be sequestered in an exoplanet’s core, we need to delve into its formation and evolution. Exoplanets are born from the same protoplanetary disks as their parent stars, where dust and gas particles collide and stick together, forming larger and larger bodies. Once these bodies reach a critical mass, they can attract more material through gravity and become planets. However, the process is not smooth and uniform, and various factors can affect the composition and structure of the planets.

One of these factors is the distance from the star, or the planet’s orbital location. Exoplanets can be classified into different types based on their size, density, and temperature, among other parameters. For example, there are gas giants like Jupiter, rocky planets like Earth, and mini-Neptunes that fall in between. The closer a planet is to its star, the hotter it gets, and the more likely it is to lose its volatile components, such as water, to the intense radiation and winds from the star. Therefore, exoplanets that are farther away from their stars, in the so-called habitable zone, are considered more favorable for hosting liquid water and potentially life.

However, the new study suggests that even exoplanets that are not in the habitable zone could have significant amounts of water, albeit in a different form. According to Dorn and her colleagues, when a planet forms, it can accumulate water in its mantle, the layer between the core and the crust, through various mechanisms, such as impacts from comets or asteroids, or outgassing from the interior. This water can then migrate towards the core, where it can react with the iron and other elements to form minerals that can store the water. The process is similar to how water is stored in the Earth’s mantle, where it is estimated to be several times more abundant than in the oceans.

The researchers used computer simulations to model the fate of water in exoplanets of different sizes and compositions, assuming different scenarios of water delivery and retention. They found that for planets with masses between 2 and 20 times that of the Earth, which are common in the galaxy, up to 95% of their water could be hidden in the core. This means that even if a planet appears dry on the surface, it could still have a significant amount of water inside, which could affect its geology, chemistry, and habitability.

The implications of the study are twofold. First, they suggest that exoplanets could be more diverse and complex than we thought, with a wide range of water contents and distributions. This could have implications for the search for life beyond the Solar System, as water is considered a key ingredient for life as we know it. Second, they challenge the traditional view that the habitable zone is the only criterion for assessing a planet’s potential habitability. If a planet can store water in its core, it could maintain a stable climate and surface conditions over long periods, even if it is not in the habitable zone or if its star is unstable or variable.

Of course, the study has some limitations and uncertainties. For example, the simulations assume simplified models of planet formation and water transport, and do not account for the effects of plate tectonics, magnetic fields, or other factors that can affect the water cycle. Moreover, the study does not provide direct evidence of water in exoplanet cores, but rather suggests a plausible mechanism for its existence. Therefore, more observations and measurements are needed to confirm or refute the hypothesis.

Nonetheless, the study opens up new avenues for exploring the diversity and habitability of exoplanets, and highlights the importance of interdisciplinary research and collaboration in this field. As Dorn notes, “We need to combine the expertise of astronomers, geophysicists, geochemists, and planetary scientists to fully understand the nature and evolution of exoplanets, and to assess their potential for hosting life.” With the growing number of exoplanets discovered and characterized in the coming years, we may be able to unlock more secrets of these fascinating worlds and our place in the universe.