Researchers at the University of Washington have found that a rocky planet similar in size to Earth would need at least 20% to 50% of the water in Earth's oceans to maintain a stable carbon cycle and moderate temperatures over time.
The search for extraterrestrial life typically focuses on planets in the habitable zone around their star, where temperatures could allow liquid water to exist on the surface. But a new study from the University of Washington suggests that just being in the right orbit isn't enough. Even if a planet is in the habitable zone, it may be too dry to sustain stable conditions for life over time. According to the researchers, a rocky planet similar in size to Earth would need at least 20% to 50% of the water in Earth's oceans to maintain a balanced geological carbon cycle.
The study, published in The Planetary Science Journal, examines arid planets, that is, worlds with very little water on their surface. For years, such worlds were considered possible candidates for life, as long as they were in the habitable zone. The assumption was that even a small amount of water could be sufficient, and perhaps even make it easier to maintain a stable climate because less water means less water vapor in the atmosphere. The new study challenges this assumption and shows that water is not only a raw material for life, but also part of the planet's climate-regulating mechanism. .
The key is the geological carbon cycle. On Earth, carbon dioxide is released into the atmosphere by volcanoes, among other things. It then dissolves in rainwater, reacts with rocks on the surface, washes into the oceans, sinks to the seafloor, and returns over millions of years to the depths and to the surface through plate tectonics. This cycle acts as a kind of natural thermostat: when the atmosphere warms, the weathering of rocks can remove more carbon dioxide and moderate warming.
If there is no water, the weathering process weakens.
But on a planet that’s too dry, that thermostat can break down. If there’s not enough water to make rain, the chemical weathering of rocks slows down. Volcanoes continue to spew carbon dioxide, but there’s no effective mechanism to remove it from the atmosphere. The result is a buildup of greenhouse gases, increasing warming, evaporation of the remaining water, and ultimately a transition to a hot, dry state that’s unsuitable for life. According to the arXiv study, rocky, Earth-like planets with less than 20% to 50% of the mass of Earth’s oceans fail to maintain a balanced carbon cycle over 4.5 billion years. (arXiv)
The researchers reached this conclusion using computer models of the carbon cycle on arid planets. Previous models focused mostly on colder, wetter worlds, and included evaporation due to starlight, but did not always represent desert conditions well. The study's lead researcher, Haskell White-Gianella, a doctoral student in Earth and space sciences at the University of Washington, adapted the models to drier conditions and improved estimates of evaporation and precipitation.
The study is particularly important because scientists have already confirmed more than 6,000 planets outside our solar system, and many more are expected to be discovered. Some are in the habitable zone, but most are very difficult to observe directly. That means astronomers have to sift through candidates and decide which worlds are worth investing valuable telescope time in. The new finding suggests that scientists should be more cautious about dry planets, even if they are at the right distance from their star.
Like Venus
Venus is used in the study as a nearby and troubling example. It is similar in size to Earth, was probably formed at about the same time, and may have once had more water. But today its surface is very hot and its atmosphere is dense and rich in carbon dioxide. The researchers suggest that if Venus started out with less water than Earth, because it was closer to the sun, its carbon cycle may have gotten out of balance relatively early. The buildup of carbon dioxide would have heated the surface, causing further water loss, and making Venus far from any possibility of stable surface life.
This doesn’t mean that every dry planet is completely unsuitable, but rather that its chances of sustaining life over time are smaller than previously thought. The research is also not about actually discovering life, but rather about what conditions allow a planet to remain habitable over geological timescales. This is an important distinction: a particular world might be habitable for a short time, but lose its stability before life can emerge, establish itself, and leave signs that we can detect.
The next step will be to compare the models to future observations, mainly in missions to Venus and studies of the atmospheres of planets outside the solar system.
The broad conclusion is that Earth has a more subtle advantage than it might seem at first glance. It’s not just that it’s at the right distance from the sun. It also holds enough water not only for oceans and rain, but also for a geological cycle that stabilizes climate over time. If the new study is correct, many of the worlds that seem promising at first glance may be too dry to maintain that delicate balance.
More of the topic in Hayadan:
