Planets with Water Aren’t Always Earth-Like

Planets with water are the main targets for scientists’ search for life on other worlds. Find out how a new study shows that these planets can be very different from Earth.

In my student days, I spent several summers working in construction to pay my way through university. It was hard, gritty work, but it paid well for a young man with no credentials or experience.

Physical labour outdoors in the hot sun can work up an insatiable thirst. The big water jugs we all brought for ourselves often ran out by the end of the day , so the company always provided big tanks of water for the workers as well.

We can’t live without water and apparently neither can any other living things on Earth. Everywhere we look, if there’s water, there’s life.

We Can Always Find Living Things Thriving in Water

Even if the water is boiling hot or solid ice, we can always find living things thriving in it. On the other hand, if there’s virtually no water at all, the habitat is practically lifeless.

That’s why scientists spend so much time, effort and money looking for earth-like planets with water. Their working assumption is that water on rocky planets like ours is a prerequisite for life on other worlds.

Scientists can tell if they’ve found planets with water in two ways. The first technique they came up with involved watching the planet as it passed in front of its sun.

analyze How Planets’ Gravity Affects Their Stars

More recently, astronomers have found ways to analyze planets based on how their gravity affects the way their star moves around. Then, they can use a spectroscope on the infrared light to identify the presence of water.

However, since we haven’t found life on any other worlds yet, this is all just an educated guess. How sure are we that looking for earth-like planets with water is the best way to look for extraterrestrial life?

A new study published in the journal Nature Astronomy challenges the assumption that water is most common on planets like Earth. They point out that water seems to be abundant throughout the Universe and that it exists on worlds very different from our own.

We Believe Life on Earth First Arose in the Oceans

Since we believe life on Earth first arose in the oceans, it seems safe to assume that we should be looking for water out there. However, the researchers from the University of Bern, the University of Zurich and the National Centre of Competence in Research (NCCR) PlanetS have learned that water can exist for billions of years on planets that aren’t remotely like our own.

Ravit Helled is a professor of theoretical astrophysics at the University of Zurich and a co-author of the study. He explained, “One of the reasons that water can be liquid on Earth is its atmosphere. With its natural greenhouse effect, it traps just the right amount of heat to create the right conditions for oceans, rivers and rain.”

Even so, our atmosphere was very different when the Earth was young. Instead of nitrogen and oxygen, its primordial atmosphere consisted mainly of the simpler elements hydrogen and helium.

More Massive Planets Retain Primordial Atmospheres

Our atmosphere evolved over time, but that doesn’t necessarily happen with more massive planets. It doesn’t even mean that the greenhouse effect depends on the gases in our modern atmosphere.

 “Such massive primordial atmospheres can also induce a greenhouse effect – much like Earth’s atmosphere today,” Professor Helled explained. “We therefore wanted to find out if these atmospheres can help to create the necessary conditions for liquid water.”

Using computer models, the research team simulated the development of a wide range of exoplanets over billions of years. They considered atmospheric conditions on these planets, the radiation from the stars they orbited and the heat generated inside the planets themselves.

“The Right Conditions for Liquid Water Can Occur”

The lead author of the study is Marit Mol Lous, a PhD student at the University of Bern and the University of Zurich. He explained that, “What we found is that in many cases, primordial atmospheres were lost due to intense radiation from stars, especially on planets that are close to their star. But in the cases where the atmospheres remain, the right conditions for liquid water can occur.”

Geothermal heat isn’t strong enough to contribute to our planet’s greenhouse effect. However, Mol Lous explains that, “in cases where sufficient geothermal heat reaches the surface, radiation from a star like the Sun is not even necessary so that conditions prevail at the surface that allow the existence of liquid water.”

Not only can water exist on those planets. It can also persist there for billions of years.

“To Many, This May Come as a Surprise”

Co-author Christoph Mordasini added that, “To many, this may come as a surprise. Astronomers typically expect liquid water to occur in regions around stars that receive just the right amount of radiation: not too much, so that the water does not evaporate, and not too little, so that it does not all freeze.”

The team’s discovery challenges the conventional idea of a “goldilocks zone,” a region in space whose distance from a star is “just right” for water to form. It now appears that planets with water might exist outside this zone or that there could be free-floating rogue planets with water that don’t orbit a star at all.

The unknown factor in this new model is the volume of these planets’ atmospheres, which is essential for planets with water to last for billions of years. Scientists don’t know how common the correct atmospheric volume is on exoplanets.

Culture View Water as the Sacred Symbol of Life

It seems reasonable to take it on faith that water drives life, at least until scientists find otherwise. Cultures throughout the world view water as the sacred symbol of life.

Finding out if life exists on other worlds is vital to the new, science-based story we need to explain the Universe of which we’re a part and our place in it. Finding planets with water is an important step in that direction, but there are no guarantees.

Professor Mordasini concluded with the comment that, “”Even under the right conditions, it is unclear how likely it is for life to emerge in such an exotic potential habitat. That is a question for astrobiologists. Still, with our work we showed that our Earth-centred idea of a life-friendly planet might be too narrow.”

We always have more to learn if we dare to know.
Learn more:
Long-term liquid water also on non-Earth-like planets?
Potential long-term habitable conditions on planets with primordial H–He atmospheres
Astrobiology: 3 Questions We Need to Answer
Planets Without Suns: Can They Sustain Life?
Planetary Formation in the News



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