Habitable planets will most likely be cold and dry “pale yellow dots”

Remember all the habitable planets we’ve seen in science fiction movies? There’s the winter Hoth, for example, and the overwhelmingly hot Dune. People inside Interstellar visited an ocean world and a desolate rocky world. Despite all their differences, these places were still what they call Star Trek Habitable M-class worlds. Of course they weren’t all like Earth, but that made them excitingly alien to the life forms they supported. In the real universe, it appears that alien worlds not quite like ours could be the norm. Earth could be the real alien world.

According to a couple of researchers in Europe, blue spots like our planet are probably not all that common. Instead, many habitable planets may be colder and drier than ours. Also, as they may not have that much water, these places may look more like pale yellow spots.

Planetary scientists Tilman Spohn and Dennis Höning modeled possible exoplanets to see how the evolution of continents and planetary water cycles could shape the development of habitable worlds. They concluded that the planets are about 80% likely to be mostly covered by the earth. That is, they would have mostly continental landscapes. Another 20 percent of possibly habitable worlds would likely be primarily oceanic. A small percentage (less than one percent) would be similar to the Earth’s land-water distribution.

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I pianeti abitabili di tipo terrestre possono evolversi in tre scenari di distribuzione terra/oceano: coperti da terre, oceani o un mix uguale di entrambi.  Il pianeta coperto di terra è lo scenario più probabile (circa l'80%), mentre la nostra Terra
Habitable terrestrial-type planets can evolve in three land / ocean distribution scenarios: covered by land, ocean, or an equal mix of both. The planet covered with earth is the most likely scenario (around 80%), while our Earth “equal mix” (<1% chance) is even more unique than previously thought. Modeling shows that the probabilities of three types of very different looking terrestrial planets vary widely and affect their climate and habitability. Credit: Europlanet 2024 RI / T. Ruggero.

“We earthlings enjoy the balance between land areas and oceans on our home planet,” said Spohn, who is executive director of the International Space Science Institute in Bern, Switzerland. “It is tempting to assume that a second Earth would be just like ours, but our modeling results suggest that this is likely not the case.”

Differences in habitable planets

So why would habitable planets be so different from Carl Sagan’s “pale blue dot”? The “look and feel” of each exoplanet depends on various characteristics. These range from their structure to the star on which they orbit. On Earth, the growth of continents by volcanic activity and their erosion by atmospheric agents is quite well balanced. Life thrives here. Many plants, for example, grow well on land. That’s where they get access to the friendly Sun to do photosynthesis. This process allows them to transmit energy and nutrients along the food chain. Life also thrives in the oceans and they provide a huge amount of water which improves rainfall. Ocean water resources prevent Earth’s current climate from becoming too dry.

Geology also plays an important role, according to the researchers. The main driver of Earth’s plate tectonics is internal heat. “This drives geological activity, such as earthquakes, volcanoes and mountain construction, and results in the growth of continents,” Spohn said. “The erosion of the territory is part of a series of cycles that exchange water between the atmosphere and the interior. Our numerical models of how these cycles interact show that today’s Earth could be an exceptional planet and that the balance of the continental mass could be unstable for billions of years. While all the modeled planets could be considered habitable, their fauna and flora could be very different ”.

Not all life-bearing planets are the same

The good news here is that continental mass-ocean ratios allow for a rather broad definition of “habitable”. An ocean world, with less than 10 percent land, for example, could turn out to be a hot, humid planet. It may be similar to Earth after it recovered from the impact that helped kill the dinosaurs. This makes sense since the models that Spohn and Höning worked on show that the average surface temperatures on these worlds would be more similar to those of Earth. Such a world could teem with life forms.

Planets with less than 30% oceans would have colder temperatures and drier climates. They might have cold deserts and maybe some ice caps. We know from similar regions here on Earth that life can thrive in such environments.

Here’s another intriguing thought. The Earth we know today is different from how it was at various other stages of its history. For example, there may be worlds with conditions similar to what our planet experienced during the ice ages. Life flourished here in those days, and such a world would be quite habitable. Interestingly, people who lived on our planet around that time 10,000 years ago would have found these places comfortable and familiar.

The count of confirmed known exoplanets is now over 5,000. Some are habitable. Others don’t. Some are super-Earths, others are gaseous supergiants. But it’s only a matter of time before planetary scientists find a pale spot of a world. It is interesting to think that, blue or yellow, it could welcome life.

For more information

Earth-like exoplanets are unlikely to be another “pale blue dot”
Spohn, T. and Hoening, D .: Land / Ocean Surface Diversity on Earth-like (Exo) planets: Implications for Habitability, Europlanet Science Congress 2022, Granada, Spain, 18–23 September 2022, EPSC2022-506, 2022.

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