Habitable planets will most likely be cold, dry "pale yellow dots"

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

Do you remember all the habitable planets we’ve seen in sci-fi movies? There’s wintery Hoth, for example, and extremely hot Dune. The people in Interstellar visited an ocean world and a desolate rocky world. Despite all their differences, these places were still what they call star trek M-class habitable worlds. Sure, they weren’t all like Earth, but that made them extremely alien to the lifeforms they supported. In the real universe, it seems alien worlds not quite like ours might be the norm. Earth could be the real alien world.

Pale blue dots like our planet probably aren’t that common, according to a pair of researchers in Europe. Instead, many habitable planets could be colder and drier than ours. Also, since they may not have as much water, these spots might look more like pale yellow dots.

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 have about an 80% chance of being mostly covered by land. That is, they would have mainly continental landscapes. Another 20% of potentially habitable worlds would likely be primarily oceanic. A tiny percentage (less than one percent) would be similar to Earth’s land-water split.

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Terrestrial-type habitable planets can evolve in three scenarios of land/ocean distribution: covered by lands, oceans, or an equal mix of both. The land-covered planet is the most probable scenario ( around 80%), while our “equal mix” Earth (<1% chance) is even more unique than previously thought. Modelling shows that the probabilities of three very-different looking types of terrestrial planets vary widely, and impact their climate and habitability. Credit: Europlanet 2024 RI/T. Roger.
Earth-like habitable planets can evolve under three land/ocean distribution scenarios: covered by land, ocean, or an equal mix of the two. The land-covered planet is the most likely scenario (approximately 80%), while our “equally mixed” Earth (<1% chance) is even more unique than previously thought. Modeling shows that the probabilities of three very different types of terrestrial planets vary widely and impact their climate and habitability. Credit: Europlanet 2024 RI/T. Roger.

“We Earthlings appreciate the balance between land areas and oceans on our home planet,” said Spohn, executive director of the International Institute of Space Science in Bern, Switzerland. “It’s tempting to assume that a second Earth would be like ours, but our modeling results suggest that’s not likely to be 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 around which they orbit. On Earth, the growth of continents by volcanic activity and their erosion by weathering is fairly well balanced. Life thrives here. Many plants, for example, do well on land. This is where they have access to the friendly sun to do photosynthesis. This process allows them to transmit energy and nutrients to the food chain. Life also thrives in the oceans and they provide a huge amount of water which enhances rainfall. Ocean water resources keep Earth’s current climate from becoming too dry.

Geology also plays an important role, according to the researchers. The primary driver of Earth’s plate tectonics is internal heat. “It stimulates geological activity, such as earthquakes, volcanoes and mountain building, and drives the growth of continents,” Spohn said. “Soil erosion is part of a series of cycles of water exchange between the atmosphere and the interior. Our numerical models of how these cycles interact show that present-day Earth may be an exceptional planet, and that the balance of landmass may be unstable for billions of years. While all of the planets modeled could be considered habitable, their flora and fauna can be quite different.

Not all life-bearing planets are alike

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

Planets with less than 30% ocean would have colder temperatures and drier climates. They might have cool deserts and maybe 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 what it was at various other stages in its history. For example, there could be worlds with conditions similar to those our planet endured during the Ice Ages. Life flourished here at that time, and such a world would be quite habitable. Interestingly, people who lived on our planet at that time 10,000 years ago would find these places comfortable and familiar.

The number of known confirmed exoplanets is now over 5,000. Some are habitable. Others are not. Some are super-Earths, others are gaseous supergiants. But it’s only a matter of time before planetary scientists discover a faint speck of a world. It is interesting to think that whether it is blue or yellow, it can be welcoming to life.

For more information

Earth-like exoplanets likely won’t be another ‘pale blue dot’
Spohn, T. and Hoening, D.: Land/ocean surface diversity on Earth-like (exo)planets: Implications for habitability, Europlanet 2022 Scientific Congress, Granada, Spain, 18-23 September 2022, EPSC2022-506 , 2022.

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