Habitable Planets Will Most Likely be Cold, Dry “Pale Yellow Dots”

Remember all the habitable planets we’ve seen in science fiction movies? There’s wintry Hoth, for example, and overwhelmingly hot Dune. The folks in Interstellar visited an ocean world and a desolate rocky world. For all their differences, these places were still what they call on Star Trek M-class habitable worlds. Sure they weren’t all like Earth, but that made them excitingly alien for the lifeforms they did support. In the real universe, it seems that alien worlds not quite like ours could be the norm. Earth could be the real alien world.

According to a pair of researchers in Europe, pale blue dots like our planet probably aren’t all that common. Instead, many habitable planets could be colder and dryer than ours. Moreover, because they might not have as much water, these places could 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 planets have approximately an 80 percent probability of being mostly covered by land. That is, they’d have mostly continental landscapes. Another 20 percent of possibly habitable worlds would likely be mainly oceanic. A tiny percentage (less than one percent) would be similar to Earth’s land-water distribution.

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.

“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 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 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. That’s where they get access to the friendly Sun to do photosynthesis. That process allows them to transmit energy and nutrients up the food chain. Life also thrives in the oceans and they provide a huge amount of water that enhances rainfall. The oceanic water resources prevent Earth’s present 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. “That drives geologic activity, such as earthquakes, volcanoes, and mountain building, and results in the growth of continents,” Spohn said. “The land’s erosion 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 present-day Earth may be an exceptional planet and that the equilibrium of landmass may be unstable over billions of years. While all the planets modeled could be considered habitable, their fauna and flora may be quite different.”

Not all Life-bearing Planets are Alike

The good news here is that the landmass-to-ocean ratios allow for a pretty wide definition of “habitable”. An ocean world, with less than 10 percent land, for example, could turn out to be a moist, warm planet. It might be similar to Earth after it recovered from the impact that helped kill off the dinosaurs. This makes sense since the models Spohn and Höning worked on show that average surface temperatures on these worlds would be more like Earth’s. Such a world could be teeming with life forms.

The planets with less than 30 percent oceans would have colder temps and dryer climates. They might have cool deserts and possibly some ice sheets. 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 in its history. For example, there could be worlds with conditions similar to those our planet endured during the Ice Ages. Life did flourish here during those times, and such a world would be quite habitable. Interestingly, people who lived on our planet during that time 10,000 years ago would find these places comfortable and familiar.

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

For More Information

Earth-like exoplanets 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 Sep 2022, EPSC2022-506, 2022.

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