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

Remember all those habitable planets we saw in science fiction movies? There is, for example, the wintry Hoth and the overwhelmingly hot dune. the people inside Interstellar visited a sea world and a desolate rock world. For all their differences, these places were still what they sought out star trek M-class habitable worlds. Sure, they weren’t all like Earth, but that made them excitingly alien to the life forms they supported. In the real universe, alien worlds not quite like ours seem to be the norm. Earth could be that real strange world.

Pale blue dots like our planet are unlikely to be all that common, according to two researchers in Europe. Instead, many habitable planets could be colder and drier than ours. Also, since they may not have as much water, these locations may 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 might affect the evolution of habitable worlds. They concluded that there is about an 80 percent chance that planets are mostly covered by land. That means they would have mostly continental landscapes. Another 20 percent of potentially habitable worlds would likely be mostly oceanic. A tiny percentage (less than one percent) would resemble Earth’s land-water distribution.

“We Earthlings enjoy the balance between land and ocean on our home planet,” said Spohn, executive director of the International Space Science Institute in Bern, Switzerland. “It’s tempting to assume that a second Earth would be just like ours, but our modeling results suggest that’s probably not the case.”

Differences in habitable planets

So why should habitable planets be so different from Carl Sagan’s “pale blue dot”? The “look and feel” of each exoplanet depends on various properties. These range from their structure to the star they orbit. On Earth, the growth of continents from volcanic activity and their erosion from weathering is fairly balanced. Life thrives here. For example, many plants do well on land. There they gain access to the friendly sun to carry out photosynthesis. This process allows them to transfer energy and nutrients up the food chain. Life also thrives in the oceans and they provide a vast amount of water that increases precipitation. Oceanic water resources prevent Earth’s current climate from becoming too dry.

According to the researchers, geology also plays an important role. The main driver of Earth’s plate tectonics is internal heat. “This drives geological activities such as earthquakes, volcanoes and mountain building and leads to the growth of continents,” said Spohn. “Land 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 Earth today may be an extraordinary planet and that the balance of the landmass may be unstable for billions of years. While all of the modeled planets could be considered habitable, their fauna and flora could be quite different.”

Not all life-bearing planets are the same

The good news here is that the landmass-to-ocean ratio allows for a fairly broad definition of “habitable.” For example, a marine world with less than 10 percent land might turn out to be a humid, warm planet. It could be similar to Earth after recovering from the impact that helped kill the dinosaurs. This makes sense because the models that Spohn and Höning worked on show that the average surface temperatures on these worlds would be closer to Earth’s. Such a world could teem with life forms.

The planets with less than 30 percent oceans would have colder temperatures and drier climates. You could 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 than at various other times in its history. For example, there could be worlds with conditions similar to our planet during the ice ages. Life thrived here in those times, and such a world would be perfectly livable. Interestingly, people living on our planet at that time 10,000 years ago found 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 gas supergiants. But it’s only a matter of time before planetary explorers find a pale spot on a world. It’s interesting to imagine if it’s blue or yellow, it could welcome life.

For more informations

Earth-like exoplanets probably aren’t 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.