Artist’s image of the exoplanet LHS 1140b, with its parent star, a red dwarf, in the background of the image
What is it that doesn’t have days (at least, as we know them), doesn’t rotate, is both intensely hot and cold, and both dark and luminous? If you didn’t get it right, we tell you: it’s an “eyeball” planet.
These static hypothetical worlds got their name because they are planets that, seen from a distance, could resemble a gigantic eye in space. The idea may seem like something out of a science fiction film, but it results from a well-documented astronomical phenomenon: the tidal lock.
The phenomenon occurs when a celestial body rotates on itself at the same rate as it orbits another body, explains . In this situation, always keeps the same face facing the object it orbits. This is what happens with the Moon in relation to the Earth, which is why, from our planet, we always see the same lunar side.
In the case of some exoplanets, scientists consider it plausible that they are tidally locked to their stars. This would mean that one side would be permanently exposed to starlight, while the other would remain in a eternal night.
This extreme difference in conditions could create a world with very contrasting appearances between the day side and the night side. In one of the scenarios, known as “hot eye”, the face facing the star would be dry, arid, with the water evaporated or eliminated by stellar radiation. On the other side, in the darkness, there could be a huge ice cap.
But the most curious thing is what is (or could be) between these two equally frightening faces. Between these two extreme regions a band of permanent twilight, where day meets night and where melting ice could make liquid water available.
According to a 2013 study in Astrobiology, this intermediate zone could, in theory, bring together habitability conditionseventually allowing the development of vegetation.
Another possibility is that of an “icy eye”, located further away from the heat of its star. In this case, the night side would still be covered in ice, but the illuminated side could contain a liquid ocean rather than a dry surface. This ocean, similar to the seas on Earth, could also be potentially habitable.
Astronomer Sean Raymond points out that these two hypothetical scenarios represent extreme cases. Any tidally locked planet could have large differences between its two sides, whether due to the distribution of clouds, the freezing of water on the dark side, or thawing on the light side.
Of course, When we say “possibly habitable”, we don’t actually mean habitable for humans. NASA defines the “habitable zone” as a region where liquid water could exist on the surface, and many tidally locked planets around red dwarfs may be in this zone. These “blocked” planets may be habitable for simple life, but for humans the bar is much higher. To live there, we might need closed habitats, radiation protection, oxygen production, pressure control and atmospheric filtration.
The classic problem for these planets is atmospheric collapse: the night side can become so cold that gases such as CO₂ or even other atmospheric components can freeze or condense.
So far, NASA is “keeping its eye” on a major candidate, , about 49 light years away. Observations by James Webb and recent models suggest that this “eyeball” could be a water-rich world, perhaps with a nitrogen-dominated atmosphere and possibly a liquid ocean on the side facing the star.
Tomás Guimarães, ZAP //
