Enlarge / High pressure ices near the crust are a feature of water-rich worlds.` (credit: Benoit Gougeon (University of Montreal))

The possibility that there is liquid water on an exoplanet’s surface usually flags it as “potentially habitable,” but the reality is that too much water might prevent life from taking hold.

“On Earth, the ocean is in contact with some rock. If we have too much water, it creates high-pressure ice underneath the ocean, which separates it from the planet’s rocky interior,” said Caroline Dorn, a geophysicist at ETH Zurich, Switzerland, who led new research in exoplanet interiors.

This high-pressure ice prevents minerals and chemical compounds from being exchanged between the rocks and the water. In theory, that should make the ocean barren and lifeless. But Dorn’s team argues that even exoplanets that have enough water to form such high-pressure ice can host life if the majority of the water is not stored in the surface oceans but is held much deeper in the planet’s core. The water in the core can’t sustain life—it’s not even in its molecular form there. But it means that a substantial fraction of a planet’s water isn’t on the surface, which makes the surface oceans a little more shallow and prevents high-pressure ice from forming at their bottom.

Enlarge / Image of Epsilon Indi A at two wavelengths, with the position of its host star indicated by an asterisk. (credit: T. Müller (MPIA/HdA), E. Matthews (MPIA))

We have a couple of techniques that allow us to infer the presence of an exoplanet based on its effects on the light coming from its host star. But there's an alternative approach that sometimes works: image them directly. It's much more limited, since the planet has to be pretty big and orbiting far away enough from its star to avoid having light coming from the planet swamped by the far more intense starlight.

Still, it has been done. Massive exoplanets have been captured relatively shortly after their formation, when the heat generated by the collapse of material into the planet causes them to glow in the infrared. But the Webb telescope is far more sensitive than any infrared observatory we've ever built, and it has managed to image a relatively nearby exoplanet that's roughly as old as the ones in our Solar System.

Looking directly at a planet

What do you need to directly image a planet that's orbiting a star light-years away? The first thing is a bit of hardware called a coronagraph attached to your telescope. This is responsible for blocking the light from the star the planet is orbiting; without it, that light will swamp any other sources in the exosolar system. Even with a good coronagraph, you need the planets to be orbiting at a significant distance from the star so that they're cleanly separated from the signal being blocked by the coronagraph.

Enlarge / Renditions of a possible composition of LHS 1140 b, with a patch of ocean on the side facing its host star. Earth is included at right for scale. (credit: BENOIT GOUGEON, UNIVERSITÉ DE MONTRÉAL)

Of all the potential super-Earths—terrestrial exoplanets more massive than Earth—out there, an exoplanet orbiting a star only 40 light-years away from us in the constellation Cetus might be the most similar to have been found so far.

Exoplanet LHS 1140 b was assumed to be a mini-Neptune when it was first discovered by NASA’s James Webb Space Telescope toward the end of 2023. After analyzing data from those observations, a team of researchers, led by astronomer Charles Cadieux, of Université de Montréal, suggest that LHS 1140 b is more likely to be a super-Earth.

If this planet is an alternate version of our own, its relative proximity to its cool red dwarf star means it would most likely be a gargantuan snowball or a mostly frozen body with a substellar (region closest to its star) ocean that makes it look like a cosmic eyeball. It is now thought to be the exoplanet with the best chance for liquid water on its surface, and so might even be habitable.