I wasn’t surprised to learn that the number of confirmed exoplanets had finally topped 6,000, a fact recently announced by NASA. After all, new worlds keep being added to NASA’s Exoplanet Science Institute at Caltech on a steady basis, all of them fodder for a site like this. But I have to admit to being startled by the fact that fully 8000 candidate planets are in queue. Remember that it usually takes a second detection method finding the candidate world for it to move into the confirmed ranks. That 8000 figure shows how much the velocity of discovery continues to increase.

The common theme behind much of the research is often cited as the need to find out if we are alone in the universe. Thus NASA’s Dawn Gelino, head of the agency’s Exoplanet Exploration Program (ExEP) at JPL:

“Each of the different types of planets we discover gives us information about the conditions under which planets can form and, ultimately, how common planets like Earth might be, and where we should be looking for them. If we want to find out if we’re alone in the universe, all of this knowledge is essential.”

I sometimes think, though, that the emphasis on an Earth 2.0 is over-stated. The search for other life is fascinating, but deepening our scientific knowledge of the cosmos is worthwhile even if we learn we are alone in the galaxy. What nature creates in bewildering variety merits our curiosity and deep study even on barren worlds. With ESA’s Gaia and NASA’s Roman Space Telescope in the mix, exoplanet detections will escalate dramatically. And a little further down the road is the Habitable Worlds Observatory, assuming we have the good sense to green-light the project and build it.

Bear in mind that almost all the known exoplanets are within a few thousand light years of Sol. We are truly awash in immensity. If there will ever be a complete catalog of the Milky Way’s planets, it will likely be from a Kardashev Type III civilization immensely older than ourselves. It’s fascinating to think that such a catalog might already exist somewhere. But it’s also fascinating to consider that we may be alone, which raises all kinds of questions about abiogenesis and the possible lifetime of civilizations.

Image: Scientists have found thousands of exoplanets (planets outside our solar system) throughout the galaxy. Most can be studied only indirectly, but scientists know they vary widely, as depicted in this artist’s concept, from small, rocky worlds and gas giants to water-rich planets and those as hot as stars. Credit: NASA’s Goddard Space Flight Center.

The idea of a large and growing catalog of exoplanets is the kind of thing I used to dream about as a kid reading science fiction magazines. Now we’re on the cusp of biosignature detection capabilities via the deep study of exoplanet atmospheres. Fewer than a hundred exoplanets have been directly imaged, a number that is likewise expected to rise with the help of the Roman instrument’s coronagraph. With new tools to better block out the overwhelming glare of the host star, we’ll be seeing gas giants in Jupiter-like orbits. That in itself is interesting – how many exoplanet systems have gas giants in such positions? Thus far, the Solar System pattern is rarely replicated.

The Fortunes of K2-18b

The sheer variety of planetary systems brings even more zest to this work. Consider the planet K2-18b, so recently in the news as the home of a possible global ocean, and one with prospects for life given all the parameters studied by a team at the University of Cambridge. It’s a fabulous scenario, but now we have a new study that questions whether sub-Neptunes like this are actually dominated by water. Caroline Dorn (ETH Zurich), co-author of the paper appearing in The Astrophysical Journal, believes that water on sub-Neptunes is far more limited than we have been thinking.

Here’s another demonstration of how our Solar System is so unlike what we’re finding elsewhere. Lacking a sub-Neptune among our own planets, we’re learning now that such worlds – larger than Earth but smaller than Neptune and cloaked in a thick atmosphere abundant in hydrogen and helium – are relatively common in our galaxy as, for that matter, are higher density but smaller ‘super-Earths.’ A global ocean seems to make sense if a sub-Neptune formed well beyond the snowline and brought a robust inventory of ice with it as it migrated into the warmer inner system. Indeed, the term Hycean (pronounced HY-shun) has been proposed to label a sub-Neptune planet with a deep ocean under an atmosphere rich in hydrogen.

The new paper examines the chemical coupling between the planet’s atmosphere and interior, with the authors assuming an early stage of formation in which sub-Neptunes go through a period dominated by a magma ocean. The hydrogen atmosphere helps to maintain this phase for millions of years. And the problem is that magma oceans have implications for the water content available. Using computer simulations to model silicates and metals in the magma, the team studied the chemical interactions that ensue. Most H20 water molecules are destroyed, with hydrogen and oxygen bonding into metallic compounds and disappearing deep into the planet’s interior.

From the paper:

Our results, which focus on the initial (birth) population of sub-Neptunes with magma oceans, suggest that their water mass fractions are not primarily set by the accretion of icy pebbles during formation but by chemical equilibration between the primordial atmosphere and the molten interior. None of the planets in our model, regardless of their initial H2O content, retain more than 1.5 wt% water after chemical equilibration. This excludes the high water mass fractions (10–90 wt%) invoked by Hycean-world scenarios (N. Madhusudhan et al. 2021), even for planets that initially accreted up to 30% H2O by mass. These findings are consistent with recent studies suggesting that only a small amount of water can be produced or retained endogenously in sub-Neptunes and super-Earths.

The work analyzes 19 chemical reactions and 26 components across the range of metals, silicates and gases, with the core composed of both metal and silicate phases. A computer model known as New Generation Planetary Population Synthesis (NGPPS) combines planetary formation and evolution and meshes with code developed for global thermodynamics. Thus a population of sub-Neptunes with magma oceans is generated, and consistently primordial water is destroyed by chemical interactions.

Image: This is Figure 3 from the paper. Caption: Envelope H2O mass fraction as a function of semimajor axis… The left panel shows planets that predominantly formed inside the water ice line; the right panel shows those that formed outside. Classification is based on the accreted H2O mass fraction, with a threshold set at 5% of the total planetary mass. The colorbar indicates the molar bulk C/O ratio. Planets formed inside the ice line are systematically depleted in carbon due to the lack of volatile ice accretion and exhibit higher envelope H2O mass fractions. In contrast, planets formed beyond the ice line retain lower H2O content despite higher bulk volatile abundances. Each pie chart shows the mean mass fraction of hydrogen in H2 (gas), H (metal), H2 (silicate), H2O (gas), and H2O (silicate), normalized to the total mean hydrogen inventory for each population. Only components contributing more than 5% are labeled. Planets that formed beyond the ice line store most hydrogen as H2 (gas) and H (metal), while those that formed inside the ice line retain a larger share of hydrogen in H (metal), H2 (silicate), and H2O (gas + silicate).. Credit: Werlen et al.

This is pretty stark reading if you’re fascinated with deep ocean scenarios. Here’s Dorn’s assessment:

“In the current study, we analysed how much water there is in total on these sub-Neptunes. According to the calculations, there are no distant worlds with massive layers of water where water makes up around 50 percent of the planet’s mass, as was previously thought. Hycean worlds with 10-90 percent water are therefore very unlikely.”

The paper is suggesting that we re-think what had seemed an obvious connection between planet formation beyond the snowline and water in the atmosphere. Instead, the interplay of magma ocean and atmosphere may deliver the verdict on the makeup of a planet. By this modeling, planets with atmospheres rich in water are more likely to have formed within the snowline. That leaves rocky worlds like Earth in the mix, but raises serious doubts about the viability of water in the sub-Neptune environment.

From the paper:

Counterintuitively, the planets with the most water-rich atmospheres are not those that accreted the most ice, but those that are depleted in hydrogen and carbon. These planets typically form inside the ice line and accrete less volatile-rich material. While some retain significant atmospheric H2O, the high-temperature miscibility of water and hydrogen likely prevents the presence of surface liquid water—even on these comparatively water-rich worlds.

This has broad implications for theories of planet formation and volatile evolution, as well as for interpreting exoplanet atmospheres in the era of the James Webb Space Telescope (JWST), the Extremely Large Telescope (ELT), ARIEL, the Habitable World Observatory (HWO), and the Large Interferometer for Exoplanets (LIFE). It also informs atmospheric composition priors in interior characterization of transiting planets observed by Kepler, TESS, CHEOPS, and PLATO with radial velocity (RV) or transit timing variation (TTV) constraints.

K2-18b will obviously receive continued deep study. But with an aggregate 14,000 exoplanets confirmed or listed as candidates, consider how overwhelmed our instruments are by sheer numbers. To do a deep dive into any one world demands the shrewdest calculations to find which exoplanets are most likely to reward the telescope time. This aspect of target selection will only get more critical as we proceed.

The paper is Werlen et al., “Sub-Neptunes Are Drier than They Seem: Rethinking the Origins of Water-rich Worlds,” Tje Astrophysical Journal Letters Vol. 991, No. 1 (18 September 2025), L16 (full text).