Water locked in their bones — not on the surface, but deep inside.
Three small planets orbiting dim red stars have handed astronomers a puzzle: they contain less iron than they should. The gap between what was expected and what was measured is now pointing, tentatively but meaningfully, toward water — not on the surface, where these worlds are far too scorched to hold it, but locked away in their interiors.
The planets in question are GJ 1132 b, GJ 1252 b, and LTT 3780 b — all classified as hot super-Earths, all circling M dwarf stars, the small, cool, red stars that make up the majority of the galaxy's stellar population. A large international team of researchers used a new instrument called the Near Infrared Planet Searcher, or NIRPS, to measure the masses of these three worlds with fresh precision: 1.69, 1.54, and 2.34 times the mass of Earth, respectively. Those numbers matter because mass, combined with radius, tells you something fundamental about what a planet is made of.
The key concept here is the core mass fraction — the proportion of a planet's total mass that sits in its iron-rich core. Researchers have long known that a planet's composition tends to mirror, at least roughly, the composition of the star it formed around. Stars rich in iron tend to produce iron-rich planets. So if you measure the iron, magnesium, and silicon content of a host star, you can make a reasonable prediction about how much of its planet should be dense, metallic core.
NIRPS was designed partly for exactly this kind of double-duty science. The same spectra used to track the subtle wobble of a star — the radial velocity signal that reveals a planet's mass — can also be analyzed for the star's elemental fingerprint. The team did both, measuring stellar abundances and planetary masses from the same observational campaign.
When they compared the predicted core mass fractions to the ones derived from the planets' actual masses and radii, something didn't line up. Across nine hot super-Earths in total — the three newly measured worlds plus six others with precise masses already in the literature — the planets consistently came in with smaller core mass fractions than their host stars would suggest. The discrepancy was statistically significant, not a rounding error or a fluke of measurement.
The most straightforward explanation, the team argues, is water. Not surface oceans — these planets orbit close enough to their stars that any water on the surface would have long since boiled away. But water sequestered deep inside, mixed into the mantle or trapped in the core during formation, would add mass without adding iron. It would make a planet look less iron-heavy than it actually is, because the water is diluting the signal. The researchers estimate interior water mass fractions of roughly one percent — a small number in everyday terms, but geologically and chemically significant.
There is an alternative explanation worth noting: mantle stripping. If a planet suffers a giant impact early in its life, it can lose a portion of its rocky mantle, leaving behind a proportionally larger iron core. That would push the core mass fraction up, not down — the opposite of what was observed here. So stripping, at least as the dominant process, doesn't fit the data.
What the findings suggest instead is that these planets formed in a water-rich environment and held onto some of that water through the violent early period of planetary development, incorporating it into their deep interiors rather than losing it entirely to space or stellar radiation. M dwarf systems, it turns out, may be particularly good at producing this kind of world.
The paper, led by Drew Weisserman and accepted for publication in Astronomy and Astrophysics, represents the first results from the NIRPS guaranteed time observation program focused on core mass fractions. It is a beginning, not a conclusion. The team measured nine planets; the full program will eventually cover more. As that sample grows, the tentative signal seen here will either strengthen into something definitive or dissolve into noise. For now, the data are suggestive enough to take seriously — small planets around small stars, carrying water in their bones.
Notable Quotes
The core mass fractions of these planets are smaller than expected from their host stellar abundances, to a statistically significant degree — suggestive of significant reservoirs of water in their interiors.— Drew Weisserman et al., paraphrased from the study abstract
The Hearth Conversation Another angle on the story
So these planets are too hot for liquid water on the surface — why does water matter at all?
Because water doesn't have to be on the surface to change what a planet is made of. If it's locked into the interior during formation, it shifts the planet's overall density in ways you can measure from the outside.
How do you measure what's inside a planet you can never visit?
You weigh it and measure its width. Mass and radius together give you density, and density tells you roughly what mix of materials you're dealing with — iron, rock, ice, water.
And the host star is the key to knowing what to expect?
Exactly. Stars and their planets form from the same cloud of gas and dust, so a star's elemental composition is a kind of recipe for what its planets should contain. Iron-rich star, iron-rich planet — that's the baseline assumption.
But these planets have less iron than expected.
Less than the stellar abundances predict, yes. And the most natural way to explain that gap is to add something lighter — water, in this case — that contributes mass without contributing iron.
One percent water by mass sounds tiny.
It is, by everyday standards. But inside a planet, one percent water distributed through the mantle or core is a geologically active ingredient. It changes how rock flows, how heat moves, potentially how long a planet stays geologically alive.
Could there be another explanation besides water?
Mantle stripping from giant impacts is the main alternative, but that would push the core fraction up, not down. The data point the other direction, which is why water fits better.
What does this mean for the search for habitable worlds around red dwarf stars?
It suggests these systems may routinely produce water-bearing planets. Whether any of that water ever reaches a surface in liquid form depends on the planet's orbit and its star's behavior — but the raw ingredient appears to be there.