Meet Ross 176b! This small exoplanet, newly discovered just 33 light-years away, is making waves, and not just because of its size. With a density of around 3.8 g/cm³, it’s believed to be a water world. Its history and composition could offer key insights into planetary diversity, how planets form, and the many evolutionary paths they can take. Join us in today’s Astrobito to learn more about this fascinating new world!
Paper details:
Title: Discovery of a transiting hot world-water candidate orbiting Ross 176 with TESS and CARMENES
Authors: S. Geraldía-González, J. Orell-Miquel, E. Pallé, F. Murgas, G. Lacedelli, et. al.
First-author institution: Instituto de Astrofísica de Canariasd (IAC), La Lagua, Tenerife, España
Status: Acceso abierto en arXiv. Accepted for publication in A&A
In the search for exoplanets, we’ve gotten surprisingly good at finding them; thousands are now catalogued, ranging from gas giants to rocky Earth-sized worlds. But knowing what these planets are made of? That’s a much harder question. Especially for the ones that are similar in size to Earth or a bit larger, where a small change in mass or radius can mean a completely different planet underneath.
That’s what makes Ross 176 b such an interesting case. It’s a small planet orbiting a nearby K-type star, and thanks to a combination of precise measurements and clever modelling, astronomers now have a better idea of what might be inside.
In exoplanet science, the planets we know the least about are often the ones closest in size to Earth. That’s partly because of how we detect them. When a planet transits (passes in front of) its star, we can measure its radius. If it also tugs on its star enough to detect with radial velocity measurements, we can estimate its mass. But putting those together to learn about what a planet is made of is a much harder task.
That’s especially true for planets in the 1.5–2.5 Earth radii range: the so-called "radius valley". These planets are common, but they sit right at the edge between rocky worlds like Earth and larger, puffy planets with thick atmospheres like mini-Neptunes. A small change in either mass or radius can mean the difference between a dense rocky core or one surrounded by gas or water.
The radius valley in exoplanet populations is the relative paucity of planets with radii between 1.5 and 2 Earth radii. Figure adapted from Fulton et. al.
So whenever we get a well-measured planet in this size range, it's worth paying attention. Ross 176 b is one of those rare cases.
Ross 176, the star, is a quiet, middle-aged K-dwarf just 33 light-years away. Its planet, Ross 176 b, was first spotted via radial velocity measurements. Later photometry from TESS confirmed that it transits, which allowed astronomers to get both its radius and its mass.
What they found is intriguing. The planet is about 2.2 times the radius of Earth, with a mass of 9.1 Earth masses. That puts it right in the transition zone between rocky and non-rocky planets. But its density, around 3.8 g/cm³, is too low to be purely rocky. For comparison, Earth’s density is about 5.5 g/cm³. That means Ross 176 b must have something else contributing to its size: either a thick atmosphere, a large water layer, or both.
World composition models show that the radius and mass of Ross 176 b place it right in the trend for “water-worlds”. Wolf 503 b and HD 40307 b are other “water-wolrd” exoplanet candidates similar to Ross 176 b. Figure adapted from Figure 5 in the main manuscript.
To figure out which, the authors of the new study used a machine learning model trained on thousands of planetary interiors to explore different compositions that could explain the observed mass and radius. They found that the planet is most likely a water-rich sub-Neptune, with a significant portion of its mass (around 20%) in the form of water.
That makes Ross 176 b a strong candidate for what astronomers sometimes call a “water world”, though not necessarily one with oceans like Earth. Instead, it could have deep layers of high-pressure ice or supercritical water, depending on temperature and pressure conditions inside the planet.
Planets like Ross 176 b aren’t just interesting on their own, they also help fill in key gaps in our understanding of planet formation. For one, it's relatively rare to find planets in the radius valley with such well-constrained properties. Even rarer are ones that seem to have held onto large amounts of water while orbiting close to their stars.
That raises questions about how Ross 176 b formed. Did it start out farther away and migrate inwards? Did it somehow avoid losing its water during formation? Either way, it adds a valuable datapoint to population-level studies of small exoplanets, especially around K-dwarfs, which are less studied than M-dwarfs like TRAPPIST-1 but are more Sun-like in many ways.
Interestingly, Ross 176 b also appears to be the only planet in its system, at least based on current data. That’s a little unusual for systems with small planets, which often come in tightly packed multiples. If future observations confirm that Ross 176 is truly a single-planet system, that could also tell us something about how its formation history might differ from the norm.
Because it’s relatively close and transits its star, Ross 176 b is also a potential target for atmospheric observations with telescopes like JWST or Ariel. Detecting water vapour, hydrogen, or other atmospheric molecules could confirm whether the planet is water-rich… or reveal an unexpected composition altogether!
That makes this small world more than just another data point. It’s a stepping stone toward answering bigger questions about planet diversity, formation, and the different evolutionary paths planets can take, even when they’re roughly the same size. With more detailed observations in the future, Ross 176 b might help us understand not just what it's made of, but how planets like it come to be.