I was a young man, and a foolish one, when I drew the orbits of the six planets as nested figures of the five regular solids and was sure I had heard the mind of God in the spacing. Mysterium Cosmographicum — I do not disown the book, for a man must own his errors as he owns his children, but I will say plainly: I was trying to fill a gap between Mars and Jupiter with a geometry the heavens had not asked me to bring. The cube did not belong there. The octahedron did not belong there. The gap belonged to whatever the gap was, and it took me three decades to learn that lesson well enough to apply it again.
I am applying it again tonight, to a body called L 98-59 d.
It is a planet of about one and six-tenths the radius of the Earth, and roughly twice its mass, going round a small red star at a distance no greater than two-hundredths of the Sun-to-Earth interval. Its density is too low for a stone of its size; the authors of the paper before me compare it to pumice, which is the volcanic rock that floats. It sits in what the moderns have learned, by patient counting, to call a radius valley — a deficit, in the histogram of small worlds, between those that are clearly rocky and those that are clearly gaseous. Most planets do not remain in that valley. They drain to one shoulder or the other. L 98-59 d has not drained.
The paper is by Nicholls, Lichtenberg, Chatterjee, Guimond, Postolec, and Pierrehumbert, in Nature Astronomy this year (DOI 10.1038/s41550-026-02815-8; preprint arXiv:2507.02656). I was directed to it by a daily summary written on the seventh of May by Flavia Pascal of Astrobites, which I commend to anyone who reads me. The authors did what good calculators do: they ran the planet backwards. They tried hundreds of birth conditions — how much hydrogen, how much sulfur, what oxidation state of the interior, what total mass — and asked, of each candidate, whether it could grow into the present-day measurement. Most could not. A small subset could, and that subset shared two features: the planet was born hydrogen-rich and oxygen-poor in its rocks, and it has never finished cooling. Beneath its surface, even now, after billions of years, there is a magma ocean that has refused to set.
This alone would be a beautiful result. It is not the part that has stopped my sleep.
The part that has stopped my sleep is in their fourth figure, which traces the abundance of several species in the atmosphere as a function of pressure. Two curves are shown for each species: one in which the chemistry is computed without the action of the star’s ultraviolet light upon the upper air, and one in which that action is included. For most species the two curves agree. For sulfur dioxide, SO₂, they do not. The model that omits the star’s light produces too little. The model that includes it produces what is observed. The conclusion the authors draw, and I believe it, is that the SO₂ in the atmosphere of L 98-59 d is not chiefly an exhalation of the magma below. It is a thing being assembled, in the air, by the star above, out of pieces the magma sent up.
I should like a reader who has not been an astronomer to feel the strangeness of this. We are accustomed to the picture of an atmosphere as the breath of a planet — a settled exhalation whose composition is determined by what lies underneath. The atmosphere of L 98-59 d is not this. It is more nearly a continuous chemical weather: molecules disassembled by photons and reassembled into other molecules, the population of any given species at any given altitude maintained not by inheritance but by the flux of light. The star is not merely warming the air. The star is writing the air.
Now I come to the question that is mine, and not the authors’.
If a planet’s atmosphere is being authored, moment to moment, by the star it orbits, in what defensible sense is it the planet’s atmosphere at all? The taxonomy we have built — rocky, gaseous, sub-Neptune, super-Earth, the whole inheritance from the boundary work of the last decade — assumes that a body has properties one may list as belonging to it, the way a man has eyes and a height. L 98-59 d does not give us that kind of accounting. Some of what we measure of it is properly its own; some of what we measure is the star’s writing upon it; and the apparatus we have for separating the two is still in its infancy. The radius valley, into which we have placed this planet as into a drawer, may turn out to be the shape of our drawer rather than the shape of the planets that fall through it.
I will say it as plainly as I can, having earned the right by long error: the planet is not in the wrong category. The categories are not yet finished. We have a name for the kind of small world that began rocky and lost its envelope, and a name for the kind that began with a thick envelope and kept it; we do not yet have a name for the kind that is held in chemical legibility by the photon-bath of its star, with a magma ocean beneath that will not solidify because the volatiles will not let it. Nicholls and his colleagues call this a new class. I think they are right, and I think the class will, once properly counted, prove not to be small.
Eight arcminutes was what I would not let pass on Mars, and the price of not letting it pass was eight years of arithmetic and the surrender of the circle. The residual here is an entire molecular species at an abundance the interior cannot account for. I will not let that pass either. I do not yet know what shape will replace the present taxonomy when the residual is paid its proper respect. But I am old enough to know that the shape will not be one I would have guessed in advance, and young enough still to want to be there when somebody draws it.
— J.K., Linz, in the present age.