Posted by New Horizons flew past Pluto in a few hours on July 14, 2015, and then kept going. The spacecraft took more than a year to send home all its data over a link slower than old dial-up, and researchers have been chewing on that single pass ever since. Most Pluto updates you read today are not new observations at all, but new readings of 2015 data, sharpened by computer models, telescopes on Earth and, lately, the James Webb Space Telescope.
What follows is less the discovery story than the after-party: what scientists think they have learned since, and where they still disagree.
An ice sheet that turns itself over
Sputnik Planitia, the smooth western half of Pluto’s bright heart, is a basin roughly a thousand kilometres across filled with nitrogen ice, probably several kilometres deep. Its surface is divided into irregular polygons tens of kilometres wide, separated by shallow troughs. The leading explanation, worked out in papers published in 2016, is that the ice is slowly convecting. Faint warmth from below makes the nitrogen rise in the middle of each cell and sink at the edges, like a pan of very thick soup on a very low flame.
The consequence is a surface that keeps erasing itself. No impact craters have been spotted on the plain, which puts its age at no more than about 10 million years, and the cells may refresh on timescales of hundreds of thousands of years. On the eastern side, nitrogen glaciers flow down from the uplands into the basin, carving valleys as they go. Nitrogen ice at about minus 230 degrees Celsius is soft enough to flow. Water ice, at the same temperature, is as hard as rock and forms the mountain ranges that poke up around the plain’s edge.
Is there an ocean down there?
This is the most tantalising of the Pluto updates, and also the most indirect.
Sputnik Planitia sits almost exactly opposite the point that always faces Charon, close to the line connecting the two bodies. That is suspicious. A heavy load placed on a spinning body tends to drift toward that alignment, so several teams argued that the basin is unusually massive, and the simplest way to make it heavy is to have a layer of dense liquid water pushed up beneath the thinned ice shell. Long extensional faults across Pluto’s surface also suggest the crust has been expanding, which is what you would expect if an internal ocean were slowly freezing.
Keeping water liquid that far from the Sun is the obvious problem. One proposal, from a 2019 study, is that a thin layer of gas hydrates at the base of the ice shell acts like a blanket and traps the interior’s heat. No instrument has detected an ocean directly, and a flyby cannot measure gravity in the detail an orbiter could. The case is circumstantial. It is still taken seriously.
Mountains that might be volcanoes
South-west of Sputnik Planitia sit two odd mounds, Wright Mons and Piccard Mons. Wright Mons is several kilometres high and around 150 kilometres across, with a deep central depression and almost no craters. In 2022 a New Horizons team argued they are probably cryovolcanoes, built from repeated outpourings of icy, slushy material rather than molten rock. If they are right, Pluto had enough internal heat to drive large-scale volcanism relatively recently in its history, which is not what anyone expected of a body this small and this cold. Not everyone is convinced the mounds are volcanic. Alternatives are hard to come by, though.
Further east lies Tartarus Dorsa, a region of tall, closely spaced ridges of methane ice that look like a field of giant blades. They may grow and erode as methane freezes out of the atmosphere and evaporates again over Pluto’s long seasons.
A thin atmosphere with a temper
Pluto’s air is mostly nitrogen with traces of methane and carbon monoxide, and its surface pressure is roughly a hundred-thousandth of Earth’s. It is also unstable. Pluto’s orbit is elongated, and it passed closest to the Sun in 1989. Since then it has been slowly receding, and for decades astronomers expected the atmosphere to freeze out onto the ground. Instead, stellar occultations showed pressure roughly tripling between 1988 and 2015, probably because seasonal sunlight was evaporating nitrogen ice in Sputnik Planitia. Occultation campaigns since the flyby suggest the rise has stalled, and a full or partial collapse later this century is still a possibility. Amateur observers with modest telescopes regularly help with these measurements, which is one of the nicer aspects of Pluto science.
Then there is the haze. New Horizons saw more than twenty thin layers stretching far above the surface, made of organic particles that form when sunlight breaks up methane and nitrogen. Looking back at Pluto with the Sun behind it, the haze glows blue, because the small particles scatter blue light most strongly. A 2017 study predicted that these particles, rather than the gases, control the temperature of the upper atmosphere by radiating heat away into space. Observations with JWST reported in 2025 appear to back that up, which would make Pluto the only known world whose climate is set mainly by haze.
Charon gets its own chapter
Charon is about half Pluto’s diameter, and the two orbit a point in empty space between them. For years the standard explanation was a giant impact similar to the one thought to have formed our Moon. In 2025 a team proposed a softer version they called “kiss and capture”: the two bodies touched, stuck briefly and separated into the pair we see, with both staying largely intact. It is a modelling result, not a settled answer.
Charon’s dark red north pole, nicknamed Mordor Macula, now has a neat explanation. Methane leaks off Pluto, some of it is captured by Charon, freezes onto the frigid winter pole and is slowly cooked by ultraviolet light into reddish compounds. One world is painting the other. JWST has since detected carbon dioxide and hydrogen peroxide on Charon’s surface, both hints about how its ices are processed by radiation.
The four small moons are stranger still. Nix and Hydra tumble chaotically rather than spinning steadily, because they are being tugged by two large bodies at once.
Beyond Pluto, and back to it
After Pluto, New Horizons was steered toward a tiny Kuiper Belt object and flew past it on January 1, 2019. That object, Arrokoth, is about 36 kilometres long and made of two lobes that appear to have merged gently, at walking pace. It is one of the best pieces of evidence that the first building blocks of planets came together softly, not through violent collisions. The spacecraft is still running in the outer Solar System, though its future funding has been questioned more than once.
No return mission to Pluto is approved. Orbiter concepts have been studied, including one called Persephone, but anything launched in the coming years would take decades to arrive. For now the 2015 data, Earth-based occultations and JWST are what we have, and the next round of Pluto updates will most likely come from them.
If a world the size of a continent can still hide an ocean, volcanoes and a self-renewing ice sheet, it is worth asking what else is out there. SETIworld keeps track of the outer Solar System and the people trying to understand it. Come and follow along.
