Universe

Dwarf planet with atmosphere discovered beyond Pluto: why it defies some scientific theories

2002 XV93 the unpoetic provisional name given to the mini-planet discovered by the Japanese: only 480 km in diameter

by Leopoldo Benacchio

10 May 2026

La luce della stella si affievolisce lentamente avvicinandosi al pianeta 2002 XV93 – University of Tokyo

5' min read

Translated by AI

Versione italiana

5' min read

Translated by AI

Versione italiana

Jared Isaacman, administrator of Nasa, has in recent days claimed for Pluto the return to the role of planet, now a 'dwarf planet' after its cataloguing was changed years ago by the World Assembly of Astrophysicists. Here, however, the Japanese displeased him by finding another mini-planet, much smaller and much stranger, six billion kilometres away.

This story that in Prague, in August 2006, Pluto was, along with others, classified no longer as a planet but as a 'dwarf' planet is regarded by many Americans as a real insult, meaningless of course since Pluto just doesn't change because of this and peacefully continues its orbit around the sun. The mischievous claim that Isaacman, a billionaire, honourable self-made private astronaut and great space enthusiast, has perhaps brought up this alleged snub to the US again to cover up the continued delays of the Artemis lunar missions. The Artemis III mission was in fact just moved months after the success of Artemis II, and in any case nasa's role is not to discuss astrophysics but to fly rockets and, satellites and astronauts.

Un'animazione confronta l'effetto di un'atmosfera su un'occultazione stellare

But let us come to the news, which is very interesting. Last Monday, Japanese astronomers from the National Observatory of the Rising Sun announced the discovery of a small planet, 480 kilometres in diameter, to get an idea, 3475 those of the Moon and 2376 of Pluto. A small world then, located 5.9 billion kilometres away, roughly the distance of the aforementioned Pluto. It does, however, have one unexpected peculiarity: it appears to be enveloped by a layer of atmosphere, 2002 XV93 the not very poetic provisional name.

"I was genuinely surprised," said Ko Arimatsu, the scientist who led the team of Japanese astronomers who published their findings in the leading scientific journal Nature, and the reason is that the planet, even more dwarf than Pluto, has an atmosphere, and really shouldn't have.

The atmosphere of a celestial body, like that of our Earth, is composed of molecules of various elements. In the case of the one we breathe, we have a mixture of Nitrogen, 78 per cent, Oxygen, 21 per cent, and then traces of others such as Argon, Carbon Dioxide, Helium, Methane and others.

I maggiori pianeti nani oltre l’orbita di Nettuno, Plutone non è neppure il più grande / Nasa

Between the ground and an altitude of 100 kilometres, an arbitrarily set limit for reference, our atmosphere is more or less dense and the molecules are more or less agitated, depending on the temperature. If the Sun heats the atmosphere, the molecules acquire heat energy and transform it into speed. Let us also think of moving balls, an image that would horrify a physicist, but one we can understand.

On these continuously moving balls acts the force of gravitational attraction, which keeps them bound to the Earth, but if for some reason a ball receives energy and manages to reach a speed of 11.2 kilometres per second, called the escape velocity from the Earth, then it can get away, and we lose it. The smaller the body, hence less mass and less gravitational attraction, the lower the escape velocity. For example, the Moon lost its atmosphere because it has a much smaller gravitational pull than the Earth due to its much smaller mass. Mercury is small, about 4880 kilometres, plus it is very close to the Sun, so the molecules in its atmosphere, if there ever was one, received a lot of energy and escaped immediately. More energy and less force of gravity equals easier escape. In the cosmos it is very often a matter of finding a balance between opposing forces.

Now, in a solid body like a dwarf planet with a diameter of 480 kilometres, the escape velocity is very low, so the discovery of a gaseous atmosphere layer is puzzling, since, as we have just said, it would have had to be gone very soon after formation, or even, given the temperatures over there, we are in the 230 degrees below zero, any atmosphere would have had to freeze immediately and fall like ice to the ground.

Video realizzato con immagini riprese da un telescopio a Kiso in Giappone

But nothing, from the Japanese study, the presence of a gaseous atmosphere is very evident. They observed, from various observatories in Japan, that small planet on 10 January 2024, as it passed in front of a distant star. In such a situation the planet occults the star, if we want to use other words, it briefly extinguishes the light of that star, it obscures it, but it does so decisively. If, on the other hand, as has happened, the star's light fades little by little as the small planet approaches and then disappears, then it means that prior to complete occultation the light is dimmed by an atmosphere of the planet. It is a bit like what we have here on Earth at sunrise and sunset, when the sun does not go away, or appear, suddenly, but is dimmed by the atmosphere.

In short, Pluto is one of the many celestial objects towards the end of the planetary system, beyond Uranus, which until the end of the last century we thought was only populated by a strange planet, Pluto itself, with an orbit very different from the other planets.

The real enemy of Pluto is therefore not the International Astronomical Union, in which, moreover, the Americans are almost a majority, but the many trans-Neptunian objects, i.e. those that lie beyond Pluto's orbit and that have been discovered in recent years.

In fact, several dwarf planets have been discovered in this century, all in the vicinity of Pluto or slightly beyond, in astronomical terms of course. Varuna in 2000, with a curiously elongated shape, like a rugby ball, Ixion in 2001, very dark, Quaoar, in 2002, an icy world with a diameter almost half that of Pluto, Sedna in 2003, with a very elongated orbit that it travels in a good 11.400 years, Haumea in 2004, has a ring system and two small moons, Eris in 2005: almost as large as Pluto but more massive. Together with Makemake discovered in 2005 Eris was Pluto's killer as a planet. It was finally realised that this growing family of objects could not be regarded as the eight planets we learn about in school. Hence the term 'dwarf' planet for these and hence the alleged disrespect, if not crime of lese majesty to the USA. Pluto was not 'demoted' in 2006, rather it was assigned its true family, that of trans-Neptunian celestial objects. But the Americans also consider space 'their thing' due to the fact that they were first, and at the time only, on the Moon in the 1970s.

In short, Nature is always pulling new things out of her hat that we cannot imagine and which force us to revise our theories. That's science, fortunately.