Scientists have found that Pluto radiates too much in X-rays. The existing models cannot explain this, but there are still no other explanations.
Pluto, which once bore the proud title of the planet, has long been excluded from this list and entered the much less grandiose group of dwarf objects, which are many in the distant Kuiper belt. However, it has not ceased to attract the attention of specialists, and the data collected by the New Horizons probe that flew past Pluto in 2015 brings more and more new finds and mysteries. For example, American astronomers recently discovered that a dwarf planet emits unusually high levels of X-rays. Their article has been accepted for publication by the Icarus magazine, while its preprint has been posted (PDF) on the ArXiv online service.
In general, various bodies of the solar system can emit in X-rays, this radiation arises from the interaction of particles of the solar wind with inert gases of their atmospheres, such as nitrogen or argon. X-rays were recorded near Venus and Mars, a number of comets. It was theoretically expected from Pluto as well. The same New Horizons probe showed that the fragile atmosphere of the dwarf planet greatly changes in size and density depending on the local season. For 248 years, which is required for Pluto to complete a revolution around the Sun in its elongated orbit, the distance to the star changes almost twice. The amount of weak radiation reaching the dwarf also changes, causing either partial freezing or sublimation of atmospheric nitrogen and methane.
The perigee of the orbit, when the distance to the Sun is minimal and is about 4.4 billion km, and the atmosphere is maximal, Pluto passed in September 1989. In 2015, during the visit of New Horizons, the height of summer was still going on here, and the probe recorded atmospheric nitrogen, methane and carbon dioxide. Therefore, the Chandra orbiting telescope conducted observations in search of X-rays that might well be expected from Pluto at this time. However, everything turned out to be not so simple.
X-rays, which were recorded by the onboard spectrometer Chandra ACIS, turned out to be noticeably more intense than scientists expected. Small bodies of the solar system sometimes emit so actively, which reflect the X-ray photons of the Sun itself with small particles of nitrogen, carbon and oxygen. But the energy spectrum of Pluto's photons does not fit this explanation, and so far the reason for the increased radiation intensity remains unknown.
The authors of the article even put forward a very unusual hypothesis about the possibility of the existence of some kind of "focusing" mechanism of the solar wind, which enhances the flow of particles entering Pluto's atmosphere. However, scientists admit that there is no clear explanation for this yet.
Sergey Vasiliev