It turned out that the moons of Uranus are more like planets

The scientists used data from the Herschel Space Observatory, which was deployed between 2009 and 2013. Compared to its predecessors, which covered a similar spectral range, this telescope’s observations were significantly sharper. It was named after the astronomer William Herschel, who discovered infrared radiation in 1800. A few years earlier, he also discovered the planet Uranus and its two moons (Titania and Oberon), which are today explored in more detail along with three other moons (Miranda, Ariel and Umbriel).

Images of the five largest moons of Uranus – Miranda, Ariel, Umbriel, Titania and Oberon. The Voyager 2 space probe captured these images during its overflight on January 24, 1986. The diameters of the moons are shown to scale. Credit: NASA / JPL / MPIA

Cool objects such as Uranus and its five major satellites emit very bright light in the spectral range from 70 to 160 microns. Warmed by the sun, they reach temperatures of about 60 to 80 K (–213 to –193 ° C).

It turned out that the surfaces of the moons are surprisingly good at keeping temperatures and cooling relatively slowly. Astronomers are familiar with this behavior from compact objects with rough icy surfaces. This is why scientists speculate that these moons are celestial bodies, similar to dwarf planets at the edge of the solar system, such as Pluto or Haumea. Independent studies of some of Uranus’s outer, irregular satellites, which are also based on observations with PACS / Herschel, show that they have different thermal properties. These moons exhibit the characteristics of smaller and loosely coupled trans-Neptunian objects that are located in a zone outside the planet Neptune.

Initially, the five main satellites of Uranus were hardly noticed. Bright objects like this planet generate strong artifacts in the PACS / Herschel data that cause some of the infrared light in the images to spread over large areas. This is hardly noticeable when observing faint celestial objects. However, with Uranus, this is even more pronounced. “The moons, which are 500–7400 times fainter, are so close to Uranus that they merge with equally bright artifacts. Only the brightest moons, Titania and Oberon, stand out slightly from the surrounding light, ”explains Gabor Marton of the Konkoli Observatory in Budapest.

This accidental discovery prompted Ors H. Detre to make the moons more visible so that their brightness could be measured reliably. “In cases like this, such as searching for exoplanets, we use coronagraphs to mask their bright central star,” Detre explains. – Herschel didn’t have such a device. Instead, we took advantage of the photometric stability of the PACS instrument. ” Based on this stability, and after calculating the exact position of the moons during observations, he developed a method that allowed him to remove Uranus from the data. “We were all surprised when four moons appeared clearly in the images, and we were even able to spot Miranda, the smallest and innermost of the five largest moons of Uranus,” concludes Detre.

“The result demonstrates that we don’t always need complex planetary space missions to gain new insights into the solar system,” notes co-author Hendrik Linz of MPIA. – In addition, the new algorithm can be applied to further observations that have been collected in large quantities in the electronic data archive of the European Space Agency ESA. Who knows what surprise still awaits us there? “

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