Free Astronomy Magazine September-October 2018

22 SEPTEMBER-OCTOBER 2018 PLANETOLOGY The idea found a discreet consensus in the scientific community, but since it was im- possible to prove its validity at the time, it remained just an idea. Twenty years later (then around the mid-1980s), astronomers realized that the upper atmosphere of Uranus was incredibly cold: -224°C (-371°F). This temperature is what one would expect if the upper atmosphere was heated al- most exclusively by the Sun and not by in- ternal heating from the planetary core - as happens in the other giant planets. The surface thermal emission of Uranus is ap- proximately in equilibrium with the insola- This brief description, however, neglects the two most mysterious aspects that make Uranus a very interesting subject for plane- tologists: its rotation axis is practically lying on the orbital plane, and its external atmos- phere is colder than those of all the other planets – including Neptune, which is more than 1.5 billion km farther from the Sun. How are these peculiarities explained? The first hypothesis on the exceptional incli- nation of the rotation axis of Uranus dates back to the mid-60s, when the Russian as- tronomer Viktor Safronov (father of the planetesimal theory, related to the formation of the planets) claimed that the large tilt in the rotation axis was the result of a collision between Uranus and a massive planet wan- dering in the solar system. T he great British astronomer of Ger- man origin Wilhelm (William) Herschel, por- trayed in 1785 by Lemuel Francis Abbott. On the side, the historic telescope with which he discovered the planet Uranus.