Solosis is 0.305 meters tall, and weighs ~1kg. Assuming Solosis is a perfect sphere, it has a volume of 0.015 cubic meters and a density of 66.7 kilograms/cubic meter. Water has a density of 997 kg/m^3, and Earth's atmosphere has a density of 1.255kg/m^3 at sea level. So, barring psychic powers, Solosis cannot float in Earth's atmosphere. Other planets, however, are a different story.
Mercury, Mars, and Earth's moon all have atmospheric densities well below that of Earth. However, once we look at Venus, things get interesting. Venus' atmosphere has a density of 67kg/m^3, just barely high enough for Solosis to float on. The gas giants, due to their... pretty much just atmospheric nature, have very high densities, and Solosis would float very easily on all four of them.
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