Venus is truly an odd twin of Earth. Despite its similar size, the planet has such a thick, hot atmosphere that it would crush you and cook you if you were to stand on its surface. And the space oddities don’t end there. As all the other planets spin anticlockwise on their axes, Venus spins clockwise. It’s like it is upside down compared to the other major players in the Solar System. The reason is unclear, but a new proposal sees Venus capturing and then destroying a moon.

Earth is also affected by its own Moon. The Moon has stabilized and slowed down the planet's rotation. In the case of Venus, this new, hypothetical moon did even more than that. Researchers Valeri Makarov at the United States Naval Observatory in Washington DC, and Alexey Goldin at Teza Technologies in Chicago, want us to cast our eyes back to near the beginning of the Solar System. It was a time when the rocky planets had assembled most of their mass, but there were still plenty of building blocks hanging about.

From these, a planetesimal could have been captured by Venus. The research duo looked at hundreds of thousands of simulations and found scenarios that they deem both possible, and with the power to explain the properties of Venus today. First of all, the moon capture would have been retrograde. This means that was going around Venus in the opposite direction. The assumption is that Venus at that point was spinning in the same direction as the other planets.

The motion of this moon would have created tides that slowed down Venus's spin, as long as the moon could stick around for over 10,000 years. This is easier said than done. Holding on to a moon, it would have been difficult for Venus to be so close to the Sun. And this is true whether it captured one or whether it formed from a major impact, like our own Moon. The captured moon in the scenario was kept thanks to the presence of material around Venus that removed orbital energy. But this comes at a cost.

The frequent collisions, the tides, and the orbital motion eventually brought the moon too close to the planet. The gravity of Venus broke the moon apart, and its broken remains eventually fell down on the world below. 

The team called the moon Neith. Giovanni Cassini, the discoverer of Titan and the rings of Saturn, is believed to have seen such a moon around Venus, and for a few centuries people reported the presence of this hypothetical object. It's likely they were seeing stars that had serendipitously appeared near the planet.

The scenario is not the only one that set out to explain why Venus orbits the way it does: a backward day, from our point of view, and extremely slowly. It takes about 243 Earth days for Venus to do a single pirouette on its axis. Other suggestions for the weird spin relate to gravitational interactions with the Sun, the effects of its thick atmosphere, and a major impact, similar to the one that forced Uranus to orbit on its side.

The study is published in the journal Universe.

[H/T: New Scientist]