A long time ago, in a nebula that no longer exists, our newborn planet was hit with a giant impact so energetic that it melted part of the planet and the impactor and created a spinning molten glob. That whirling disk of hot melted rock was turning so fast that from the outside it would have been difficult to tell the difference between the planet and the disk. This object is called a “synestia” and understanding how it formed may lead to new insights into the process of planetary formation. The synestia phase of a planet’s birth sounds like something out of weird science fiction movie, but it may be a natural step in the formation of worlds. It very likely happened several times during the birth process for most of the planets in our solar system, particularly the rocky worlds of Mercury, Venus, Earth, and Mars. It’s all part of a process called “accretion”, where smaller chunks of rock in a planetary birth créche called a protoplanetary disk slammed together to make bigger objects called planetesimals. The planetesimals crashed together to make planets. The impacts release huge amounts of energy, which translates into enough heat to melt rocks. As the worlds got larger, their gravity helped hold them together and eventually played a role in “rounding” their shapes. Smaller worlds (such as moons) can also form the same way.