You can see more of the art up-close here: SayoStudio and read the Science News article written by Nadia Drake here (available for anyone to read now, but will only be available to subscribers once a new issue comes out): http://www.sciencenews.org.
The task was to create two illustrations, one for the cover and one for the opener, about updates to the Nice model (Nice as in Nice, France) which explains how the giant gas planets positioned themselves in our outer solar system. When the theory was first proposed in 2005 a few things were still left unexplained. The cover art focuses on a possible 5th outer planet, similar in size to Neptune and Uranus. Its presence helps explain how an early Earth’s trajectory was changed so that it didn’t go crashing into Venus. This extra gas planet played a dangerous dance with Saturn and Jupiter, eventually leading to it’s ejection from the solar system.
The opener feature art focuses on how the gas giants–Jupiter, Saturn, Neptune and Uranus–were able to so quickly attain their size. This current theory postulates that 3-4.5 billion years ago the planets were much closer together, and they lacked the standard orbits they have today leading to a lot of jostling. As the early gas giants moved, they were able to collect the debris that were thickly distributed in the younger solar system, with Jupiter even stealing some that would otherwise have allowed Mars to become larger.