Free Astronomy Magazine July-August 2016

PLANETOLOGY ject or not. The conclusion was that past sky surveys had little chance of even detecting an ob- ject with a mass twice that esti- mated for Planet Nine, and that not even the observation program most suited to that end, i.e. the Wide-field Infrared Survey Explor- er, could have recor- ded such a subtle pres- ence. According to the Swiss researchers, on- ly with the entry into service of instruments like the James Webb Space Telescope and the Large Synoptic Survey Tele- scope (both in advanced con- struction stage), or with the implementation of dedicated surveys, we will be able to de- termine whether Planet Nine actually exists or not. Even if admitting its existence would be the simplest and most sensible way to explain the orbital anomalies of some Kui- per Belt objects, not all astronomers agree on this scenario, mostly due to the diffi- culty of explaining how a planet so mas- sive could have formed at a distance from the Sun that (according to the simulation used as reference) could be between 200 and 1,500 astronomical units (the distance most widely quoted is close to 700 AU). Among those who investigated the reason for such unusual remote location so as to understand if the planet formed there or just arrived from somewhere else, there are Gongjie Li (Harvard-Smithsonian Cen- ter for Astrophysics) and Fred Adams (Uni- versity of Michigan). The two researchers performed together countless computer simulations to test three possible scenar- ios: 1) Planet Nine has migrated from a more inner region of our solar system; 2) it is a rogue planet captured by the Sun; 3) it is an exoplan- et stolen from another star by the Sun. In the first scenario, support- ed and complemented by another study conducted by Scott Kenyon (Harvard-Smith- sonian Center for Astrophy- sics) and Benjamin Bromley (University of Utah), Planet Nine would have been born not far from the gas giants of the solar system. When about 4.5 billion years ago the planetary orbits were still subject to instability and the protoplanetary disk was still present, although thinned, a se- ries of close encounters with Jupiter and Saturn would have shoved Planet Nine be- yond Neptune’s orbit. Almost certainly it would have dispersed into space if the remnants of the protoplanetary disk had not slowed it down to the point of stop- ping it in a region beyond the Kuiper Belt, perhaps at a distance in the order of 100 AU. Later, but not by much, a star would have passed close enough to our solar sys- F red Adams and Gongjie Li are the authors of a recent study that places the origin of Planet Nine in our solar system. Below, left to right, a scaled comparison be- tween Uranus, Neptune and the hypothetical ninth planet. [NASA]