Pluto is about forty times the distance from the Sun as Earth. But the Solar System is over 50000 times that length across, meaning it could be hiding some huge secrets. That's now looking like a small but real possibility.
In recent years, astronomers have discovered a bunch of planets located at least 100 astronomical units - 1 AU is the distance from the Sun to the Earth - away from their host stars. These planets are gas giants - they would have to be for us to see them at all - so this is something very different the dwarf planets like Pluto and Eris discovered in our solar system's Kuiper Belt and beyond.
There's almost no chance that these giant planets could have formed as part of their host star's planetary disc, considering their immense distance away. That strongly suggests that these are former rogue planets captured by the star's gravity. Hagai Perets of the Harvard-Smithsonian Center for Astrophysics and Thijs Kouwenhoven at Peking University's Kavli Institute for Astronomy and Astrophysics teamed up to figure out just how often we can expect stars - potentially including those like our own Sun - to capture these giant wandering planets. ScienceNOW reports their results:
Because most stars are born with others, Perets and Kouwenhoven ran computer simulations to see what happens when a star cluster contains free-floating planets. If the number of free-floating planets equals the number of stars, then 3% to 6% of the stars succeed in capturing a planet, and some stars capture two or three. Most of the captured planets end up hundreds or thousands of times farther from their stars than Earth is from the sun. Furthermore, most captured planets have orbits tilted to those of native-born planets, and half the captured planets revolve around their stars backward.
Their work depends on having a good grasp of how many rogue planets there really are, and we can't be sure our current estimates, which suggest there area as many wandering planets as there are stars, are accurate. But if these results are accurate, then our Sun, whose mass is slightly above average, had a real chance of capturing one or more planets eons ago.
The chances aren't huge by any stretch - probably only a few percent, according to Perets - but if their numbers are accurate, then the possibility of such a planet definitely exists. Coupled with some recent, admittedly far from convincing signs pointing to such a planet's possible existence in the Oort Cloud, the Solar System might just have one gigantic secret waiting for us to discover.