For the longest time, we were all taught that there were nine planets in the solar system. From closest to, and farthest from the sun, they were: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Sadly, Pluto fell off the list when astronomers determined that its small size and failure to fulfill all the gravitational functions of a bona fide planet disqualified it. So we were down to being eight fellow spheroids in our neighborhood, commonly known as the Solar System. Recently, however, two astronomers (one of whom made the case for Pluto’s ouster) have postulated the existence of a replacement ninth member, one that’s in the club but doesn’t come by for a meeting very often.
While observing the orbits of six non-planetary bodies that are situated out beyond Neptune, these guys became curious about the unusual elliptical pathways these bodies follow. Their travels do not lie in the common plane that the planets traverse (called the plane of the ecliptic) and something had to have influenced them into these abnormal orbits. A host of gravitational pressures might have pushed a given body into an odd orbit back in the early days of the solar system’s existence, but for so many to be so oriented suggested a singular influence. A whole lot of complicated physics and math led the observers to posit that a planet ten times the size of Earth, or roughly the size of Neptune, is making a wild swing around the sun about every 15,000 years.
Astronomers measure some distances by numbers of AUs. An AU is an “astronomical unit” – it’s the distance between the sun and the Earth (which, you will recall, is 93 million miles, give or take). Even when closest to the sun, this “Planet X” would still be seven times farther away than Neptune, which amounts to 200 AU. Being away for 15,000 years means that at its greatest distance it’s ending up very, very far afield indeed – perhaps 600 to 1200 AU. Do the math, and keep track of all those zeros. By any measure, that kind of orbit would expand the “neighborhood” of the solar system enormously.
Of course, while the physics seem to work out for such a scenario, it will take an actual image of this far-flung planet to be conclusive. This could be rather difficult given its extreme distance from decent lighting (ie, the sun), but it could be done, and with this good evidence in hand, the search is on. The Hubble and Keck telescopes are powerful enough to find it, but their field of view is limited. The Subaru telescope on Hawaii’s Big Island has both the power to find it and can search a much wider field. If it’s within that field, Planet X may well be found. Such a discovery is a planetary scientist’s dream – wouldn’t it be great to have the number of planets back to an even nine?