The possibility of a planet lurking in the outer reaches of the solar system has gained new ground, based on the orbits of recently discovered objects. There is a new twist to the latest evidence, however, with suggestions of not one but two large planets at mind-bending distances from the Sun.
The quest for a "Planet X" beyond Neptune has been going on for more than a century. Recently, two dwarf planets Sedna and 2102 VP113 have been identified with orbits extending to distances hundreds of times further from the Sun than our own.
Distant as these orbits are, they are too close to be part of the Oort Cloud, a collection of comets that mostly orbit at distances beyond 5000 AU.
Instead it is thought that these objects formed closer to the sun. The gravitational influence of a large planet is one explanation of how their orbits changed. The theory has its own problems – if we can’t explain how objects like these came to be orbiting at such distances, then it’s equally unclear how a theoretical planet came to be there.
Scott Sheppard, of the Carnegie Institution for Science, and the Gemini Observatory's Chad Trujillo noted a clustering in the orbits of the solar system’s most distant known entities, many of which they had discovered. Ten Kuiper Belt Objects, and minor planets Sedna and 2012 VP113, all have orbits that cross the plane of the solar system at angles that range from shallow to steep.
Yet all of these distant objects reach their closest point to the sun just when they are near the plane the planets circle in. The scientists considered this unlikely to be a coincidence, and speculate it might be a sign of a planet influencing all of their orbits.
In Monthly Notices of the Royal Astronomical Society Letters brothers Carlos and Raul de la Fuente Marcos of Complutense University of Madrid have taken this a step further. “The analysis of several possible scenarios strongly suggest that at least two trans-Plutonian planets must exist,” they conclude.
Even more recently, Lorenzo Iorio of the Italian Ministry of Education, Universities and Research has argued in the same journal that if planet X exists, it must be much further out than Trujillo and Sheppard proposed. How far it would need to be depends on its mass, but an unknown object twice as heavy as the Earth could not be less than 500 AU from the Sun, Iorio maintains.
Other astronomers are more cautious. David Jewitt of the University of California, Los Angeles told Science News, "The outer solar system can be full of all sorts of unseen and interesting things,” Jewitt says, “but the argument ... for a massive perturber is a bit puzzling.” Jewitt notes that if the Kuiper Belt Objects in the Trujillo/Sheppard study have a planet keeping them in line, it may well be Neptune. Sedna and 2012 VP113 are too far out for this to be true for them as well, but it is far easier to explain two orbits as coincidences than twelve.
While the question may only be finally settled by the discovery of a large planet lurking in space, a number of teams have redoubled their efforts to find modest sized objects whose orbits might help us lend credence to, or reject, the theories proposed so far.