Are two massive planets lurking beyond the orbit of Pluto?

Credit: Scott S. Sheppard/Carnegie Institution for Science/Bob King (astrobob.areavoices.com)

Wednesday, June 18, 2014, 4:51 PM - Regardless of whether 'Planet X' conjures up memories of Saturday morning cartoons, astronomy classes or doomsday prophecies, the idea of a planet lurking beyond the outer solar system has been around since Pluto was first discovered in 1930. However, recent discoveries of more dwarf planets beyond Pluto have some astronomers thinking that there may be more to this than just an idea, and there may be more planets out there than just one.

Back in March, astronomers working with NASA's Wide-Field Infrared Survey Explorer (WISE) satellite announced that after a survey of the outer solar system, they found no evidence of any planet that matches the current ideas of what a 'Planet X' would be like - a massive planet the size of Saturn or Jupiter. This wasn't just a limited survey either. Their scan confirmed that no planet the size of Saturn is anywhere within around 1.5 trillion kilometres of the Sun, or 10,000 astronomical units (one AU is the distance from the Sun to the Earth), and no planet the size of Jupiter was seen out to nearly 4 trillion kilometres from the Sun (or 26,000 AU). This put to rest many doomsday prophecies that still held fast to the idea that a massive planet such as these was lurking out there, on a highly elliptical orbit that would someday bring it plowing through the inner solar system to cause global extinctions here on Earth. 

Even closer in, the telescope would have been able to spot a planet roughly the size of Neptune, at a distance of up to 700 AU (according to a 2009 estimate). 

The current limits of WISE are shown in the image below:

Credit: Penn State University

However, something smaller and/or cooler could escape WISE's attention, and astronomers are pointing to the orbits of dwarf planets 90377 Sedna and 2012 VP113 - found in 2003 and 2012 respectively - as clues for the possibility that not just one, but two planets between the size of Earth and Neptune might be out beyond Pluto. 

As Sedna and 2012 VP113, along with a few other smaller objects out there, orbit the Sun, they trace out ellipses that take them from just outside the Kuiper Belt and the orbit of Pluto to far, far away from the solar system and then back. However, these ellipses apparently show some interesting patterns of alignment that suggest something is tugging on these dwarf planets and other objects, shaping those orbits. There are, apparently, similar patterns in conditions of 'orbital resonance' - as seen with the moons of Jupiter and Saturn, and even between Neptune and Pluto. 

These dwarf planets and other objects are too small to influence each other that way, so it must be something larger doing it, which orbits far beyond them. 

The first indications of this came earlier this year, when a team of astronomers led by Scott Sheppard, of the Carnegie Institution for Science, suggested that it could be a planet roughly 10 times the mass of Earth, orbiting around 250 AU from the Sun. This wasn't a definite conclusion, though, since other explanations could account for it (the pull from nearby dwarf stars, like those found by WISE, or from stars or planets thrown clear of our solar system when it first formed). 

Now, though, the ideas of Sheppard's team are getting support from two researchers from the Complutense University of Madrid in Spain - Carlos and Raul de la Fuente Marcos. Their observations of Sedna, 2012 VP113 and their rocky/icy 'colleagues' shows the same evidence for a planet 10 times the mass of Earth, at 250 AU out, but also adds in another planet, somewhere between the size of Mars and Saturn, orbiting the Sun at a distance of 200 AU. The larger end of that spectrum would likely be ruled out by WISE's survey, but it's possible something smaller could be out there. 

Will this breathe new life into the doomsday prophecies? It might. They're always looking for a new angle, it seems. However, even the researchers note that their study is only based on a small number of observations, and it's probably a little early to be proclaiming anything definite about this. 

Perhaps with NASA's New Horizons spacecraft arriving at Pluto next year, and its subsequent extended mission to some target beyond that, it will provide more, and more detailed, observations, and possibly even confirm this incredible idea.