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Has a planet been discovered in the outer solar system? Not yet, but there is compelling evidence for one to be there.
OUT OF THIS WORLD | Earth, Space And The Stuff In Between - a daily journey through weather, space and science with meteorologist/science writer Scott Sutherland

Could a 'super-Earth' planet be lurking out beyond Pluto?

Scott Sutherland
Meteorologist/Science Writer

Thursday, January 21, 2016, 11:02 AM - Is there a massive planet, bigger than the Earth, lurking unseen in the outer solar system? Based on the latest word from Caltech astronomers, evidence appears to be mounting for the existence of a Planet Nine.

Focus your telescopes on the outer solar system, well beyond the orbit of Neptune and Pluto, and there appear to be some unusual coincidences going on there.

Several icy bodies, out beyond the orbit of Neptune and Pluto - as shown below - have highly elliptical orbits. While that's not necessarily unusual on its own (consider the orbits of comets from the Kuiper Belt and Oort Cloud), the strange part, is that all of their orbits are skewed to one side of the solar system.

The orbits of the six most distant objects we know about beyond Neptune and Pluto. Credit: Caltech/R. Hurt (IPAC)

What could account for this strange coincidence?

Based on computer simulations by Caltech researchers Konstantin Batygin and Mike Brown, it may be a "super-Earth" planet, roughly 10 times the mass of Earth, orbiting far out past Neptune and Pluto in an elliptical path that skews towards the other side of the solar system. Coming as close to the Sun as about 200 astronomical units (200 times farther away from the Sun than Earth), it would likely take around 20,000 years for this "Planet Nine" to go around once in its orbit.

The orbits of these objects and the orbit of a theoretical ninth planet that could explain the coincidence. Credit: Caltech/R. Hurt (IPAC)

If it's there, and it conforms to their simulations, Planet Nine could be big enough to clear its orbit, and thus qualify as a true planet, regardless of being so far away from the Sun.

"This would be a real ninth planet," said Brown, the Richard and Barbara Rosenberg Professor of Planetary Astronomy at the California Institute of Technology. "There have only been two true planets discovered since ancient times, and this would be a third. It's a pretty substantial chunk of our solar system that's still out there to be found, which is pretty exciting."

"Although we were initially quite skeptical that this planet could exist, as we continued to investigate its orbit and what it would mean for the outer solar system, we become increasingly convinced that it is out there," said Batygin, a Caltech assistant professor of planetary science. "For the first time in over 150 years, there is solid evidence that the solar system's planetary census is incomplete."

Is Planet Nine actually there?

The paper by Batygin and Brown gives some fairly compelling evidence for the existence of Planet Nine, but this does not mean that the planet actually exists.

It is possible that there are other explanations for what we're seeing with these distant objects. It's also possible that there are other, as of yet undiscovered objects with similar elliptical orbits that are equally distributed around the rest of the solar system's neighborhood. This would eliminate the need for a large planet like Planet Nine to explain their orbits.

It will take independently-verified sightings of this planet in order to actually confirm that it's real.

If it is there, how can we find it?

According to the blog The Search for Planet Nine, started by Batygin and Brown to gather their thoughts and perhaps progress on the study of and search for this potential new world, there are certainly ways that his planet can be detected.

A search by NASA's WISE satellite has already shown that there are no planets of specific sizes in the outer solar system.

WISE detection limits compared to mass of solar system giants and distances. Credit: Penn State University

Added to the image above is the theoretical mass and orbital distance of Planet Nine. It falls well within the detection limits of WISE, but yet nothing was found.

Searches through data from skywatching surveys, such as the Catalina Sky Surveys and Pan STARRS, have similarly turned up nothing.

There are limitations on their search, however. If the planet is near its closest approach, around 200 AU out, it might already have been spotted. If it is much farther away, closer to 500 AU from the Sun, it becomes much more difficult, especially since that end of its orbit is very near the galactic plane, meaning that it could get lost in the background "noise" of the light of the Milky Way.

There is still some hope for a sighting, however. Continued work by Catalina and Pan STARRS, along with updated WISE surveys using more sensitive instruments, and a better understanding of exactly what the detection limits are for a 10 Earth-mass planet, could result in a detection at some point.

Batygin and Brown have even made a crude plot of the various surveys and what coverage of Planet Nine's position in the sky each covers.

Coverage of Planet Nine's projected brightness vs position in the sky from Pan STARRS (PS moving and PS transient), Catalina Sky Survey and WISE. Credit: Batygin and Brown

From here, the next steps are to run more computer simulations, varying the mass and distance of the theoretical planet, while continuing to monitor these surveys for a potential sighting, and as always, for more researchers to continue to explore the strange behaviour of these distant icy bodies, in case some other scenario provides an even better explanation.

What could this mean if it is found?

As I mentioned on Twitter shortly after the announcement, there is a very interesting aspect to this potential discovery.

The most abundant type of exoplanet discovered out in the galaxy so far is the "super-Earth," a planet larger than Earth but smaller than Neptune. However, there are no super-Earths in our solar system. By comparison to the vast number of planetary systems we are seeing out in our galaxy, this fact makes ours the strange one.

If Planet Nine were confirmed to exist, we would join this large "Club Super-Earth," making our solar system less weird. By extension of that connection, we could even say that with our solar system being less of the "odd" one, other aspects about our solar system (ie: the definite presence of life) may also be less odd. That would give us more hope of finding life out there in the galaxy. 

Source: Caltech | Search for Planet Nine

Watch Below: NASA’s Director of Planetary Science, Jim Green, discusses the Jan. 20 Astronomical Journal science paper that points to the possibility of a new “Planet Nine” in our solar system.

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