More evidence for ninth planet roaming solar system's outer fringes

October 19, 2016, University of Arizona
Known objects in the Kuiper belt beyond the orbit of Neptune. (Scale in AU; epoch as of January 2015.). Credit: Wikipedia/CC BY-SA 3.0

As the search for a hypothetical, unseen planet far, far beyond Neptune's orbit continues, research by a team of the University of Arizona provides additional support for the possible existence of such a world and narrows the range of its parameters and location.

Led by Renu Malhotra, a Regents' Professor of Planetary Sciences in the UA's Lunar and Planetary Lab, the team found that the four Kuiper Belt Objects with the longest known orbital periods revolve around the Sun in patterns most readily explained by the presence of a hypothetical "Planet Nine" approximately ten times the mass of Earth. Malhotra is presenting the results at the joint 48th meeting of the Division for Planetary Sciences of the American Astronomical Society and 11th European Planetary Science Congress in Pasadena, California.

According to the researchers' calculations, such a hypothetical planet would complete one around the Sun roughly every 17,000 years and, at its farthest point from our central star, it would swing out more than 660 astronomical units, with one AU being the average distance between the Earth and the Sun.

Scientists think that objects in the Kuiper Belt, a vast region of dwarf planets and icy rocks populating the fringes of our solar system beyond the orbit of Neptune, dance mostly to the tune of the giant planets, Saturn, Jupiter, Uranus and Neptune, influenced by their gravity either directly or indirectly.

However, there are a few known Kuiper Belt objects (KBOs) that are unlikely to be significantly perturbed by the known giant planets in their current orbits. Referred to as "extreme KBOs" (eKBOs) by the authors, all of these have extremely large orbital eccentricities, in other words, they get very close to the Sun at one point on their orbital journey, only to swing far out into space once they pass the Sun, on long elliptical orbits that take these strange mini worlds hundreds of AUs away from the Sun.

"We analyzed the data of these most distant Kuiper Belt objects," Malhotra said, "and noticed something peculiar, suggesting they were in some kind of resonances with an unseen planet."

In their paper, "Corralling a Distant Planet with Extreme Resonant Kuiper Belt Objects," Malhotra and her co-authors, Kathryn Volk and Xianyu Wang, point out peculiarities of the orbits of the extreme KBOs that went unnoticed until now: they found that the orbital period ratios of these objects are close to ratios of small whole numbers. An example of this would be one KBO traveling around the Sun once while another takes twice as long, or three times as long, or four times as long etc., but not, say, 2.7 times as long.

According to the authors, such ratios could arise most naturally if the extreme KBOs' orbital periods are in small whole number ratios with a massive planet, which would help to stabilize the highly elliptical orbits of eKBOs.

The findings bolster previous work by other scientists that showed that six of those bodies travel on highly eccentric orbits whose long axes all point in the same direction. This clustering of orbital parameters of the most distant KBOs suggested a large, planetary size body shepherding their orbits.

Another paper published earlier this year presented the results of numerical simulations providing a range of possibilities for the mass and orbit for such a hypothetical planet, that could account for the observed clustering of eKBO orbits.

"Our paper provides more specific estimates for the mass and orbit that this planet would have, and, more importantly, constraints on its current position within its orbit," Malhotra said.

The team's calculations also suggest two likely orbital planes for the planet: one moderately close to the mean plane of the solar system and near the mean plane of the four eKBOs at about 18 degrees, and one steeper plane, inclined at about 48 degrees.

While the results provide additional support for the idea of a potential "Planet Nine" and lay out possible scenarios, the authors stress that their paper should not be considered definitive proof of the planet's existence.

For one, the very far and faint KBOs haven't been observed for very long, and given their minuscule apparent motion along their immensely long journeys around the Sun, the estimates for their closeness to whole number ratios of orbital periods come with uncertainties that can be narrowed down only through more observations.

The authors also note that the long orbital timescales in this region of the outer solar system may allow formally unstable orbits to persist for very long times, possibly even to the age of the solar system, without the help of orbital resonances. In this scenario, orbits whose orderly parameters appear as testimony to the stabilizing influence of an unseen planet may in fact be in the process of deterioration but haven't been observed long enough for it to show.

Future observations and studies into the dynamical lifetimes of non-resonant planet-crossing orbits in the far regions of the outer solar system could help to further test the case for the existence and whereabouts of a ninth planet, Malhotra and her co-authors write.

Explore further: Kuiper Belt objects point the way to Planet 9

More information: "Corralling a Distant Planet with Extreme Resonant Kuiper Belt Objects," Renu Malhotra, Kathryn Volk & Xianyu Wang, 2016 June 20, Astrophysical Journal letters … /2041-8205/824/2/L22 , Arxiv:

Related Stories

Kuiper Belt objects point the way to Planet 9

March 14, 2016

On January 20th, 2016, researchers Konstantin Batygin and Michael E. Brown of Caltech announced that they had found evidence that hinted at the existence of a massive planet at the edge of the Solar System. Based on mathematical ...

Curious tilt of the sun traced to undiscovered planet

October 19, 2016

Planet Nine—the undiscovered planet at the edge of the Solar System that was predicted by the work of Caltech's Konstantin Batygin and Mike Brown in January 2016—appears to be responsible for the unusual tilt of the sun, ...

Newly discovered solar system objects resonate with Neptune

July 21, 2016

The search for distant solar system objects has found two more small worlds far outside the orbit of Neptune. The new objects are located beyond the Kuiper Belt, which is a belt of small icy objects just beyond Neptune, of ...

Recommended for you

Exoplanet stepping stones

November 20, 2018

Astronomers have gleaned some of the best data yet on the composition of a planet known as HR 8799c—a young giant gas planet about 7 times the mass of Jupiter that orbits its star every 200 years.


Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Oct 19, 2016
660 AU sounds like a perfect place for a FOCAL style mission. A space telescope sent there when it's at more than 550 AU distance would be in the right focal distance from the sun. Then it could use this planet (and it's moons) for gravity assisted manoeuvres and then use them as giant sunshades to block sunlight, while using the sun gravitatational lensing for directly imaging some exoplanets with decent detail. Still very difficult , but having it there could be much better than nothing.
5 / 5 (2) Oct 19, 2016
Good work. Very interesting and well written, with appropriate caveats. This is a great opportunity for the University of Arizona. Looks like they are in the hunt, among the leaders, with an excellent team and all the resources.

They just might find this wildcat--Planet 9.
not rated yet Oct 20, 2016
Let's name it "{insert your least favorite politician}". He/She is very much on the fringes of political sanity.
Mark Thomas
1 / 5 (1) Oct 20, 2016
A nice bit of math. So take the next logical step and compare the predicted results with current searches and archival data from the Catalina Sky Survey, Pan-STARRS and especially WISE.

"An infrared survey by the Wide-field Infrared Survey Explorer (WISE) in 2009 allowed for a Neptune-sized object beyond 700 AU."


At first glance, a 10 earth mass planet at "~665 AU" seems possible, but it is right at the edge of what WISE would have detected, so it is not clear how much the predicted results help or hurt the chances of Planet Nine currently existing. One has to wonder if we really missed something that big at 665 AU.

So far it has been about ten months since Brown and Batygin (B&B) announced and the clock is still ticking. B&B made a good argument that something perturbed all the orbits they highlighted to infer Planet 9, but my money is on Planet Nine being a wild goose chase and time is on my side.
Mark Thomas
not rated yet Oct 20, 2016
B&B: "We compare the allowed orbital positions and estimated brightness of Planet Nine to previous and ongoing surveys which would be sensitive to the planet's detection and use these surveys to rule out approximately two-thirds of the planet's orbit." (March 17, 2016)

We don't know yet whether Planet Nine exists or not, but eventually it will run out of places to hide.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.