[meteorite-list] Caltech Researchers Find Evidence of a Real Ninth Planet

[Ron Baalke]

http://www.caltech.edu/news/caltech-researchers-find-evidence-real-ninth-planet-49523

Caltech Researchers Find Evidence of a Real Ninth Planet
Caltech
January 20, 2016

Caltech researchers have found evidence of a giant planet tracing a bizarre, 
highly elongated orbit in the outer solar system. The object, which the 
researchers have nicknamed Planet Nine, has a mass about 10 times that 
of Earth and orbits about 20 times farther from the sun on average than 
does Neptune (which orbits the sun at an average distance of 2.8 billion 
miles). In fact, it would take this new planet between 10,000 and 20,000 
years to make just one full orbit around the sun.

The researchers, Konstantin Batygin and Mike Brown, discovered the planet's 
existence through mathematical modeling and computer simulations but have 
not yet observed the object directly.

"This would be a real ninth planet," says Brown, the Richard and Barbara 
Rosenberg Professor of Planetary Astronomy. "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."

Brown notes that the putative ninth planet - at 5,000 times the mass of 
Pluto - is sufficiently large that there should be no debate about whether 
it is a true planet. Unlike the class of smaller objects now known as 
dwarf planets, Planet Nine gravitationally dominates its neighborhood 
of the solar system. In fact, it dominates a region larger than any of 
the other known planets - a fact that Brown says makes it "the most planet-y 
of the planets in the whole solar system."

Batygin and Brown describe their work in the current issue of the Astronomical 
Journal and show how Planet Nine helps explain a number of mysterious 
features of the field of icy objects and debris beyond Neptune known as 
the Kuiper Belt.

"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," 
says Batygin, an 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."

The road to the theoretical discovery was not straightforward. In 2014, 
a former postdoc of Brown's, Chad Trujillo, and his colleague Scott Sheppard 
published a paper noting that 13 of the most distant objects in the Kuiper 
Belt are similar with respect to an obscure orbital feature. To explain 
that similarity, they suggested the possible presence of a small planet. 
Brown thought the planet solution was unlikely, but his interest was piqued.

He took the problem down the hall to Batygin, and the two started what 
became a year-and-a-half-long collaboration to investigate the distant 
objects. As an observer and a theorist, respectively, the researchers 
approached the work from very different perspectives - Brown as someone 
who looks at the sky and tries to anchor everything in the context of 
what can be seen, and Batygin as someone who puts himself within the context 
of dynamics, considering how things might work from a physics standpoint. 
Those differences allowed the researchers to challenge each other's ideas 
and to consider new possibilities. "I would bring in some of these observational 
aspects; he would come back with arguments from theory, and we would push 
each other. I don't think the discovery would have happened without that 
back and forth," says Brown. " It was perhaps the most fun year of working 
on a problem in the solar system that I've ever had."

Fairly quickly Batygin and Brown realized that the six most distant objects 
from Trujillo and Shepherd's original collection all follow elliptical 
orbits that point in the same direction in physical space. That is particularly 
surprising because the outermost points of their orbits move around the 
solar system, and they travel at different rates.

"It's almost like having six hands on a clock all moving at different 
rates, and when you happen to look up, they're all in exactly the same 
place," says Brown. The odds of having that happen are something like 
1 in 100, he says. But on top of that, the orbits of the six objects are 
also all tilted in the same way - pointing about 30 degrees downward in 
the same direction relative to the plane of the eight known planets. The 
probability of that happening is about 0.007 percent. "Basically it shouldn't 
happen randomly," Brown says. "So we thought something else must be shaping 
these orbits."

The first possibility they investigated was that perhaps there are enough 
distant Kuiper Belt objects - some of which have not yet been discovered - to 
exert the gravity needed to keep that subpopulation clustered together. 
The researchers quickly ruled this out when it turned out that such a 
scenario would require the Kuiper Belt to have about 100 times the mass 
it has today.

That left them with the idea of a planet. Their first instinct was to 
run simulations involving a planet in a distant orbit that encircled the 
orbits of the six Kuiper Belt objects, acting like a giant lasso to wrangle 
them into their alignment. Batygin says that almost works but does not 
provide the observed eccentricities precisely. "Close, but no cigar," 
he says.

Then, effectively by accident, Batygin and Brown noticed that if they 
ran their simulations with a massive planet in an anti-aligned orbit - an 
orbit in which the planet's closest approach to the sun, or perihelion, 
is 180 degrees across from the perihelion of all the other objects and 
known planets - the distant Kuiper Belt objects in the simulation assumed 
the alignment that is actually observed.

"Your natural response is 'This orbital geometry can't be right. This 
can't be stable over the long term because, after all, this would cause 
the planet and these objects to meet and eventually collide,'" says Batygin. 
But through a mechanism known as mean-motion resonance, the anti-aligned 
orbit of the ninth planet actually prevents the Kuiper Belt objects from 
colliding with it and keeps them aligned. As orbiting objects approach 
each other they exchange energy. So, for example, for every four orbits 
Planet Nine makes, a distant Kuiper Belt object might complete nine orbits. 
They never collide. Instead, like a parent maintaining the arc of a child 
on a swing with periodic pushes, Planet Nine nudges the orbits of distant 
Kuiper Belt objects such that their configuration with relation to the 
planet is preserved.

"Still, I was very skeptical," says Batygin. "I had never seen anything 
like this in celestial mechanics."

But little by little, as the researchers investigated additional features 
and consequences of the model, they became persuaded. "A good theory should 
not only explain things that you set out to explain. It should hopefully 
explain things that you didn't set out to explain and make predictions 
that are testable," says Batygin.

And indeed Planet Nine's existence helps explain more than just the alignment 
of the distant Kuiper Belt objects. It also provides an explanation for 
the mysterious orbits that two of them trace. The first of those objects, 
dubbed Sedna, was discovered by Brown in 2003. Unlike standard-variety 
Kuiper Belt objects, which get gravitationally "kicked out" by Neptune 
and then return back to it, Sedna never gets very close to Neptune. A 
second object like Sedna, known as 2012 VP113, was announced by Trujillo 
and Shepherd in 2014. Batygin and Brown found that the presence of Planet 
Nine in its proposed orbit naturally produces Sedna-like objects by taking 
a standard Kuiper Belt object and slowly pulling it away into an orbit 
less connected to Neptune.

[Graphic]
A predicted consequence of Planet Nine is that a second set of confined 
objects should also exist. These objects are forced into positions at 
right angles to Planet Nine and into orbits that are perpendicular to 
the plane of the solar system. Five known objects (blue) fit this prediction 
precisely.
Credit: Caltech/R. Hurt (IPAC) [Diagram was created using WorldWide Telescope.]

But the real kicker for the researchers was the fact that their simulations 
also predicted that there would be objects in the Kuiper Belt on orbits 
inclined perpendicularly to the plane of the planets. Batygin kept finding 
evidence for these in his simulations and took them to Brown. "Suddenly 
I realized there are objects like that," recalls Brown. In the last three 
years, observers have identified four objects tracing orbits roughly along 
one perpendicular line from Neptune and one object along another. "We 
plotted up the positions of those objects and their orbits, and they matched 
the simulations exactly," says Brown. "When we found that, my jaw sort 
of hit the floor."

"When the simulation aligned the distant Kuiper Belt objects and created 
objects like Sedna, we thought this is kind of awesome - you kill two 
birds with one stone," says Batygin. "But with the existence of the planet 
also explaining these perpendicular orbits, not only do you kill two birds, 
you also take down a bird that you didn't realize was sitting in a nearby 
tree."

Where did Planet Nine come from and how did it end up in the outer solar 
system? Scientists have long believed that the early solar system began 
with four planetary cores that went on to grab all of the gas around them, 
forming the four gas planets - Jupiter, Saturn, Uranus, and Neptune. Over 
time, collisions and ejections shaped them and moved them out to their 
present locations. "But there is no reason that there could not have been 
five cores, rather than four," says Brown. Planet Nine could represent 
that fifth core, and if it got too close to Jupiter or Saturn, it could 
have been ejected into its distant, eccentric orbit.

Batygin and Brown continue to refine their simulations and learn more 
about the planet's orbit and its influence on the distant solar system. 
Meanwhile, Brown and other colleagues have begun searching the skies for 
Planet Nine. Only the planet's rough orbit is known, not the precise location 
of the planet on that elliptical path. If the planet happens to be close 
to its perihelion, Brown says, astronomers should be able to spot it in 
images captured by previous surveys. If it is in the most distant part 
of its orbit, the world's largest telescopes - such as the twin 10-meter 
telescopes at the W. M. Keck Observatory and the Subaru Telescope, all 
on Mauna Kea in Hawaii - will be needed to see it. If, however, Planet 
Nine is now located anywhere in between, many telescopes have a shot at 
finding it.

"I would love to find it," says Brown. "But I'd also be perfectly happy 
if someone else found it. That is why we're publishing this paper. We 
hope that other people are going to get inspired and start searching."

In terms of understanding more about the solar system's context in the 
rest of the universe, Batygin says that in a couple of ways, this ninth 
planet that seems like such an oddball to us would actually make our solar 
system more similar to the other planetary systems that astronomers are 
finding around other stars. First, most of the planets around other sunlike 
stars have no single orbital range - that is, some orbit extremely close 
to their host stars while others follow exceptionally distant orbits. 
Second, the most common planets around other stars range between 1 and 
10 Earth-masses.

"One of the most startling discoveries about other planetary systems has 
been that the most common type of planet out there has a mass between 
that of Earth and that of Neptune," says Batygin. "Until now, we've thought 
that the solar system was lacking in this most common type of planet. 
Maybe we're more normal after all."

Brown, well known for the significant role he played in the demotion of 
Pluto from a planet to a dwarf planet adds, "All those people who are 
mad that Pluto is no longer a planet can be thrilled to know that there 
is a real planet out there still to be found," he says. "Now we can go 
and find this planet and make the solar system have nine planets once 
again."

The paper is titled "Evidence for a Distant Giant Planet in the Solar 
System."

Written by Kimm Fesenmaier

Contact: 
Deborah Williams-Hedges
(626) 395-3227
debwms at caltech.edu