Caltech Researchers Find Evidence of a Real Ninth Planet

Source: https://astrobiology.nasa.gov/news/caltech-researchers-find-evidence-of-a-real-ninth-planet/

   Jan. 20, 2016
   Feature Story


   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 Shepherd
   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.

   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.]
   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.”