This
artistic rendering shows the distant view from Planet Nine back towards
the sun. The planet is thought to be gaseous, similar to Uranus and
Neptune. Hypothetical lightning lights up the night side. Credit: Caltech/R. Hurt (IPAC)
The
six most distant known objects in the solar system with orbits
exclusively beyond Neptune (magenta) all mysteriously line up in a
single direction. Also, when viewed in three dimensions, they tilt
nearly identically away from the plane of the solar system. Batygin and
Brown show that a planet with 10 times the mass of the earth in a
distant eccentric orbit anti-aligned with the other six objects (orange)
is required to maintain this configuration.Credit: Caltech/R. Hurt (IPAC); [Diagram created using WorldWide Telescope.]
Caltech's
Konstantin Batygin, an assistant professor of planetary science, and
Mike Brown, the Richard and Barbara Rosenberg Professor of Planetary
Astronomy, discuss new research that provides evidence of a giant planet
tracing a bizarre, highly elongated orbit in the outer solar system.Credit: Caltech AMT
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.]
Caltech
professor Mike Brown and assistant professor Konstanin Batygin have
been working together to investigate distant objects in our solar system
for more than a year and a half. The two bring very different
perspectives to the work: Brown is an observer, used to looking at the
sky to try and anchor everything in the reality of what can be seen;
Batygin is a theorist who considers how things might work from a physics
standpoint. Credit: Credit: Lance Hayashida/Caltech
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.
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
