From Newsgroup: sci.misc
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
planetrCOs existence through mathematical modeling and computer simulations
but have not yet observed the object directly.
rCLThis would be a real ninth planet,rCY says Brown, the Richard and Barbara
Rosenberg Professor of Planetary Astronomy. rCLThere have only been two true
planets discovered since ancient times, and this would be a third. ItrCOs a
pretty substantial chunk of our solar system thatrCOs still out there to be
found, which is pretty exciting.rCY
Brown notes that the putative ninth planetrCoat 5,000 times the mass of
PlutorCois 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 planetsrCoa fact that Brown says makes it rCLthe most planet-y of
the planets in the whole solar system.rCY
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.
rCLAlthough 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,rCY says Batygin, an assistant professor of planetary science. rCLFor
the first time in over 150 years, there is solid evidence that the solar
systemrCOs planetary census is incomplete.rCY
The road to the theoretical discovery was not straightforward. In 2014, a
former postdoc of BrownrCOs, 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 perspectivesrCoBrown 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 otherrCOs ideas
and to consider new possibilities. rCLI would bring in some of these
observational aspects; he would come back with arguments from theory, and
we would push each other. I donrCOt think the discovery would have happened
without that back and forth,rCY says Brown. rCL It was perhaps the most fun
year of working on a problem in the solar system that IrCOve ever had.rCY
Fairly quickly Batygin and Brown realized that the six most distant
objects from Trujillo and ShepherdrCOs 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.
rCLItrCOs almost like having six hands on a clock all moving at different
rates, and when you happen to look up, theyrCOre all in exactly the same
place,rCY 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 wayrCopointing 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. rCLBasically it
shouldnrCOt happen randomly,rCY Brown says. rCLSo we thought something else must
be shaping these orbits.rCY
The first possibility they investigated was that perhaps there are enough
distant Kuiper Belt objectsrCosome of which have not yet been discoveredrCoto
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. rCLClose, but no cigar,rCY
he says.
Then, effectively by accident, Batygin and Brown noticed that if they ran
their simulations with a massive planet in an anti-aligned orbitrCoan orbit
in which the planetrCOs closest approach to the sun, or perihelion, is 180
degrees across from the perihelion of all the other objects and known
planetsrCothe distant Kuiper Belt objects in the simulation assumed the
alignment that is actually observed.
rCLYour natural response is rCyThis orbital geometry canrCOt be right. This
canrCOt be stable over the long term because, after all, this would cause
the planet and these objects to meet and eventually collide,rCOrCY 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.
rCLStill, I was very skeptical,rCY says Batygin. rCLI had never seen anything
like this in celestial mechanics.rCY
But little by little, as the researchers investigated additional features
and consequences of the model, they became persuaded. rCLA good theory
should not only explain things that you set out to explain. It should
hopefully explain things that you didnrCOt set out to explain and make
predictions that are testable,rCY says Batygin.
And indeed Planet NinerCOs 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 rCLkicked
outrCY 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.
rCLSuddenly I realized there are objects like that,rCY 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. rCLWe plotted up the positions of those objects and their orbits,
and they matched the simulations exactly,rCY says Brown. rCLWhen we found
that, my jaw sort of hit the floor.rCY
rCLWhen the simulation aligned the distant Kuiper Belt objects and created
objects like Sedna, we thought this is kind of awesomerCoyou kill two birds
with one stone,rCY says Batygin. rCLBut 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 didnrCOt realize was sitting in a nearby
tree.rCY
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 planetsrCoJupiter, Saturn, Uranus, and Neptune. Over
time, collisions and ejections shaped them and moved them out to their
present locations. rCLBut there is no reason that there could not have been
five cores, rather than four,rCY 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 planetrCOs orbit and its influence on the distant solar system.
Meanwhile, Brown and other colleagues have begun searching the skies for
Planet Nine. Only the planetrCOs 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 worldrCOs largest telescopesrCosuch as the twin
10-meter telescopes at the W. M. Keck Observatory and the Subaru
Telescope, all on Mauna Kea in HawaiirCowill be needed to see it. If,
however, Planet Nine is now located anywhere in between, many telescopes
have a shot at finding it.
rCLI would love to find it,rCY says Brown. rCLBut IrCOd also be perfectly happy if
someone else found it. That is why werCOre publishing this paper. We hope
that other people are going to get inspired and start searching.rCY
In terms of understanding more about the solar systemrCOs 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 rangerCothat 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.
rCLOne 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,rCY says Batygin. rCLUntil now, werCOve thought
that the solar system was lacking in this most common type of planet.
Maybe werCOre more normal after all.rCY
Brown, well known for the significant role he played in the demotion of
Pluto from a planet to a dwarf planet adds, rCLAll 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,rCY he says. rCLNow we can go and find
this planet and make the solar system have nine planets once again.rCY
The paper is titled rCLEvidence for a Distant Giant Planet in the Solar
System.rCY
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