Planet
X9 from Outer Space?
This
week’s (Jan 20, 2016) Astrophysical Journal has a paper by two
researchers (K. Bayugin and M. Brown) announcing the possible
discovery of another planet in the solar system. I say possible,
because the planet’s existence is deduced from the orbits of
several dwarf planets, beyond the orbit of Pluto (Kuiper Belt objects
or KBOs). They argue that the orbits of those bodies are best
explained by a large (Neptunish sized) planet in the far reaches of
the solar system, which is perturbing the orbits of the smaller dwarf
planets.
This
is a well-established planet hunter technique, going back to the
discovery of Neptune, which was based on orbital anomalies in the
orbit of Uranus. The search for Pluto was also motivated by
gravitational arguments, though it turns out to be too small to have
the effects thought to have been seen in the orbit of Neptune.
Mostly, its discovery was fortuitous – gravitational arguments just
happened to lead Claude Tombaugh to look in the general area where
Pluto happened to be. However, many extra-solar planets have also
been discovered via gravitational means, usually the effect of large
planets close in to their star, which creates a sort of jitter in the
stars motion, which can be detected from Doppler shifts in the star’s
spectrum.
I
am calling this planet X9, as a bit of an astrophysics/science
fiction gag on my part. The X can stand for “unknown”. But it
can also stand for 10, as in Roman numerals, since it would be the
10th
planet under the old system, which had Pluto as the 9th.
But, since Pluto has been demoted to “dwarf planet”, the new
planet is really number 9. Thus, via a sort of planet naming
mash-up, I will call it Planet X9. Plus it reminds one of “Plan 9
from Outer Space”, one of the all-time best good-bad science
fiction movies.
I
will do my best to interpret their paper in the text below, though
with only a
humble B.Sc. in physics,
I am no expert on orbital mechanics (or automobile mechanics, for
that matter).
One
reason for thinking there may be another planet out there, is evident
in the following picture, from their paper:
The
image can be thought of as looking at the solar system from a view
above the plane of the ecliptic (the common plane in which the
planets orbit around the sun), rather like looking at the Earth from
far above the north pole might be. The ellipses (ovals) show the
orbits of some large KBOs. As you can see, their perihelia (the
closest point of the orbit to the sun) are all clustering on the same
side of the sun, in a fairly narrow range. The authors of the paper
have calculated that this is a very unlikely configuration, an
almost certainly not based
on pure chance, so there must be something behind the process. They
think that a large planet in the far reaches of the solar system
would produce this result.
There
is some possibility that this result is due to observational bias
(the tendency to see more things in one area of the sky because
that’s where you spend the most time looking), but the authors say
that is unlikely – though some bias might be present due to there
being an observational bias towards the plane of the ecliptic, that
doesn’t explain the clustering in ecliptic longitude that is seen.
The
paper then goes into both an analytical analysis and a numeric
analysis of what gravitationally perturbing bodies might account for
the behaviour of the KBOs. The former involves a lot equations based
on standard gravitational theory, which demonstrate that the observed
behaviour can be produced by the presence of an unseen planetary
mass. However, they note that the analytical solution has many
simplifying assumptions built in to it, so some skepticism is
warranted.
They
then offer a numeric solution via computer simulations - in other
words direct N-body simulations of the solar system. With this
method, they could vary inputs to the model, see how the computer
simulation evolved, then compare that to the known observations.
They could then derive such parameters as the distance to the
perturbing planet, the planets orbital eccentricity and relationship
to the KBOS, and planetary mass. The surprising result of this
analysis, is that the perturbing planet is favoured to be opposite to
the KBOs, as this image from a CBC story shows:
They
tried simulations using various masses for the unseen planet. Though
an Earth size mass could work, a larger mass planet (10
Earth masses) fit
the simulations much better and is therefore considered a more
realistic choice.
Simulations
also show that the perihelion (closest approach to the sun) of such a
planet would be very distant – something on the order of 250 AU
(astronomical units, equal to the distance of the Earth’s orbit
from the sun). For comparison, Pluto’s perihelion is about 30 AU,
so this planet would be about 8 times farther away.
Finally,
the simulations indicate that the unknown planet’s orbit may be
inclined to the plane of the ecliptic, but probably not by a very
large amount. In other words, it may be orbiting in a different
plane than the Earth but not radically different (within a few tens
of degrees). Again for comparison, Pluto is inclined to the plane
of the ecliptic by about 17 degrees.
The
paper notes that this theory has some testable implications, as more
KBOs are discovered, in terms of their inclinations, and so forth.
It also notes that the ultimate test of the theory is to actually
observe the possible planet. The paper acknowledges that the search
space is still very wide, based on the theory developed. But, who
knows, once the search starts in earnest, we may have a result sooner
than we expect.
As
for the origin of such a large
planet at this
great distance from the sun, they speculate that it may be that this
is a gas giant
that was ejected from the inner solar system (inner relative to 250+
AU) during the early stages of its evolution.
Here's
a link to the article:
There
are now thought to be potentially billions of rogue planets in the
galaxy, based on micro-lensing findings. Could this be one of these?
The paper doesn’t speculate about that, but it seems to
me that's a
possibility worth
thinking about.
One
would expect a world so far from the sun to be extremely cold, but
some of the theoretical work on rogue planets has indicated that they
could in fact have warm interiors under a layer of ice, perhaps even
liquid oceans. Maybe this planet is the same, if it exists. But it
would take an awfully long time for a space probe to get there and do
some close-in measurements, should we ever pin its location down.
----------------------------------------------------------------------
And
here’s a science fiction novel which includes plenty of action on
an unexplored planet, in
the far reaches of the galaxy.
It's only
$3.99 :)
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