Astronomy

Kuiper Belt objects away from the Kuiper Belt. And, in
a result we did not expect, it would alter these elongated
orbits, twisting them to be perpendicular to the
plane of the planets and driving their perihelia inside
of Saturn’s orbit. We didn’t know off the tops of our
heads if any such strange orbits existed because,
embarrassingly, we had overlooked Gomes’ paper,
which had come out just as we were in the intensive
last phases of our analysis. But when we saw the
objects pointed out by Gomes, we grew excited. Our
planet theory not only predicts objects like that, but it
also predicts exactly how those orbits should be
aligned. We quickly plotted the locations of these distant,
twisted orbits, and, to our astonishment, the
orbits were precisely where we predicted them to be.
For us, this moment marked moving from working
on an interesting hypothesis about some unusual orbital
alignments to instantly realizing that we were talking
about something that was really out there. This was
something waiting to be found, something that both
explained the old observations we were working on and
also crystallized correct predictions about things we
were completely unaware of. We like to think of this as
the day that Planet Nine was born.
With the many effects that Planet Nine is having on
the outer solar system, we can infer many things about
its properties. In practice, because the solar system is a
complicated place, understanding these properties has
involved massive amounts of computer simulation. We
simulate a slightly larger planet, a slightly closer planet,
a slightly more inclined planet, and each time we compare
the results of our simulations with observations of
the solar system that we know.
From these constraints we have determined that
Planet Nine is about 10 times the mass of Earth, that
its orbit is inclined by approximately 30 degrees to the
plane of the planets, that it has an average distance of
something like 600 AU from the Sun, and that when it
is at its most distant point from the Sun, it lies toward
the outstretched arm of the constellation Orion.
All of this relatively detailed knowledge might
make it seem like we could, like
Le Verrier, simply say to the world, “Go
look; it will be THERE!” But we can’t.
Le Verrier had the advantage of being
able to analyze the full orbit of Uranus
around the Sun to see its deviations. If
we waited 10,000 years to fully track
Sedna around its orbit, we, too, would be able to pinpoint
Planet Nine.
Instead, though, we have only a snapshot of the
orbits of a variety of different objects, and we must
infer what should have happened in the past. In practical
terms, that means that although we know the
orbital path of Planet Nine through the sky, we don’t
know where it is in its orbit. We no longer have to
search the entire sky to find Planet Nine, but there’s
still a lot of work to do.
The search will not be as hard as it might have
been, however, as many sky surveys over the past
few years have covered large swaths of the sky and
might have detected Planet Nine had it been in their
region. We know, for example, that when Planet Nine
is at its perihelion, it is as bright as 18th magnitude,
lying in the southern sky near the constellation Ophiuchus.
Such an object would have been detected years
earlier. Most likely, Planet Nine is now closer to its
aphelion, where it would glow dimly, likely close to
25th magnitude.
While that is very faint, detecting such an object is
well within the capabilities of the 8-meter Subaru Telescope
on Mauna Kea and its impressive Hyper
Suprime-Cam, a mosaic of 112 CCD cameras covering
nearly two square degrees of sky with every exposure.
We have already begun our search using this telescope.
Other astronomers are likely to follow.
Is Planet Nine really out there? It’s always wise
to be skeptical, but still, we are quite convinced that
the answer is yes. Something must be responsible for
all the unusual orbits that we now see in the outer
solar system. Planet Nine is by far the most likely
explanation.
So if it is really out there, when will we find it? The
world has been alerted, and multiple teams are on the
hunt. Perhaps during the next five years, someone, at
some telescope somewhere, will spot a faint blip in the
sky that moves to a slightly different spot the next
night. When they first see it, they will gasp. Then
they’ll recheck all the data and gasp again. They’ll
scramble to beg and borrow a few hours on big telescopes
here and there to confirm the blip’s slow march
across the sky. Finally, after checking and double
checking and checking 10 more times, they’ll make a
dramatic announcement to a now-anticipating world:
Planet Nine is found; Planet Nine is real!
An artist’s illustration
of Planet Nine,
looking from the
back side toward
the Sun, depicts the
planet as a gaseous
ball with lightning
visible on its night
side. CALTECH/R. HURT (IPAC)
For us, this moment marked moving from working on
an interesting hypothesis to instantly realizing that we
were talking about something that was really out there.
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