Astronomers believe there is a giant planet lurking in the outer solar system,
and we're going to learn more about it starting right now.
Welcome back to Launch Pad, I'm Christian Ready, your friendly neighborhood
astronomer. In 2016, astronomers Mike Brown and Konstantin Batygin at Caltech
announced evidence of a giant planet in the distant outer solar system. In this
video, I'd like to tell you about how they arrived at this claim and where we
stand on the hunt for Planet 9. Now claims of new planets in our solar
system are nothing new. In 1906, Percival Lowell believed that there was a giant
planet that was disturbing the orbits of Uranus and Neptune. Lowell calculated that
the planet must be at least the mass of Jupiter but perhaps even larger. He spent
the last decade of his life searching for Planet X to no avail. When Clyde
Tombaugh discovered Pluto in 1930, people thought that Lowell's giant planet was found.
But Pluto wasn't a giant. In fact it's not even a Mercury sized planet. Pluto
turned out to be a member of a new region of trans-Neptunian objects called
the Kuiper belt. As for Lowell's calculations, well, it turns out he was wrong. He used the
wrong values for the masses of Neptune and Uranus, and when Voyager 2 flew past
both planets in the 1980s, their masses were precisely calculated and the
perturbances of the other orbits that Lowell discovered went away. Still
that didn't stop astronomers from invoking Planet X on 573 different
occasions, only to find that claim disproven on
every single attempt. So why are astronomers so convinced that there is a
new planet out there? In order to understand that, we need to go back to
2003 when Mike Brown and his colleagues discovered Sedna.
It's a 1000 kilometer-wide dwarf planet that orbits far beyond Neptune. But
Sedna's orbit isn't just far, it's shaped differently than anything that had been
discovered before. Sedna has a close approach to the Sun - or
perihelion - of 76 astronomical units, and swings out to an aphelion of
934 astronomical units. This highly eccentric orbit was unlike anything that
we'd had ever discovered in the solar system before, and astronomers
immediately wanted to know just what Sedna was doing out there on such a
strange orbit. In 2014 Chad Trujillo and Scott Sheppard discovered that a new
object, 2012 VP 113, was also on a really eccentric orbit that was not unlike
Sedna's. Both of these objects are so distant that they are completely
oblivious to the gravity of the inner planets. In other words, they're detached
from the rest of the solar system. Soon additional objects were discovered that
are a little closer to the Kuiper belt, but are otherwise on very eccentric
orbits like the Sednoids. But there was something else really
peculiar about these objects: not only were their orbits eccentric but they
were all pointing in roughly the same direction. Mike Brown and Konstantin
Batygin analyzed these orbits and they discovered that not only were they
aligned but they were also inclined by roughly the same amount with respect to
the rest of the solar system. In fact they even calculated that the chances of
these alignments occurring by random chance were something like 0.007%.
they concluded that something must be shepherding the
orbits of these objects. And that something turns out to be a giant planet
about 10 times Earth's mass on a 10,000 to 20,000 year orbit around the Sun. They
dubbed this new world "Planet 9". But they also realized something else: over time
Planet 9 could twist up the orbits of inner Kuiper belt objects and fling them
into highly eccentric perpendicular orbits to the rest of the solar system.
Sure enough, a population of exactly those types of objects on exactly those
types of orbits were discovered year prior. And Planet 9 explained something else that
has puzzled astronomers for a very long time about our solar system: all of the
planets orbit the Sun and roughly the same plane, inclined by about
degree of each other. But the Sun is tilted by about six degrees, and that's a
significant shift from the plane of the planets. But Planet 9's orbit is highly
inclined with respect to the solar system. That means that Planet nine can
act as a kind of a lever arm, and gradually tilt the orbits of the planets.
In other words, it's not the Sun that's tilted but the orbits of the planets
themselves that have been tilted, courtesy of Planet nine. And Planet nine
gives our solar system something that we didn't even realize we were missing
until relatively recently. 2000 planetary systems have been discovered so far with
planets ranging in size from smaller than Mars to larger than Jupiter. But the
most common type of planet found to date ranges in size between Earth and
Neptune. These are called "Super Earths", and we don't seem to have anything like
that in our solar system. But now with Planet 9 we do! Planet 9 is estimated
to be about 10 times Earth's mass which would put it firmly in the super-Earth
category. But why would our super-Earth be so far away from the rest of the
other planets? It turns out that if you simulate the early solar system with
four giant cores, you end up with the four giant planets that you have today.
But if you simulate five giant cores forming in the early solar system, the
least massive of these cores gets ejected while the remaining four cores
goes on to form the four giant planets. The ejected core's growth is stunted and
it only grows to about ten Earth masses. So Planet 9 explains so much about our
solar system. It explains the orbital clustering of distant outer
bodies. It explains the apparent tilt of the Sun. It even gives our solar system
its very own super-Earth. Only one teeny little problem is that we haven't
actually found Planet 9 yet. But astronomers are looking for it, and the
good news is that they're pretty sure it should be found somewhere near the
constellation Orion. The bad news is that it's still an enormous chunk of the sky
to have to search through, so astronomers have formed teams in a friendly
competition to see who can find planet nine first. And in October 2018,
Scott Sheppard and Chad Trujillo announced that a new object in the outer
solar system has an orbit that fits perfectly into the Planet 9 model.
This object is called 2015 TG387, and because it was discovered near Halloween
and because of "TG" in its name, it's been nicknamed "The Goblin". The Goblin is tiny,
only 300 kilometers across and is most likely a frozen chunk of ammonia and
methane ice. The Goblin's orbit is the largest and most eccentric of any object
of its kind ever discovered. Its perihelion is "only" 65 au, so it does come
in a little bit closer than Sedna. But it swings out to an aphelion of, get this,
2,300 au, making this the most distant object of its type ever discovered. And
best of all, The Goblin's orbit is exactly where it would be expected to be if it
were being shepherded by Planet 9. And it gets even better because astronomers
have found another set of objects in orbits that are anti-aligned with the
previous orbits, but they would fit perfectly inside of Planet 9's
predicted orbit, allowing it to be held stable by the giant planet. Now all this
makes for some compelling evidence that there is a distant giant planet lurking
in the outer solar system, and we're going to learn how astronomers are
searching for Planet 9 in our next video. So make sure that you subscribe, and ring
that notification bell so that you don't miss out when our next video is posted.
Until next time keep watching the skies.