Translator: Cihan Ekmekçi
Reviewer: Leonardo Silva Thank you, David. Thank you, everybody for being here,
thank you to everybody who's tuned in. So, let me get started. I have a limited amount of time,
the clock's running. (Laughter) The solar system is old,
we have little time here. (Laughter) So I'm a planetary dynamicist. That's a mouthful
and a pretty esoteric profession. But as a planetary dynamicist,
I'm especially tuned to two mathematical things about planets; the shapes of their orbits
and their period ratios. So I like numbers. So about two years ago,
when astronomers started noticing some peculiar patterns in the Kuiper Belt, this belt of small planets
beyond Neptune - we call them minor planets
or Kuiper Belt objects - I tuned into their orbit shapes
and their period ratios, and I stumbled onto a new idea that may help us discover a new planet
in the distant solar system. Searching for a distant planet
in the solar system is a story about human
imagination and curiosity, and increasing intellectual
and technological sophistication. It's also about being human,
about our curiosity about the universe, about seeking knowledge
of our place in the cosmos. So I'm going to start with a brief history
of this very human enterprise. Ancient civilizations had
a very simple concept of the cosmos. (Laughter) So, we live on Earth, the Sun and the Moon rule the sky
and our daily lives, and the afterlife
is in heaven above or hell below. Very simple. With more data of the long
time cycles in the sky, humans developed a more
sophisticated model of the universe. a more sophisticated conceptual
model of the universe. There were the distant fixed stars,
the seven wandering stars all gods and demons
that ruled over our lives and over the cosmos. In time, with even more data
and even more mathematics, a more precise model came to be accepted. That's the Ptolemaic model
that you see up there. This was a combination of circular paths. Everything in the universe was perfect;
circles are perfect, spheres are perfect. And this was a highly successful model, it was really accurate
in predicting the cycles in the sky, and it was accepted
for more than a thousand years. It predicted the seasons, everything that we needed
for human life, agriculture, and so on. Then, in the dawn of the 17th century, Galileo upset the elders
with a new technology. He pointed a little telescope to the sky and he discovered that not everything
in the heavens turns around Earth. There are worlds turning around Jupiter. The Sun was not perfect, it was blotchy. The Moon bore mountains
and valleys, like Earth, and the evening star, Venus,
ran phases right the Moon. So, seeding the center to the Sun, the heliocentric cosmos
grew in acceptance. A mechanical model of the cosmos emerged, beginning with Kepler's laws
of planetary motion and leading to Newton's law
of gravitation. The cosmos, it was finally realized,
was ruled by natural laws. Shortly after that,
with a bigger better telescope, Sir William Herschel, in England, discovered a new planet
in the solar system, Uranus, a planet that was unknown to the ancients. And this discovery truly opened
human imagination to the possibility of more planets
and more objects in the cosmos, more than we could see
with the eyes before. Meticulous analysis
of the slow movement of Uranus to seek the telltale fingerprints
of the gravity of planets now, of even more planets, led a French mathematician,
Urbain Le Verrier, to predict the exact location
of the unseen 8th planet on the night of the 23rd
of September, 1846. On that night, one German astronomer
turned his telescope to the sky and actually found that planet
at the location that was predicted on that same night. And this was a triumph
of 19th-century mathematics applied to the cosmos. Now, the most famous search for Planet 9 -
I'm here to talk about Planet 9 - the most famous search for Planet 9
was that of Percival Lowell. This was his observatory. He was convinced that the movements of Uranus and Neptune
were sufficiently aberrant, that there was a large planet
beyond Neptune, and he started a systematic search in 1906 at his observatory in Flagstaff, Arizona. That's my adopted home state. And this was the first observatory
located remotely to do the best astronomical
photography ever. Now, Lowell died in 1916,
but the search continued. The sough-for planet was found in 1930
and was named Pluto. A shout-out for Pluto!
Pluto fighters, here? (Cheers) (Applause) Well, more than two decades
of searching for this planet is truly a story of persistence. A man's life and generations
of astronomers have been persistently looking
for objects in the sky. The size and mass of Pluto, however,
took even longer to settle. Not until 1978, when its moon Charon was discovered and alas, then it became very clear that Pluto is smaller
than our own moon, than Earth's moon, and hence only a distant cousin
of this planet that Lowell was searching. Well, the hard work of following
Pluto's movements over many decades revealed many surprises
about this little planet, the shape of its orbit
and its ratio with Neptune's, the things that I tune into,
that my mind tunes into. Pluto's orbit is very elliptical, OK?
It overlaps Neptune's orbit. But these two never collide. These two planets are
in a celestial partnership known as orbital resonance. So, Pluto makes
two revolutions around the Sun in the same time that Neptune makes three, and in such a way the two
are never in the same place when Pluto was at perihelion. So, that makes them never collide.
They kind of dance around each other. So calculations of the three-dimensional
geometry revealed this is - Oh, look at this
beautiful picture of Pluto. This is the symmetric grand past
that Pluto makes if you're on a carousel,
rotating around the Sun with Neptune's orbital period. Watch the symmetry of Pluto's orbit. This is this resonant orbit and we'll run into resonances
constantly again. So, the three-dimensional
geometry of Pluto's orbit is also really fascinating. There is actually an additional
resonance, more subtle. Pluto reaches perihelion
when it's farthest away from Neptune's orbit plane. So Pluto's orbit is tilted. It reaches perihelion when it's far up
away from the plane of the solar system. And over time, it only wobbles slightly
around this geometry. So altogether, Pluto is in what's called
the periodic orbit of the third kind, one of a class of orbits identified by the 19th-century
French mathematician Henri Poincare. Such orbits have a resonance period
and a specific tilt to the planet. The gravitational forces - so these are really fascinating orbits - the gravitational forces
of the planets on each other have a special respect
for these resonance patterns and we'll run into this
again with Planet 9. These peculiarities of Pluto's orbit kept celestial mechanicians
really busy for many decades. And as a graduate student,
I learned about these puzzles of Pluto and I wondered, ''How did Pluto get to be so... this peculiar planet?'' And I proposed an answer. I proposed this answer
that you're seeing in this animation here, a giant planet migration that the giant planets Jupiter, Saturn,
Uranus and Neptune formed in a very narrow annulus around the Sun, and then later spread apart. I calculated that as Neptune
slowly spiraled outward, an originally circular coplanar, Pluto,
was shepherded into this resonance and transformed
into this elliptical, tilted orbit. Now, lucky for me, for an astronomer, it's really lucky
that subsequent discoveries over the few years
after I proposed this idea, discoveries of the Kuiper belt, many small planets beyond Neptune
that are in resonance with Neptune, have bolstered this hypothesis
of giant planets migrating. So our planets didn't form
where we see them today. Computer simulations suggest that Uranus and Neptune
and possibly additional giant planets were shaken up, scattered for a long time,
for millions of years, before settling into the orbits
we observe today. Now, more recently,
hints of a massive planet in the very distant solar system
have been noticed. The orbits of the most distant
Kuiper belt objects are kind of clustered, they are not randomly oriented
as we might expect. So if a distant planet - the true Planet 9, not the little Pluto - if a distant Planet 9 is shepherding
these Kuiper belt objects, we would like to know
how massive it is, where is it orbiting. Now, computer simulations tell us that the planet is up
to 10 times Earth's mass at somewhere between
300 to 900 times Earth's distance. But these simulations tell us nothing
about its position in the sky, where it is in its orbit
at the current time, where we would go look for it. Now, when I looked carefully at the data,
I noticed another intriguing pattern; numbers again. You didn't expect to see
tables here, did you? (Laughter) So what I noticed was that the ratios of the orbital periods
of the most distant objects are close to small whole numbers. Now, this could be mere numerology
or it could be a real clue. It could be a real clue
if a massive planet - and it could be a real
and valuable clue, I'll show you - were indeed shepherding these Kuiper belt
objects and orbital resonances - If a planet were indeed shepherding
these Kuiper belt objects and resonances, then we'd have some clue
as to where the planet is, just like Pluto's resonance with Neptune. If we know where Pluto is,
we know where Neptune is not. It's not near Pluto's perihelion. So I searched for an orbit of Planet 9 that would be resonant
with the distant Kuiper belt, these Kuiper belt objects. There are only a small number
of possibilities, of likely possibilities. The choice most consistent
with the orbital clustering we've seen is an orbit of period near 17,000 years, a very long period. So this planet is moving
really slowly in the sky. This singular choice
of Planet 9's orbit period leads to simple resonant geometries of the distant Kuiper belt objects
relative to Planet 9 like the shapes that you see
on the screen now. These shapes of the resonant orbits
show us the portions of the orbital path where this hypothetical planet
cannot be at the current time. In addition, there are only two choices,
or two approximate choices, of the tilt of this planet's orbit. One is it's either close to the planes
of these distant Kuiper belt objects or it follows these Poincaré's
periodic orbits of the third kind that I mentioned before,
so it has a specific tilt. And those two choices turn out to be one that's just 18 degrees
tilted to the plane of Earth's orbit; the other one is tilted 48 degrees
to our planet's orbit. It remains now to refine
our mathematical predictions, and test them with telescopes
to prove that this planet really exists. or if perhaps the apparent patterns
in the distant Kuiper belt are a fluke, a statistical fluke of a small sample. We only have about a dozen objects
giving us these clues. So should we find Planet 9? It will be a triumph of both mathematics
and advanced astronomical technology. Again, yet again, what will it mean
to discover a distant large planet? Well, we will have
a more complete inventory and a new self-image of the solar system. It may make horoscopes
more accurate finally. (Laughter) (Applause) Well, for scientists, it will stimulate
new research on the origin of comets. This Planet 9 roams
in this reservoir of frozen bodies, the remnants of our solar system,
far out in the distant solar system. How does it affect the frequency
of comets that come to us, so we can see them in the sky? It's going to change our ideas
about how comets arrive in the inner solar system. It will also stimulate new research
on the formation of the solar system. How did the solar system manage
to look like this clockwork mechanism that Newton and Kepler believed in despite having this history
of scattering of planets? Almost surely this planet was scattered
from maelstrom closer in. Are there even more planets
roaming in the distance? How did the scattering of this planet
affect the course of our own planet? It almost surely did. So Planet 9 will provide a new challenge and a new destination
for future exploration. And I thank you. (Applause)
I've been trying to figure who actually was the first to publicly make the hypothesis of Planet 9. Does anyone know? Is there anyone else who made the proposal?