In 1820, French astronomer Alexis Bouvard noticed something strange. The gravity of the inner planets couldn't account for the orbital movement of the outermost planet, Uranus. His hypothesis: There must be a massive object influencing it. That eventually led mathematicians to correctly calculate the location of our
eighth planet, Neptune. And this discovery fueled a mad hunt to discover more planets
using this method. To give it an approximation, everybody and their brother have looked for planets beyond Neptune. I mean, the moment Neptune was discovered, everyone wanted to do it because it was this clear demonstration that you can use math to discover objects. Unfortunately, since then, no major planet in our
solar system has been found. But thanks to modern telescopes, thousands of smaller objects have been discovered in the past few decades beyond Neptune called trans-Neptunian objects, or TNOs, like Sedna, one nicknamed The Goblin and another nicknamed Biden. And planetary scientists at the California Institute of Technology have noticed some anomalies
with these objects. The first is the fact that the orbits all lie in the same plane. The second is that they all swing out into the same direction. Now, the third is where
it gets interesting. A large fraction of these orbits that swing out into what look like they've been arranged are detached from the orbit of Neptune. Their hypothesis? A giant theoretical planet beyond Neptune, which accounts for the anomalies of these objects: Planet Nine. And thanks to a new telescope equipped with the largest digital camera ever built, we may be one sky survey away from finally finding it. It will be a game changer because it's big and it can detect very faint objects. We will find objects all the way from near-Earth asteroids out to trans-Neptunian objects and beyond. After the discovery of Neptune, scientists thought there had to be yet another planet to account for the remaining orbital
anomalies of Uranus. They called it Planet X. There were hundreds of attempts by scientists to find this missing planet over the next few decades. But as it turns out, they were wrong. Astronomers had incorrectly estimated Neptune's mass, later confirmed by Voyager in 1989, which did account for
these remaining anomalies. The reason Pluto was initially considered a planet is because it was an object that was sort of in the right part of the sky where Clyde Tombaugh was
looking for Planet X, a predicted object that was supposed to resolve remaining
orbital anomalies of Uranus. Although Pluto would later be classified as a dwarf planet, since it was too small and too far away to have any influence on the outer planets. This was the start of a new era for discoveries of bodies in the outer solar system. Pluto is one of what's thought to be hundreds of thousands of TNOs. And the more that are discovered, the more data seems to
point to a ninth planet. We see this grouping of trajectories, all these little pieces of debris that are hanging out in this population of icy planetesimals beyond Neptune. All of their orbits tend to be aligned in a very specific way that is difficult to ascribe to chance or to describe in any other way other than the existence of yet another planetary mass body hanging out in the distant solar system. The aligned orbits hint at a large body pulling them
in the same direction. They also found each TNO to be tilted on its axis in approximately the same way. And they don't appear to
be put there by Neptune. These orbits should all hug the orbit of Neptune
and many of them don't. Why not? Well, the here in Planet Nine once again comes back into the fold. You need Planet Nine's gravity, as it turns out, to kind of untether these objects from the orbit of Neptune. There's other circumstantial
evidence as well. If you compare the sizes
of discovered exoplanets, planets outside our solar system, the majority fall into categories known as super-Earths or mini-Neptunes. On average, each star, if you look up in the night sky, has at least one planet. And most planets that inhabit this galaxy, if you will, are super-Earths, these objects that are a few Earth masses. It's actually quite remarkable that the solar system, as efficient as it was
at creating planets, appears to be missing these short period super-Earth type objects, which are very common elsewhere. So if Planet Nine does exist and is super-Earth size, it would make our solar system similar to the majority we've observed. To make all these calculations, and determine the potential
orbit of Planet Nine, the team at Caltech put collected data into computer simulations. As far as the math allows us to calculate, it's about five Earth masses. So it falls in between an Earth-type planet and an ice giant, such
as Uranus or Neptune. They also estimate that it's on a 20,000-year
orbit around the sun. That's 10 times further away than Neptune is from the sun. We can calculate the orbit and the mass of Planet Nine to a pretty good approximation. What we cannot do is tell you where it is on the night sky. Fundamentally, it's all just gravity. It's a different type of dynamics which allows us to infer the existence of Planet Nine than was the case with Neptune. With Neptune, you could say it's right there and get
the orbit kind of wrong. Here you have information about the orbit, not so much the information of where it is on the night sky. So that's why we're stuck kind of observing a whole swath of the sky, which takes a long time and is indeed difficult to do. That massive swath of sky makes finding such a faint, far away object like finding a needle in a haystack. Planet Nine is millions of times less bright on the night sky than Neptune. That's really, really dim. I mean, if you really think about it, Neptune is not something you can observe with your naked eye. You need a telescope to see, and Planet Nine is millions
of times less bright. Critics say that the team at Caltech have an observational bias, having not looked in enough quadrants of the sky at different times of year to conclude that there is in fact orbital clustering. Essentially, more data is needed. The good news is we are kind of entering the next generation of such surveys now with the commissioning and the construction of the Vera Rubin telescope in Chile. So I think that's going to be a game changer in this problem. The Vera Rubin Observatory is a new telescope that'll be coming online in the next couple of years to complete the widest, most detailed survey of
the sky ever conducted. This Legacy Survey of
Space and Time, or LSST, may finally tell us once and for all, if Planet Nine is out there. Rubin Observatory is a next-generation survey telescope that is set to observe the entire
night sky every few nights, repeatedly for 10 years. The idea being we can build up not only a deep image of the sky, but also a multicolor movie. Unlike telescopes like the Hubble Space Telescope, the Rubin Observatory's field of view is extremely wide. And thanks to its giant
3-gigapixel camera, which is the size of a small car, it can also detect incredibly faint objects lurking in the outer regions of the solar system, so then narrow field of view telescopes like Hubble can take a closer look. Because it's a survey telescope, we have a plan to go and do this survey across the night sky, but then also process all the images and reduce those to catalogs so that astronomers can come along and then they can look at the catalogs and they can do searches
for unusual objects or things that match
what they're looking for just using the catalogs
that we're producing. That'll help scientists
solve the mysteries of dark matter, dark energy, map the Milky Way and inventory moving objects throughout the solar system like TNOs. So, for example, in the solar system, almost every population of object that you can think of in the solar system, we will detect about 10
to 100 times more of. We will take 2 million
images over 10 years, which is actually 4 million actual pictures of the night sky. So each image is 3.2 gigapixels. And we take about 1,600 images per night. That's about 20 terabytes
of data per night. To find objects within our solar system, the telescope will take two images of every patch of sky, separated by an hour. And from the motion of the points of light over that hour, they'll be able to distinguish between faint stars and objects within our own solar system. That's a problem for Planet Nine though. Objects that are far beyond Neptune will be difficult to detect from the motion of only an hour. Another problem is Planet Nine may be in the Northern Hemisphere sky. Due to the frequent clear skies and low light pollution, Rubin Observatory is strategically located below the equator in Chile and will mostly see
Southern Hemisphere skies. But if it is there, Rubin Observatory is equipped to find it. I have colleagues who
believe in Planet Nine, and I have colleagues who don't believe in Planet Nine. It's really hard to say definitively that it's out there. We have to go and look. So one of the things that LSST can do is actually contribute by surveying more of the sky. So people are already looking for Planet Nine. But given the fact that you have to look at a large amount of sky and you need to look for very faint kind of objects, that kind of can be a slow process so LSST will be able to do that much more quickly. So within the first year
of surveying the sky, LSST could see if there was a Planet Nine within its survey footprint down to the magnitudes that we can see. The other thing though that we can do is provide further constraints on what the orbit could be, because a lot of the constraints so far come from these
also relatively rare, very distant objects. And we'll find a lot more of those too. We'll also be able to tell if it really is just a bias in the observing data. Even if the planet isn't found, it may give astronomers more data to find it during future surveys. Or it may show that there are other explanations for the orbits and tilts of these TNOs as some scientists have argued. Either way, a hunt to find Planet Nine will undoubtedly lead
to the location of new, undiscovered TNOs and whatever else is in our neighborhood. From the perspective of daily life, no one's going to change their daily habits, you know, start brushing their teeth more or whatever, you know, because there's a planet hundreds of times as far away from the sun as is the Earth. But at the same time, look, I mean, it is part of
human nature to explore. It is part of human
nature to ask the question what's out there? What lies beyond the ocean? What lies beyond this hill? What lies beyond this planet? What lies beyond the
edge of the solar system? There is something deeply satisfying about asking and answering these questions that I think is really important to kind of our collective
human experience.
