0:00:02 > 0:00:05When I was growing up, there were nine planets in the solar system.
0:00:05 > 0:00:09Then poor old Pluto got the boot, and we went down to eight.
0:00:09 > 0:00:12Now scientists claim they have found a new ninth planet
0:00:12 > 0:00:15orbiting far beyond Pluto.
0:00:15 > 0:00:17Tonight, The Sky At Night investigates
0:00:17 > 0:00:18the case for Planet Nine.
0:00:47 > 0:00:50On 20th January, two astronomers from this university,
0:00:50 > 0:00:53Caltech, in Pasadena, southern California,
0:00:53 > 0:00:55made an extraordinary announcement.
0:00:55 > 0:01:00They claimed they had compelling evidence that a new, as yet unseen, planet
0:01:00 > 0:01:03was orbiting the sun, far beyond the existing planets.
0:01:05 > 0:01:08So far this is a theoretical discovery. The planet's
0:01:08 > 0:01:10existence has been predicted by computer models.
0:01:10 > 0:01:14But if it does exist, it throws up many questions.
0:01:14 > 0:01:16Where is it? What does it look like?
0:01:16 > 0:01:19And most crucially, how did it get there?
0:01:19 > 0:01:22We'll be talking to astronomers here in California,
0:01:22 > 0:01:26and in the UK, to answer those questions and find out what
0:01:26 > 0:01:30a discovery of Planet Nine would mean for our understanding of the solar system.
0:01:33 > 0:01:37But first, let's put Planet Nine in context,
0:01:37 > 0:01:43and see how it fits in to our understanding of the rest of the solar system.
0:01:43 > 0:01:45This is what we thought was the extent of the solar system
0:01:45 > 0:01:48for thousands of years.
0:01:48 > 0:01:51And it's restricted to the planets we can see with the naked eye.
0:01:51 > 0:01:55So in the centre, of course, we have the sun, then Mercury,
0:01:55 > 0:01:57Venus, Earth, Mars,
0:01:57 > 0:02:02Jupiter. And 1.5 billion kilometres from the sun sits Saturn.
0:02:02 > 0:02:06And this is all we thought there was until the 18th century.
0:02:06 > 0:02:09Since then, the solar system has gradually expanded.
0:02:11 > 0:02:15In 1781, Uranus was discovered.
0:02:15 > 0:02:17Followed in 1846 by Neptune.
0:02:19 > 0:02:22Tiny Pluto was added in 1930,
0:02:22 > 0:02:25pulling the edge of the solar system out to 5.9 billion kilometres
0:02:25 > 0:02:27away from the sun.
0:02:28 > 0:02:32Pluto was counted as a planet for over 70 years.
0:02:33 > 0:02:36But now we know it is just one object in a huge doughnut-shaped
0:02:36 > 0:02:39belt of billions of icy bodies
0:02:39 > 0:02:42and comets called the Kuiper belt.
0:02:42 > 0:02:47It was thought that the Kuiper belt marked the outer edge of the realm of the planets.
0:02:47 > 0:02:51But now it seems that might not be the case.
0:02:51 > 0:02:54For some of us, it's an extraordinary thought,
0:02:54 > 0:02:56but many astronomers have long suspected there may be
0:02:56 > 0:03:00other planets lurking in the dark expanse beyond the Kuiper belt.
0:03:00 > 0:03:03And we're getting the first evidence that suggests
0:03:03 > 0:03:05they may be right.
0:03:05 > 0:03:10To get a feel of just how far away Planet Nine is, if it exists,
0:03:10 > 0:03:14we need to plot its position in relation to the rest of the planets.
0:03:15 > 0:03:21On this scale, every centimetre is about 35 million kilometres.
0:03:21 > 0:03:26The Earth is 150 million kilometres from the sun.
0:03:26 > 0:03:29Pluto is 40 times further away,
0:03:29 > 0:03:33and the outer edge of the Kuiper belt is still further.
0:03:33 > 0:03:36But Planet Nine is far beyond that.
0:03:36 > 0:03:39At the point of closest approach in its orbit, it sits
0:03:39 > 0:03:42200 times further from the sun than the Earth.
0:03:42 > 0:03:46That's a whopping 30 billion kilometres away from the sun.
0:03:47 > 0:03:51But unlike the other planets, the proposed orbit of Planet Nine is
0:03:51 > 0:03:52not circular.
0:03:54 > 0:03:58Its highly eccentric path takes it much, much deeper into space.
0:04:01 > 0:04:05At its furthest possible point, its aphelion,
0:04:05 > 0:04:08it could be as much 1,200 times further from the sun than the Earth.
0:04:12 > 0:04:15On this scale, it's 54 metres away.
0:04:15 > 0:04:19In reality, it would be 180 billion kilometres from the sun.
0:04:24 > 0:04:27And because the orbit is inclined by 30 degrees to the
0:04:27 > 0:04:30rest of the solar system, it means it's also above,
0:04:30 > 0:04:34or below, the plane of the rest of the planets.
0:04:34 > 0:04:37It seems almost unbelievable that there could be a planet
0:04:37 > 0:04:41so far out in the solar system.
0:04:41 > 0:04:43A mysterious world.
0:04:43 > 0:04:45Ten times more massive than the Earth...
0:04:47 > 0:04:50..on a orbit so long, it takes between 10,000
0:04:50 > 0:04:53and 20,000 years to complete one lap of the sun.
0:04:56 > 0:04:59But how was the discovery made?
0:04:59 > 0:05:03Chris drew the...short straw and flew to California to find out.
0:05:10 > 0:05:13One of the key people in the Planet Nine story is an astronomer
0:05:13 > 0:05:16here at Caltech. He's got history
0:05:16 > 0:05:19when it comes to changing our views of the solar system.
0:05:19 > 0:05:23Ten years ago, he was instrumental in demoting poor old Pluto.
0:05:23 > 0:05:27He even wrote a book about it. His name is Mike Brown.
0:05:30 > 0:05:33Brown's speciality is the Kuiper belt.
0:05:34 > 0:05:38In 2003, he discovered Eris, a Kuiper belt object bigger
0:05:38 > 0:05:40than Pluto,
0:05:40 > 0:05:44a discovery that was instrumental in Pluto's demotion.
0:05:44 > 0:05:48A month later, he co-discovered Sedna.
0:05:48 > 0:05:52This was the first of what became six unusual Kuiper belt objects
0:05:52 > 0:05:55that have been found with highly eccentric orbits that take them
0:05:55 > 0:05:57far out into space.
0:05:57 > 0:06:00They were six of the most distant objects ever
0:06:00 > 0:06:03found in the solar system.
0:06:03 > 0:06:05It was studying their orbits that led Brown to the
0:06:05 > 0:06:08idea that there might be another planet out there.
0:06:08 > 0:06:12So the story starts with the outer bits of the solar system, the Kuiper belt.
0:06:12 > 0:06:16What we found is that the most distant objects that are part
0:06:16 > 0:06:19of the Kuiper belt, so these ones that come into the Kuiper belt
0:06:19 > 0:06:22but swing out really far, the very most distant ones,
0:06:22 > 0:06:26if you look at their orbits, all of their orbits are pointing off
0:06:26 > 0:06:30in one direction, instead of being randomly distributed around the sky.
0:06:30 > 0:06:32So how tight is that alignment?
0:06:32 > 0:06:36The ones that we know for sure that are aligned are six objects.
0:06:36 > 0:06:42And they are aligned within about a 107-degree swath of the sky.
0:06:42 > 0:06:43- And that's what? - Something like this.
0:06:43 > 0:06:46OK, and it's not just the alignment in direction as well,
0:06:46 > 0:06:48the orbits have other things in common.
0:06:48 > 0:06:50So, first you see the alignment in direction.
0:06:50 > 0:06:54Then you realise that these six very most distant objects are all
0:06:54 > 0:06:59tilted in the same direction downward and a little bit to the side.
0:06:59 > 0:07:02They are within about seven degrees of being in the same
0:07:02 > 0:07:05skewed plane of each other.
0:07:05 > 0:07:09And the combination of those two is what really shows
0:07:09 > 0:07:11you that there is something strange going on.
0:07:11 > 0:07:15You could get the six of them lined up, just due to chance,
0:07:15 > 0:07:19it's about a 1% chance you would get it due to chance. 1%, you know,
0:07:19 > 0:07:22it's a small number, but it's not that small of a number.
0:07:22 > 0:07:24But if you take also the fact that they're tilted
0:07:24 > 0:07:27like that, that's another approximately 1% chance.
0:07:27 > 0:07:32So it's about a 0.01% chance that this is all just randomly aligned.
0:07:32 > 0:07:35So is that what got your attention and got you working on the problem?
0:07:35 > 0:07:39That was the big clue that something is going on in the outer solar system.
0:07:41 > 0:07:44This was an extraordinary finding.
0:07:44 > 0:07:47The tight clustering of the six orbits might have been caused by the
0:07:47 > 0:07:51gravitational influence of something massive in the outer solar system.
0:07:54 > 0:07:58For Mike Brown, the question now became, what was out there
0:07:58 > 0:08:00and how could he find it?
0:08:01 > 0:08:05Working down the corridor from Mike is theoretical astronomer
0:08:05 > 0:08:09Konstantin Batygin. Mike and Konstantin have worked together for ages,
0:08:09 > 0:08:12and it always helps to have a friendly theorist on hand.
0:08:12 > 0:08:16And so Mike popped in to see if Konstantin could help with his problem.
0:08:17 > 0:08:21Konstantin's expertise is building computer models of planetary systems.
0:08:23 > 0:08:25- Hey, how's it going?- How are you? Yeah, nice to see you.
0:08:25 > 0:08:29'And so Mike challenged him to build a computer model of the solar system
0:08:29 > 0:08:33'that would replicate the strange orbits of the six Kuiper belt objects.'
0:08:33 > 0:08:36When you started looking into this problem of these strange
0:08:36 > 0:08:40orbits, I know a planet wasn't the first thing you considered.
0:08:40 > 0:08:43- Indeed, yes.- What else could have caused this effect?
0:08:43 > 0:08:45We had a whole slew of explanations
0:08:45 > 0:08:48that we decided to look into first. You know, so gravity of
0:08:48 > 0:08:51the Kuiper belt, so the mass of the disc itself
0:08:51 > 0:08:54holding itself together in a coherent fashion.
0:08:54 > 0:08:57All the bodies pulling on each other, essentially.
0:08:57 > 0:08:59That's right, to create almost a fluid-like effect.
0:08:59 > 0:09:02As it turns out, the Kuiper belt doesn't have enough material
0:09:02 > 0:09:06for this to be relevant. If it was 1,000 times more massive,
0:09:06 > 0:09:08that model would have some pull.
0:09:08 > 0:09:13We looked into whether or not a passing star could perturb
0:09:13 > 0:09:19the structures, leave this long tail of orbits that we are now
0:09:19 > 0:09:20here to observe.
0:09:20 > 0:09:23Such encounters don't happen very often,
0:09:23 > 0:09:26and the alignment would go away over time,
0:09:26 > 0:09:29so this would have had to be recent.
0:09:29 > 0:09:32And there is no sign of a very nearby star that has just done this.
0:09:32 > 0:09:36That's right. So I think that's a very uncomfortable route to go.
0:09:36 > 0:09:40And so with all of that out the way, you start to think about planets.
0:09:40 > 0:09:44We were careful to ensure that we're not chasing a red herring here.
0:09:44 > 0:09:48That this data is indeed real, the alignment is not some observational bias,
0:09:48 > 0:09:51it's not...it can't be a coincidence. All of that.
0:09:51 > 0:09:54Before going down the path of introducing additional
0:09:54 > 0:09:57planets into the solar system.
0:09:57 > 0:10:01When he started adding planets to his models, Batygin experimented
0:10:01 > 0:10:04with thousands of possible orbits - until he found one that
0:10:04 > 0:10:09replicated the strange orbits of the six unusual Kuiper belt objects.
0:10:09 > 0:10:13Suppose the orbit of Neptune is about this big.
0:10:13 > 0:10:17- OK.- Most conventional Kuiper belt objects,
0:10:17 > 0:10:19Pluto included...
0:10:20 > 0:10:22..have orbits that
0:10:22 > 0:10:26sort of hug the orbit of Neptune and are oriented every which way.
0:10:26 > 0:10:30But importantly, if you look at the whole Kuiper belt from afar,
0:10:30 > 0:10:34it looks like a more or less axisymmetric disc.
0:10:34 > 0:10:39This rule of thumb is broken when you look at the most distant orbits.
0:10:39 > 0:10:42So these are the famous, the now famous six?
0:10:42 > 0:10:45That's right. If you look at the now famous six,
0:10:45 > 0:10:49they all tend to swing out into the same overall direction.
0:10:51 > 0:10:54So your simulation's predicted a nice elongated,
0:10:54 > 0:10:56distant Planet Nine orbit.
0:10:56 > 0:10:59Planet Nine has a sort of similar sized orbit,
0:10:59 > 0:11:03but it is facing the wrong way.
0:11:03 > 0:11:07You might as well draw the planet on. Go on. We haven't seen it yet, but just for satisfaction.
0:11:07 > 0:11:12The planet, I am pretty sure, is in this part of its orbit.
0:11:12 > 0:11:15- Cos otherwise we would have found it.- We would have found it.- Right.
0:11:15 > 0:11:19- So there's our planet.- There's our planet. It's fascinating.
0:11:19 > 0:11:23Most of the time, the objects are on opposite sides of the solar system.
0:11:23 > 0:11:27They are completely unaware of each other throughout most of their orbits.
0:11:27 > 0:11:31It is only when they come around at
0:11:31 > 0:11:34- perihelion...- When they are closest to the sun.- When they are closest to the sun,
0:11:34 > 0:11:38they temporarily interact. And the mechanism, of course,
0:11:38 > 0:11:43is just gravitational interactions, and that's what maintains this lock step.
0:11:43 > 0:11:47So every time they come around, they get this kick? Or every two times...
0:11:47 > 0:11:51No, it gets complicated. For some orbits, it's every
0:11:51 > 0:11:5633 times of this orbit per 19 times of this orbit.
0:11:56 > 0:12:00But, fundamentally, it's not too different from tugging on something
0:12:00 > 0:12:01repeatedly in a coherent way.
0:12:03 > 0:12:06Batygin's model had passed the first test. It had successfully
0:12:06 > 0:12:10replicated the orbits of the famous six.
0:12:10 > 0:12:13But there was another, unexpected effect.
0:12:13 > 0:12:16The model also predicted a second set of Kuiper belt objects,
0:12:16 > 0:12:19with even stranger orbits.
0:12:19 > 0:12:22Trouble was, they didn't seem to exist.
0:12:22 > 0:12:27The biggest twist to the whole story came when our
0:12:27 > 0:12:30numerical models consistently would generate orbits
0:12:30 > 0:12:34that looked more or less like this.
0:12:34 > 0:12:38- So sort of coming in and out of the blackboard here.- That's right. They are on their sides.
0:12:38 > 0:12:42They are perpendicular to the plane of the solar system.
0:12:42 > 0:12:46This is just so weird. You know, the fact that the simulation is producing these
0:12:46 > 0:12:49orbits that should be perpendicular, they should be readily
0:12:49 > 0:12:54observable. And the fact that we don't know of them is big trouble.
0:12:54 > 0:12:57So it's not just that they might be there. They have to be there.
0:12:57 > 0:13:01We initially thought of this as counter evidence to the existence of the planet.
0:13:01 > 0:13:03Fine, it explains alignment,
0:13:03 > 0:13:06but it also predicts stuff that's clearly not there.
0:13:08 > 0:13:12This was the key moment where we looked at it and said, "This makes no sense."
0:13:12 > 0:13:14But then I started looking more carefully at the other
0:13:14 > 0:13:17objects in the dataset that I had not been paying much
0:13:17 > 0:13:19attention to in the last couple of years.
0:13:19 > 0:13:23There are five of these objects on perpendicular orbits that I didn't...
0:13:23 > 0:13:26I remember when one of them was discovered a few years ago
0:13:26 > 0:13:29and thinking, "That's just weird." Nobody had any explanation for them.
0:13:29 > 0:13:32And I said, "Konstantin, I'm going to go and plot these right now
0:13:32 > 0:13:36"and we're going to see where they are. And if they're sitting at these two spots right here,
0:13:36 > 0:13:38"my head's going to explode." And we plotted them.
0:13:38 > 0:13:40One of them is right here,
0:13:40 > 0:13:44four of them are exactly right here where we predicted. And we both sat
0:13:44 > 0:13:47there and stared at that, and I think my jaw hit the floor.
0:13:47 > 0:13:51It really was an honest, blind prediction of something we didn't know was there.
0:13:51 > 0:13:56And that...that... When I look at those object on those wings,
0:13:56 > 0:13:59and then I see those aligned objects, too,
0:13:59 > 0:14:04I...think I'm really pretty convinced
0:14:04 > 0:14:06it's really out there.
0:14:08 > 0:14:13If Planet Nine does exist, it would be a remarkable discovery.
0:14:13 > 0:14:18The first new planet to be found in our solar system in 170 years.
0:14:20 > 0:14:23That isn't to say it was completely unexpected.
0:14:23 > 0:14:25Because there are places we have found planets
0:14:25 > 0:14:27that are similar to Planet Nine.
0:14:29 > 0:14:34I went to University College London to meet exoplanet expert
0:14:34 > 0:14:36Professor Giovanna Tinetti.
0:14:36 > 0:14:38Hello, Giovanna.
0:14:39 > 0:14:41Giovanna, if Planet Nine does exist,
0:14:41 > 0:14:45it seems to have some very exotic characteristics, compared with the rest of our solar system.
0:14:45 > 0:14:48In your work in exoplanets, have you seen anything similar to
0:14:48 > 0:14:50these characteristics out there?
0:14:50 > 0:14:53We're talking about an object that is about tens times
0:14:53 > 0:14:54the mass of the Earth.
0:14:54 > 0:14:58These king of objects are classified as super-Earths, which is
0:14:58 > 0:15:01a very exotic name. The NASA Kepler satellite tells us
0:15:01 > 0:15:06basically that these are among the most frequent objects in our galaxy.
0:15:06 > 0:15:10The fact that we didn't have any objects of this particular size was a bit puzzling,
0:15:10 > 0:15:16in a certain sense. But if Planet Nine does exist, then of course we are ticking the box
0:15:16 > 0:15:18and we also have our own super-Earth.
0:15:18 > 0:15:21- So we had one missing, and now it looks as if we've got one.- Exactly, exactly.
0:15:21 > 0:15:26But Planet Nine, if it's out there, is a whopping 200 astronomical units
0:15:26 > 0:15:28away from Earth - at its closest approach.
0:15:28 > 0:15:31So, that just seems an awful long way away.
0:15:31 > 0:15:33Have you got examples of planets that far away from their sun?
0:15:33 > 0:15:36Actually we do. And what you see now is
0:15:36 > 0:15:41GU Psc b, which is located at 2,000 astronomical units from its star.
0:15:41 > 0:15:45So that's the further reaches of Planet Nine, potentially.
0:15:45 > 0:15:49- This is a real image, by the way.- I know. The fact you can actually take an image like this is amazing.
0:15:49 > 0:15:51- It's really very cool.- OK.
0:15:51 > 0:15:54So, we've looked at the size, we've looked at the distance
0:15:54 > 0:15:55from its local star.
0:15:55 > 0:15:57But now it's also a very eccentric orbit.
0:15:57 > 0:16:01- And that seems quite exotic for our solar system.- It definitely is.
0:16:01 > 0:16:04Most of the planets in our own solar system are orbiting in orbits
0:16:04 > 0:16:06which are almost circular.
0:16:06 > 0:16:09But actually, out there, we are seeing planets that are orbiting in extremely
0:16:09 > 0:16:13elliptical orbits. And we have some examples, like
0:16:13 > 0:16:17this planet that is called HD 80606b.
0:16:17 > 0:16:20And, as you can see from this picture, we have planets
0:16:20 > 0:16:23out there that are orbiting in very elliptical,
0:16:23 > 0:16:25what we call eccentric, sort of orbits.
0:16:25 > 0:16:30So, if Planet Nine does exist, it is not as exotic as we perceive it to be.
0:16:30 > 0:16:33Because there are lots of other examples out there that are similar to the
0:16:33 > 0:16:37- characteristics of Planet Nine. - You're right, Maggie, and if Planet Nine were proposed
0:16:37 > 0:16:42and discovered before extra-solar planets were found,
0:16:42 > 0:16:46probably we would dismiss this case, saying, "This is way too eccentric,
0:16:46 > 0:16:48"this is way too odd.
0:16:48 > 0:16:50"We can't have a planet like this."
0:16:50 > 0:16:53And the fact that, on the contrary, now we know that
0:16:53 > 0:16:56planets like this can indeed exist is really making
0:16:56 > 0:16:59the case for Planet Nine even more interesting.
0:16:59 > 0:17:04- Well, thank you very much, it's been fascinating speaking to you.- It was a pleasure.
0:17:04 > 0:17:08In theory, there is no reason why Planet Nine should not be out there.
0:17:09 > 0:17:12But there is only one way to prove its existence.
0:17:12 > 0:17:15And that is to directly observe the planet.
0:17:16 > 0:17:20Mike Brown took Chris to the famous Griffith Observatory to look
0:17:20 > 0:17:22at the skies above LA and to point out
0:17:22 > 0:17:25where he thinks Planet Nine is.
0:17:25 > 0:17:29OK, well, clear sky. Sort of.
0:17:29 > 0:17:31Where is it? Where is this planet?
0:17:31 > 0:17:35Well, we know its path across the sky, we know its orbit,
0:17:35 > 0:17:37so we know the path. And the path across the sky
0:17:37 > 0:17:41goes from somewhere over here, up across here, right through the middle of Orion and
0:17:41 > 0:17:46Taurus, and then down south across this way, and then of course all the way back around.
0:17:46 > 0:17:50We actually also know that around Orion and Taurus is where
0:17:50 > 0:17:53it is the most distant from the sun.
0:17:53 > 0:17:56And we think that that is where it is. We think the places that are closer
0:17:56 > 0:17:59to the sun, it would be too bright, we would have seen it already.
0:17:59 > 0:18:02OK. So, in some sense, you're already getting some observational
0:18:02 > 0:18:04data on the star. You know where it isn't.
0:18:04 > 0:18:06And you know other things as well.
0:18:06 > 0:18:08The other thing that we would know is that if it is too big,
0:18:08 > 0:18:12if it is the size of Saturn or Jupiter, it would have been
0:18:12 > 0:18:16detected by the WISE spacecraft a couple of years ago.
0:18:16 > 0:18:19They did a survey of the whole sky, looking for things that move,
0:18:19 > 0:18:23and we now know that there is nothing as big as Jupiter or Saturn
0:18:23 > 0:18:27out to many, many times the distance of where we think Planet Nine is.
0:18:27 > 0:18:30So it has to be small, small meaning the size of Neptune
0:18:30 > 0:18:32or maybe a little bit bigger.
0:18:32 > 0:18:35And it has to be pretty far away still, and somewhere off from here.
0:18:35 > 0:18:39And pretty faint. We're not talking amateur telescopes here to find this.
0:18:39 > 0:18:41No, in the end, it is going to take us
0:18:41 > 0:18:43some time on the biggest telescopes in the world.
0:18:43 > 0:18:46But with the biggest telescopes in the world, it is well within
0:18:46 > 0:18:48range, and we'll be able to find it.
0:18:48 > 0:18:51- It will be exciting.- Yes. - So, I know you're looking for it.
0:18:51 > 0:18:54Lots of other people will as well, I'm sure they will now.
0:18:54 > 0:18:56Does it bother you who is going to find it?
0:18:56 > 0:19:00I would love to find it myself. I would love to have that moment which
0:19:00 > 0:19:05I've gotten to have before on these big objects in the Kuiper belt,
0:19:05 > 0:19:09like Eris, where you see it and for a moment you are the only person
0:19:09 > 0:19:13in the history of humanity who has seen this object so far away.
0:19:13 > 0:19:17I would love that, I will not lie.
0:19:17 > 0:19:21At the same time, I want it to be found.
0:19:21 > 0:19:25And so if someone else finds it first
0:19:25 > 0:19:29because they were searching hard for it and helping us out on the search,
0:19:29 > 0:19:34yes, I will feel, my heart will sink a little bit, but I think
0:19:34 > 0:19:38that will be overcome by the fact I'm so excited it was found.
0:19:38 > 0:19:41- It's up there somewhere, probably. - Right there.
0:19:43 > 0:19:46One of the big questions about Planet Nine that we will have to
0:19:46 > 0:19:50answer if people are to take the claims seriously is,
0:19:50 > 0:19:53how did it get to its current orbit?
0:19:53 > 0:19:56To get to the bottom of that question, I've come to
0:19:56 > 0:19:59Queen Mary University of London to meet Professor Richard Nelson.
0:20:00 > 0:20:04Richard, what's the problem with the proposed orbit of Planet Nine?
0:20:04 > 0:20:06Well, it's very difficult to form a planet on an orbit like that.
0:20:06 > 0:20:09We believe, according to current models, that it would take
0:20:09 > 0:20:11almost the age of the solar system itself to form
0:20:11 > 0:20:14- a planet all that far out.- Because it has to accrete onto the body.
0:20:14 > 0:20:17That's right. The accretion process which brings small
0:20:17 > 0:20:21agglomerations of material together and builds larger and larger objects,
0:20:21 > 0:20:24leading eventually to a planet all the way out there just takes far too
0:20:24 > 0:20:28long because it's moving around very, very slowly and there's a very low density
0:20:28 > 0:20:31- of material there as well. - So, if it didn't form out there,
0:20:31 > 0:20:34at the outer limits of the solar system, where did it form?
0:20:34 > 0:20:37Well, our best guess is that it probably formed among the existing giant
0:20:37 > 0:20:40planets - Jupiter, Saturn, Uranus and Neptune. So, there
0:20:40 > 0:20:43are theoretical models already which indicate that if you run
0:20:43 > 0:20:47computer simulations of the early solar system and its evolution, and
0:20:47 > 0:20:50you have a dynamical instability, having a fifth planet there actually
0:20:50 > 0:20:53helps those models reproduce what we see in the solar system today.
0:20:53 > 0:20:57- So that fifth planet could be Planet Nine?- It could be Planet Nine, which is no longer
0:20:57 > 0:21:00- there but orbiting further out. - So, what are we seeing here?
0:21:00 > 0:21:03What we are seeing here is a computer model of the early stages of the solar system.
0:21:03 > 0:21:07But with five planets. So the white planet there is Planet Nine.
0:21:07 > 0:21:09The other planets are Jupiter, Saturn, Uranus and Neptune.
0:21:09 > 0:21:14- You'll see the planets are going around on circular orbit.- As you'd expect.- As you'd expect.
0:21:14 > 0:21:16They're all interacting, they're all perturbing one another.
0:21:16 > 0:21:20But Planet Nine, in this simulation, is a bit too close to Saturn when it starts.
0:21:20 > 0:21:24And at some point, we're going to see it have an encounter which gives it
0:21:24 > 0:21:28quite a dramatic little kick, and you will see it change.
0:21:28 > 0:21:32- There it goes.- That's right, so you see that. And this keeps going, on and on.
0:21:32 > 0:21:35Jump forward, you can see the planet's orbit has expanded a lot,
0:21:35 > 0:21:38and there you have a very eccentric and a very inclined orbit.
0:21:38 > 0:21:41Does that do it? Does this give us the orbit of Planet Nine?
0:21:41 > 0:21:44Unfortunately not, because Planet Nine has an orbit where the closest approach to the sun
0:21:44 > 0:21:47is about 200 or 300 astronomical units away.
0:21:47 > 0:21:50So, as you will have seen here, Planet Nine keeps coming
0:21:50 > 0:21:52back into the inner solar system
0:21:52 > 0:21:54and continues to interact with the giant planets there.
0:21:54 > 0:21:58Eventually, one of those encounters will be strong enough to throw it out of the solar system.
0:21:58 > 0:22:01How can you stop it from pinging out into the galaxy?
0:22:01 > 0:22:04If Planet Nine is actually proven to be there, it's going to tell us
0:22:04 > 0:22:07something very interesting and important about the origin of our solar system.
0:22:07 > 0:22:11So, what we've looked at so far is a solar system forming in isolation.
0:22:11 > 0:22:15But now let's imagine that the solar system formed within a star cluster.
0:22:15 > 0:22:18If we look out within the galaxy, we see that stars do tend to
0:22:18 > 0:22:20form within groups and clusters of stars.
0:22:20 > 0:22:24If Planet Nine was thrown out onto one of these very eccentric orbits
0:22:24 > 0:22:27while the star cluster was still there, then gravitational
0:22:27 > 0:22:30perturbations from the star cluster could have caused Planet Nine to
0:22:30 > 0:22:33change its orbit when it was its most distant position from the sun.
0:22:33 > 0:22:35So, I have a simulation which shows that here.
0:22:35 > 0:22:38When it goes out on this orbit this time round, it's not
0:22:38 > 0:22:40going to come back in any more. There's going to be
0:22:40 > 0:22:43an interaction with a nearby star that changes the shape of the orbit.
0:22:43 > 0:22:45Woo! So, suddenly it expands outward
0:22:45 > 0:22:48and stops going back into the inner solar system.
0:22:48 > 0:22:51So, that's what saves it from being ejected from the solar system.
0:22:51 > 0:22:53So, a lot of your model does seem to tie in with
0:22:53 > 0:22:58the evidence we're seeing for Planet Nine. It can explain the huge orbit, the inclined
0:22:58 > 0:23:01orbit and why it's still there and not out there in the galaxy.
0:23:01 > 0:23:04That's right. That's why I said at the beginning there, I think proving that
0:23:04 > 0:23:09Planet Nine is present within our solar system, with the orbit suggested, would also indicate
0:23:09 > 0:23:12very strongly that our solar system didn't form in a lonely place,
0:23:12 > 0:23:14but was part of an earlier star cluster.
0:23:14 > 0:23:18- Pretty fantastic stuff.- Wonderful. - Thank you, it's been fascinating.
0:23:18 > 0:23:20A pleasure.
0:23:20 > 0:23:23And that leaves one final question.
0:23:23 > 0:23:26What sort of planet is Planet Nine?
0:23:26 > 0:23:30And what are the conditions like so far out in space?
0:23:31 > 0:23:35At the far reaches of its orbit, the sun would appear tiny.
0:23:35 > 0:23:38Just a point of light.
0:23:38 > 0:23:42But it would still be the brightest star in the sky.
0:23:42 > 0:23:45Although the sun would be a million times dimmer than it is here
0:23:45 > 0:23:51on Earth, it would be 1,200 times brighter than Venus appears to us.
0:23:53 > 0:23:55That far from the sun is very cold.
0:23:55 > 0:24:01Some estimates have suggested it could be below -250 Celsius.
0:24:01 > 0:24:04As for the qualities of the planet, we don't really know.
0:24:04 > 0:24:07The computer models suggest it should
0:24:07 > 0:24:10be ten times more massive than Earth.
0:24:10 > 0:24:12But it could take different forms.
0:24:13 > 0:24:16One possibility is that Planet Nine could have been ejected
0:24:16 > 0:24:19from the inner solar system early in its development,
0:24:19 > 0:24:22when it was just the core of a giant planet.
0:24:22 > 0:24:25This would make it predominantly rocky and icy.
0:24:25 > 0:24:28Now, with ten times the mass of Earth, it would be about twice
0:24:28 > 0:24:32the diameter. And it would probably appear to be a dark,
0:24:32 > 0:24:33pinkish red in colour.
0:24:35 > 0:24:38But if the planet was ejected when it was more fully formed,
0:24:38 > 0:24:41then it would have a chance to build up the gassy layers,
0:24:41 > 0:24:43like Neptune and Uranus.
0:24:43 > 0:24:46Then it would be much less dense, and so bigger.
0:24:46 > 0:24:49Maybe four times the diameter of Earth.
0:24:50 > 0:24:54It's thought the planet may have an atmosphere rich in hydrogen
0:24:54 > 0:24:55and helium.
0:24:56 > 0:24:58In these cold temperatures,
0:24:58 > 0:25:03it's likely it will appear as a bland, featureless sphere.
0:25:03 > 0:25:06Unless the planet is still producing some internal heat, in which
0:25:06 > 0:25:10case we may see structure in the atmosphere.
0:25:10 > 0:25:15Clouds of methane ice, ammonia and even water ice.
0:25:18 > 0:25:23It all seems so exciting, and yet despite the possibilities,
0:25:23 > 0:25:26so far Planet Nine is no more than a prediction in a computer model.
0:25:28 > 0:25:30And they have been proven wrong before.
0:25:33 > 0:25:36So, very unscientific question.
0:25:36 > 0:25:38But the one that keeps coming up.
0:25:38 > 0:25:40How sure are you that this exists?
0:25:40 > 0:25:43Oh, one quadrillion percent.
0:25:43 > 0:25:44HE LAUGHS
0:25:44 > 0:25:47- You've got an unscientific question, but a scientific answer. - CHRIS LAUGHS
0:25:47 > 0:25:50- Right back at you.- But as a theory, you believe your models?
0:25:50 > 0:25:53Actually, we try very hard not to believe our models.
0:25:53 > 0:25:55But look, if this thing is not out there,
0:25:55 > 0:25:58if the search comes up empty, then the outer solar system
0:25:58 > 0:26:02has some really, really substantial explaining to do.
0:26:02 > 0:26:05And you've ruled out the other explanations, right?
0:26:05 > 0:26:08At least for me, it's difficult at this point to argue for a
0:26:08 > 0:26:12solar system that makes sense without Planet Nine.
0:26:12 > 0:26:13Mike, what about you?
0:26:13 > 0:26:18So, I'm slightly lower than a quadrillion percent.
0:26:18 > 0:26:21The human brain is really good at seeing patterns.
0:26:21 > 0:26:26When I can't sleep at night, it's because I am worried
0:26:26 > 0:26:30that we saw a pattern and made up an explanation.
0:26:30 > 0:26:34And then as more observations...
0:26:34 > 0:26:37come in, the pattern disappears. I don't think it's going to happen,
0:26:37 > 0:26:42I'm actually very convinced. But that doesn't mean I sleep all through the night.
0:26:42 > 0:26:45I sleep all through the night.
0:26:45 > 0:26:49I am really eager to see it, to confirm it, to learn what it is like,
0:26:49 > 0:26:51to see if it has moons and rings.
0:26:51 > 0:26:54I just want to know all these things.
0:26:54 > 0:26:56And do you have any sense of what that will feel like?
0:26:56 > 0:26:59- You know, I bought a cigar... - CHRIS LAUGHS
0:26:59 > 0:27:03..about a year and a half ago and it's still sitting in my desk.
0:27:03 > 0:27:08You know, every time something dramatic happens, I always forget to smoke it.
0:27:08 > 0:27:11So maybe I won't forget...that time.
0:27:11 > 0:27:15I'm going to go and buy a good bottle of champagne, and I'm going to put
0:27:15 > 0:27:17it in my office, put it in the fridge in my office, so keep it
0:27:17 > 0:27:21cold. And when it happens, we're just going to drink it right then.
0:27:21 > 0:27:24And between now and the cigars and champagne,
0:27:24 > 0:27:28while we are waiting for the discovery, what are you two working on next?
0:27:28 > 0:27:30We are looking for the planet,
0:27:30 > 0:27:33but we are also looking for more of these objects that are aligned
0:27:33 > 0:27:36or perpendicular, that aren't aligned or perpendicular, to prove
0:27:36 > 0:27:39ourselves wrong. If anyone is going to prove us wrong,
0:27:39 > 0:27:42we would rather do it ourselves than to have someone else do it.
0:27:42 > 0:27:44We are trying very hard to prove ourselves wrong.
0:27:44 > 0:27:47- It's an exciting time.- It is. Let's hope it gets more exciting.
0:27:47 > 0:27:49- Thanks a lot.- Thank you.
0:27:52 > 0:27:57Today's been incredibly exciting. The team's belief in Planet Nine is contagious.
0:27:57 > 0:28:01And I've become increasingly convinced that it really is out there
0:28:01 > 0:28:04somewhere in the outer wastes of the solar system.
0:28:04 > 0:28:06I'm sure I'm not the only one. There will be teams
0:28:06 > 0:28:10and telescopes from across the world racing to be the first to see it.
0:28:10 > 0:28:14And if it's found, when it's found, it will be THE astronomical
0:28:14 > 0:28:16discovery of the century.
0:28:18 > 0:28:19That's it for this programme.
0:28:19 > 0:28:22Don't forget, check out February's star guide on the website.
0:28:22 > 0:28:25Next month, we'll be looking at the top five photographs
0:28:25 > 0:28:27of the solar system ever taken.
0:28:27 > 0:28:31But in the meantime, get outside and...get looking up.