Pluto is a captivating member of the solar system,
but whenever I make an Astrum video about it, for all that I cover, I can never cover
everything. Sometimes my videos leave you in the comments with questions unanswered,
like: Why is Pluto so hard to see from Earth, when telescopes like Hubble can see distant
galaxies? How is it that New Horizon’s photos of Pluto were so illuminated, when it’s supposed to
be so dark out there, so far from the Sun? I love these sorts of questions, as the answers to them
give us a fascinating insight into the way the universe works. So if you’ve ever had questions
about Pluto, this might just be your moment. It’s time for you to get some answers. I’m Alex McColgan, and you’re watching Astrum.
And I’m going to answer four of your top questions about Pluto. The answers to these questions
will be intriguing, unique, and even emotive. But let’s jump right into it. How can Hubble
see distant galaxies clearly, but not Pluto? Hubble has taken some of the most
breathtaking images of space that we know of, giving us clear views of galaxies
millions of light years away. And yet, it struggles to take clear images
of celestial bodies much closer to us like Pluto and other trans-Neptunian
objects. Surely that doesn’t make sense, how can it see something so clearly so far away,
and yet the most detailed view of Pluto is this? The answer is a lot simpler than you may think,
it’s all about the angular diameter of the object in question. This fantastic image is the Andromeda
galaxy, a galaxy found roughly 2 million light years away from us. You might think that this
distance would make it relatively small from our perspective. But even though it is so far away,
use a high-exposure camera and you’ll find that it is actually 6 times bigger than the Sun or a
full moon in our sky. This is because it is big, 220 thousand light years across, and at that size,
2 million light years away from us is not enough to make it seem tiny. Its size and distance from
us means that it covers 3° across the night sky. It is so big in fact, that when Hubble looked at
it, it couldn’t see the whole thing in one go; it simply couldn’t zoom out enough. So,
this image is actually a mosaic of hundreds of Hubble exposures, and even then, they
couldn’t fit the whole galaxy in at once. This is true of other galaxies as well. Now, most
galaxies aren’t as close or as big as Andromeda, but they are still big enough for Hubble to image
them and see interesting details while it’s at it. Here’s the Pinwheel galaxy, 21 million light
years away. Now notice how it covers the gap between some foreground stars found in our galaxy.
That’s to give you some idea of how big it is in the sky. If you could see it clearly with your
naked eye, it would appear just smaller than the Moon. I hope this is giving you some perspective.
Even though it is so far away, it is huge, and so, still has a large apparent size in our sky. As you
can imagine, this makes it much easier to image. Just as a side note, I can only
imagine how beautiful it would be if all galaxies local to us were bright
enough to be visible to the naked eye. But what about Pluto? We might need to clarify our
measurements. To describe the size of something in the sky, astronomers use degrees, arcminutes
and arcseconds. 60 arcminutes in a degree, and 60 arcseconds in an arcminute. So Andromeda
was three degrees across, and the Moon is about 30 arcminutes across. The Pinwheel galaxy is about
28 arcminutes across. Pluto is a minute 0.11 arcseconds across at its closest approach to us.
That makes it ridiculously small in our night sky, a wonder that Hubble can resolve any details on
it at all. But again, closer and larger planets, which have bigger apparent sizes, can be resolved
more easily by Hubble. That’s why sending New Horizons to Pluto back in 2015 was so exciting,
we really didn’t know what to expect before we got there as we don’t have a telescope powerful enough
to resolve any details on Pluto, and so everything New Horizons sent back was a revelation.
Here's one last example from right here on Earth, to really hit the idea home. In this picture,
you can see objects at various distances and sizes. I want you to think of this flower
as Pluto. It’s small and blurry at this distance. And yet, there’s some buildings in the
background, much further away than the flower, which are clear in this image. Imagine
these are the galaxies Hubble images. So, that should clear up the mystery
of our first question. On to number 2. Why does Pluto and its big moon Charon orbit
around a point in empty space? Not all of you would know that Pluto was doing this, but those
who had heard of it might have cause to wonder. Is there something invisible and hugely
dense there, perhaps some kind of tiny, lurking black hole? As the New Horizons
probe approached Pluto and Charon in 2015, it saw this orbiting phenomenon up close and in
detail. But in reality, no black hole explanation is necessary. Scientists were expecting this
before New Horizons even arrived, even though they had never detected anything present at
that point. This is because everything in space, not just Pluto, orbits around a barycentre.
A barycentre is the centre of mass between two objects. To help visualise this, if I get
a nice butternut squash on one end of a stick, and a kiwi on the other end, we can see that the
centre of mass is more towards the butternut. For most of the planets, with their much
greater mass than the moons orbiting them, the centre of mass in these situations resides
within the planet itself, meaning it wobbles as its moon’s gravity tugs on it. With Pluto and
Charon though, they are much more similar in mass, which means the point in which they orbit is
outside of Pluto, making it appear like they are swinging around an invisible object in space!
The only other place that this happens with all the other big celestial objects in the solar
system is, surprisingly, the Sun and Jupiter! So, another enigma about Pluto
solved. Now what about number 3? A lot of you have asked: how are the
New Horizon’s images of Pluto so bright, seeing as Pluto is so far away from the Sun?
Surely Pluto would be pitch black in reality, right? Well the answer is, with your
naked eye if you were to look at Pluto, it would indeed be darker than in these images,
but interestingly not as dark as you may suspect. Thankfully, we don’t have to try and imagine it,
because twice a day on Earth at dawn and dusk, the illumination of our sky matches how
bright Pluto can get at its high noon. NASA calls this “Pluto time,” and you can check
this website out to find out when it will happen next where you are. As you can see, it’s Pluto
time here, and my mobile phone camera can still easily take a video. Pluto is an average of 40
times further away from the Sun than the Earth, and only has a 16 hundredth of the sunlight.
Direct sunlight on Earth is 100,000 lumens per square metre, so on Pluto it would
be around 60, the equivalent to an indoor stairway or corridor lighting. Even if that
level was a problem for our cameras, we can still see darker objects by utilising longer
exposure times. Here’s night time where I am, and with a 15 second exposure, my camera can pick
up a lot of details. And if it still appears dark, I can always up the brightness
afterwards in some editing software. So, there you go! Why Pluto appears
so bright in New Horizon’s photos. We’ve got one last question to go, and I think
this perhaps is the biggest question the general public ask about Pluto. It is part a search
for answer, part a cry at the injustice of it all. But its answer actually comes with
some fascinating nuance. It is, of course, this: Why is Pluto not a planet anymore?
Before I explain, did you know that we once were considered to have a hundred planets in
our solar system? It’s true. Throughout the ages, the number of planets in our solar system has gone
up and down quite a lot. And no, before you ask, this is not because planets suddenly appeared
and disappeared, but rather due to how they were discovered. Since antiquity, there were five
planets beyond Earth, which are the ones visible to the naked eye, Mercury, Venus, Mars, Jupiter
and Saturn. The ancient Greeks added the Sun and the moon to their list of planets, but that
idea never really caught on. In the late 1700s, Uranus was discovered, which was revolutionary
at the time as no-one considered there could be more planets beyond the visible five. After
this discovery, more and more planet candidates were being found, like Ceres, Vesta, Pallas and
Juno, which are all found in what we now know as the asteroid belt, or the large belt of asteroids
between the orbit of Mars and Jupiter. But these were not initially listed as asteroids.
In science textbooks in the early 1800s, they were all listed as planets. In the middle
of the 1800s, Neptune was also discovered, and within just a few decades, we had quite a
list of planets on our hands. There just wasn’t a clear distinction between the eight planets
we know today and the large asteroids that were being discovered. By the 1860s, over one hundred
asteroids had been found, and they were finally given their own distinct classification
as asteroids. Asteroids were described as objects that couldn’t be distinguished from
stars, bar the fact that they move across the sky. So you might as well ask why Pallas isn’t
considered a planet, or demand justice for Juno. A lot of people in the world feel like Pluto
has been hard done by. But Pluto was just another victim of this ruthless refinement of
categorisation. In 1930, Pluto was discovered, and with the excitement of finding something
so large and distant, it was classified as a planet. However, within a few decades, it was
discovered that it didn’t follow the conventions of traditional planets. The planets we know
have circular orbits that align roughly with the plane of the solar system. Pluto, on the other
hand, orbits at an angle to the solar system, and its orbit is so elliptical that at some
points during its year, it’s closer to the Sun than Neptune. But it was also unique, so
no-one minded it being called a planet. But then, in the 1970s, Pluto was discovered to be a
lot tinier than expected. It turned out that its mass is only one sixth of our moon. This made
things a little awkward. Why call Pluto a planet, but not our Moon when it was so much bigger than
Pluto? Trouble continued for Pluto in the 90s, when telescopic technology had greatly
increased, and other worlds were being discovered in and around the orbit of Pluto.
Just like the discovery of the asteroid belt, it quickly became apparent that there was another
belt beyond the orbit of Neptune, now known as the Kuiper Belt. These objects are still being
discovered, the most famous ones being Eris, Sedna, Haumea and Makemake. But seriously, they
have found a lot, just look at this list! In 2006, the International Astronomical Union knew that
something had to be done. It was time to come up with a new definition for the word planet. They
finally classified a planet as: a celestial body that: (a) is in orbit around the Sun, (b) has
sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic
equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.
So, although Pluto fits the bill for the first two categories, because it lies in the Kuiper
Belt, it has not cleared its orbit and thus is not a planet. I have heard argument that lots
of the planets actually have many asteroids in their orbits too, but it is a bit different to
have some asteroids around you compared to being in an actual belt. Pluto is instead known as a
dwarf planet, which is almost the same except it’s not massive enough to have cleared its orbit. We
shouldn’t see this as a slight against Pluto, but rather because of these new discoveries, we have
a whole new category of celestial objects to learn about! Also, because of the classification change,
there is a silver lining for the once-planet Ceres - it got promoted from being an asteroid to
becoming a dwarf planet! So, better than nothing. So there you have it. 4 questions about Pluto
solved…. But how many more questions have now arisen? Perhaps as you’ve watched the icy
beauty of Pluto and learned of its mysteries, all that’s happened is it's got your mind turning
in even more directions. Like the mythical hydra, maybe this is a beast that never can be truly
slain with just chopping off its heads. But that’s part of the beauty of the universe.
We can learn so much, and there is still so much more to learn. It’s the journey that is
the most satisfying. If watching this video has sparked in your mind any more questions
about Pluto, its motions and characteristics, or about our solar system in general, let
me know in the comments below. I might do more videos like this one going forward,
and would love to know what you think. But for now, whatever you were wondering about
Pluto, hopefully you now know a little bit more. Finding answers to your questions about space can
be rewarding, but here on Earth you might have questions too. Christmas is fast on its way. Have
you found yourself struggling to figure out what to get? Don’t worry, we at Astrum and the sponsor
of today’s video Displate have come together to suggest some great ideas. I’m pleased to announce
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check out this Christmas offer while it lasts. Thanks for watching! If you want to see a recap of
the New Horizons mission to Pluto, check out this video here. A big thanks to my patrons and members
for your support. If you want your name added to the end of every Astrum video too, check the
links below. All the best, and see you next time.
