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
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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.