Date: August 14, 2011

Title: Flock of Worlds

Podcaster: Adrian Morgan


Earlier episode referenced in this podcast:

Description: In the January 14 episode of 365 Days of Astronomy, Bob Hirshon asked listeners to write songs about the solar system for inclusion in a future episode. Sadly, not enough people sent in songs, so that never happened.

This episode features the song I wrote, and asks what you’d get if you used the lyrics as an outline for an astronomy lesson. In the song, the planets and other bodies orbiting the sun are compared to a flock of birds.

Bio: Adrian Morgan lives in Adelaide, South Australia, and has no special qualifications – simply an interest in the universe around us.

Sponsor: This episode of “365 Days of Astronomy” is sponsored by — NO ONE. We still need sponsors for many days in 2011, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this web page, or contact us at


Welcome to the 365 Days of Astronomy for August fourteenth, two thousand and eleven. I’m Adrian Morgan, and I hope I can bring you an enjoyable and informative ten minutes. The story behind this episode goes back to an earlier one, January fourteenth with Bob Hirshon, where he asked listeners to write songs about the solar system, and send them in to be played in a future episode. Sadly, not enough people sent in songs, so that never happened, but I sent in mine and that’s my topic for today. See, back then I posted the song to my blog, and a friend of mine commented that it was ‘quite educational’. Now, I wasn’t thinking of it as educational when I wrote it, but the comment got me thinking: what would you get if you used the lyrics as an outline for an astronomy lesson? Well, let’s find out.

The sun’s in the middle of an orbiting swarm
In the distance it’s cold, but closer it’s warm
And humanity’s carried like fleas on a bird
Though the first time you hear it, it may sound absurd.

So here’s the big picture of the solar system and humanity’s place in it. But there are so many directions we could take this, I think we’re better off not dwelling on the overview, so I’ll just make a couple of brief points. If you ask what sort of a swarm the solar system is, one answer is that it’s smaller and less chaotic than it used to be, because over billions of years, many original members have been eliminated one collision or ejection at a time. Also, there are some differences between planets and birds, for example flocks of birds don’t follow Kepler’s Third Law.

We’ve always looked above and seen lights in the sky
We’ve been watching them move and wondering why
And we’ve found they’re worlds in our very own flock
With their own sorts of clouds and mountains and rock.

This verse introduces the history of astronomy from when we looked up with curious eyes. Now, most lights in the night sky are stars, not planets, of course, but the key is in the words: we’ve been watching them move. You probably know the word ‘planet’ comes from Greek for ‘wanderer’, because to ancient observers the planets were distinguished by the fact that they move differently from the stars. And from pinpoints of light, how far we’ve come to see the planets as worlds. On Venus, clouds are made from concentrated sulphuric acid. And Jupiter’s Great Red Spot is a storm that hasn’t stopped since we’ve had telescopes to watch. Mountains can grow taller on small planets with less gravity to keep them down, and Olympus Mons on Mars is the tallest of all at three times the height of Everest. On Venus, the mountains seem to be coated with a kind of metallic snow that condenses from the atmosphere at high elevations – most likely lead and bismuth sulphides. And only on own planet do we find plate tectonics, a process that dominates everything about our geology and is made possible by our size and our oceans. As for rocks, there are differences in both chemistry and shape, and Saturn’s moon Titan is the only other place where we find cobblestones weathered smooth after tumbling about in a stream. But these rocks are made of water ice, and the streams are liquid methane. On Earth we have ice, and we have rock, but we don’t exactly have ice as rock.

These worlds in the sky, there are several sorts
If some are like parrots, some are like hawks
Yeah, the biggest are planets, these are birds of prey
And everyone else stays out of their way.

The International Astronomical Union in 2006 declared that to be a planet, an object has to have cleared the region surrounding its orbit. Mike Brown prefers to say that a planet dominates that region. But however you put it, the idea is the overwhelming majority of stuff nearby is eventually flung out, captured, sent to a lagrange point, or destroyed in a collision. The last line in the verse is a reference to this. As for birds of prey, let’s just say that whatever the asteroid that killed the dinosaurs did to the earth, it’s nothing compared to what the earth did to the asteroid. I first used the metaphor of planets as birds in a 2006 blog post where I said that asteroids and kuiper belt objects are small with big flocks, like parrots, whereas a planet is big and solitary, like an eagle.

Mercury and Venus and Earth and Mars
The terrestrial planets, one of them ours
Venus is an oven with a terrible wind
And Mars is frozen, its atmosphere thin.

So here we start talking about individual planets. But let’s focus on the latter half, because there’s a line here I’m slightly uncomfortable with. See, I wanted to say, Venus is an oven with a terrible atmosphere, but it didn’t scan. What’s terrible is the pressure, which is like a being kilometre below water on Earth. And the temperature, because Venus takes the greenhouse effect to impressive extremes. Our best-armoured probes survive for maybe hours if they’re lucky before being melted and crushed. But the surface wind is actually very calm, which is not surprising, because the planet rotates at literally walking pace, and besides, there’s not much of a temperature gradient under all that atmosphere. Up in the middle cloud layer, there are winds faster than anything on Earth, but that’s a whole other story. Mars is our neighbour on the opposite side, and a complete contrast to Venus. Average temperatures are like Antarctica at its very coldest, and it gets much colder. The atmosphere is thin, but if you were caught in one of the dust storms that envelop the planet every few years, you’d agree it’s not insignificant.

An asteroid is smaller but no less a part
Of that orbiting swarm that flies through the dark
They’re not like planets that each fly alone
They tend to stick together in a ring of their own.

Asteroids are found throughout the solar system, but mostly in the main belt between Mars and Jupiter, where Jupiter’s gravity makes it impossible for a planet to form. There’s a lot more to be said, but for the sake of time I’m going to reluctantly move on.

The greatest of the planets are made of gas
You could drop the Earth in, like throwing out trash
And we’d sink through the clouds then far below
There’s a dense kind of fluid like nothing you know.

I’ll admit to a touch of poetic license here, because if you dropped the Earth into a gas giant it would do more than just sink. We’ve seen what happens when large comets get too close to Jupiter, and one thing they do is break apart. The tidal forces would be much greater if you dropped the earth in, because the earth is bigger, and what happens when you break it apart I can scarcely imagine, what with all the magma and so forth, but it would be spectacular. Jupiter has layers of ammonia clouds, followed by ammonium hydrosulphide, and then water, and underneath that is the bottomless pit of ever-increasing temperature and pressure. A thousand kilometres down, pressure forces hydrogen and helium into liquid form, though many properties we associate with liquids don’t apply because instead of a surface, there’s just a gradual transition from the gaseous state. Twenty thousand kilometres down, with the temperature around ten thousand degrees celsius, electrons and protons are separated from each other, forming an ionic soup that drives Jupiter’s enormous magnetic fields.

Jupiter and Saturn are the first of these
They were known long before old Herakleides
Uranus and Neptune took a while to spy
‘Cos they’re too far away for a natural eye.

I’ve used Herakleides as a stand-in for the ancient world in general, a way of saying that Jupiter and Saturn have been known since forever. Herakleides was an ancient Greek philosopher; he studied under Plato, and was a predecessor to Aristarchus. He was one of the first to propose that the earth rotates on its axis, and is often credited with proposing that Mercury and Venus orbit the Sun. Some people think he even proposed a full-blown heliocentric system, which Aristarchus certainly did, moments before astronomy as a science went out of fashion for the next few centuries. Uranus and Neptune were discovered in 1781 and 1846 respectively, although Galileo’s known to have seen Neptune in 1613, and in ideal conditions you can see Uranus with your eye if you know precisely where to look.

And round the big planets where they can’t break free
There are even more worlds for variety
Triton, Enceladus and Ganymede and more
But the moons play havoc with my metaphor.

OK, let’s look at the three moons that I’ve chosen, each of which orbits a different planet. Ganymede’s bigger than Mercury, and the only moon with a magnetic field. It may be a good place to stand if you visit Jupiter, because its field would provide some protection from Jupiter’s own. Enceladus has been a focus of attention since 2005, when the Cassini orbiter saw jets of ice crystals, mixed with some intriguing chemicals, being shot from geysers to become part of Saturn’s E-ring. The source is thought to be a subsurface ocean with ammonia antifreeze. This activity’s particularly surprising on a moon so small; its width is about the distance from Penzance to Dover and its circumference about the length of Sweden. Triton’s the only moon of Neptune that’s big enough to be spherical, and my vote for most aesthetically pleasing moon in the solar system. It orbits backwards, which tells us it’s a kuiper belt object that got captured by Neptune’s gravity. One more thing about moons; it’s not just the big planets that have them. Many kuiper belt objects are part of binary systems, for example Pluto has Charon and two smaller moons.

In the dark, empty fog so far from the sun
It gets harder to see what’s going on
There was one bird flying, that was all we knew
We didn’t find more till 1992.

Kuiper belt objects are hard to find partly because they’re faint, and partly because they’re lost in the myriad of equally faint stars. But because they orbit the sun, they’re detected by comparing images taken at different times to see what’s moved. Pluto and its moon Charon were the only ones known for decades, while the rest were just a hypothesis to explain such things as short period comets. The object found in 1992 was two hundred kilometres wide, but it proved there was more out there, and by the end of the decade, hundreds of objects had been found. Following the discovery of Eris in 2005, which is heavier than Pluto and about the same size, the International Astronomical Union voted that Pluto would no longer be considered a planet. This is analagous to how the biggest asteroids were planets when first found in the early eighteen hundreds, but assigned to a new category when it became clear they were part of a populous belt. Pluto’s reassignment is usually described as a demotion, but I end the song by challenging the assumption that being a planet is desirable.

Now we’ve discovered Pluto’s far from alone
Though its vast horde of friends are mostly unknown
If you hear of a planet club, never mind that
They’re rejects from the kuiper club, too big and fat.

End of podcast:

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