Astronomy Lesson #3

[isabellapeters]

"They're called gas giants because they're made out of gas," Bella said. "Jupiter's the biggest gas giant and the biggest planet. The only planet that isn't a gas giant apart from the four rocky planets is pluto."

[Kaiia Blackthorn]

"Yes. The other three gas giants are Saturn, Uranus and Neptune. Incidentally, gas giants are also known as Jovian planets. Jupiter and Saturn are traditional gas giants - they're made primarily of hydrogen and helium, and lack a clearly defined surface, although it's thought that they have rocky or metalic cores. Uranus and Neptune are sometimes considered a separate subclass of gas giant, known as ice giants. What hydrogen and helium they have is mostly in the outer part of the planet, but for the most part, they are composed of water, ammonia, and methane."

She displayed the images of the four planets under discussion. "Can someone tell me another feature that the four gas giants have in common?"

[Caeleb Dracus]

"Well compared to the other planets they have loads of moons." Caeleb said. "And they're really big. In fact some of their moons are bigger than the Earth." he added.

[Kaiia Blackthorn]

"Yes, the large number of moons is a feature common to all the gas giants. Ganymede is the largest moon in the solar system - it orbits Jupiter - but it has a diameter of 5262 km so it's not actually larger than Earth. It is, however, larger than both Pluto and Mercury. The other common feature that the gas giants share is rings. Everyone knows about Saturn's rings, they can be seen from earth with a relatively small telescope. The rings of Uranus and Jupiter were discovered in the late 1970s, and the rings of Neptune not until 1989."

Kaiia watched as the rings appeared around the planets in the diagram. "Do you notice anything about the orientation of Uranus's rings?"

[Caeleb Dracus]

Caeleb tilted his head to one side.
"Yeah...they're kinda...tilted the wrong way." he said realising what a stupid thing to say that was. He felt like an idiot. "Why are they like that?" he asked.

[Kaiia Blackthorn]

"Exactly. Uranus has an axial tilt of 97.77 degrees from the plane of the solar system. In effect, it's lying on its side, and we see the rings almost vertically, rather than horizontally. No one is entirely sure how it came to be like this, though one theory is that it was caused by a collision with some massive object early in its history."

Kaiia let the students watch the miniature solar system rotate for a moment, then said, "The final planet in the solar system is Pluto - although there's a lot of disagreement among muggle astronomers as to whether Pluto should even be considered a planet at all." She tried not to smile at the idea of roomfuls of muggles arguing about what qualified as a planet. She set the tiny orb rolling in the diagram. "Watch the orbital path of Pluto. Does anyone notice anything unusual?"

[Max Randolf]

"Yeah, it's orbit is eccentric, and highly inclined, instead of round like the others, and crosses Neptune's, causing it to be interior to Neptune, every now and again," Max said, leaning against the door frame. "Sorry I'm late Professor. I had a meeting with the Headmistress." He pushed himself upright, then sauntered in and sat in a desk next to Caeleb. He nodded to him, then turned back to Professor Blackthorn, giving her his best good boy smile.

[Kaiia Blackthorn]

Kaiia looked up at the sound of Max's voice. When he explained that he'd been talking to the headmistress, she nodded curtly, and waited until he'd taken his seat. "Yes, Mr Randolf. The textbook gives Pluto's distance from the sun as 5.9 billion km. That, however is the average. Pluto's orbit takes it from as close as 4.4 billion km to as distant as 7.4 billion km. And as you say, this means that Pluto spends a small part of its orbital period closer to the sun than Neptune. It is also inclined approximately 17 degrees to the plane of of the solar system." She added Pluto to the revolving diagram, so that the students could see its unusual orbit.

"What else is in the outer region?"

[angel]

Angel raised her hand knowing this was what she had answered earlier. "There's the Oorf Cloud, ice dwarfs and Kupier's belt." She ignored the new addition to the class, honestly no one else would've dared to walk into a class late if it was being taught by Professor Blackthorn.

[Kaiia Blackthorn]

"Correct. Two points to Hufflepuff," Kaiia said, nodding at Angel. She knew the girl had mentioned them earlier in the class. "Can you define those features for us?"

[angel]

Angel thought for a moment trying to recall bits of the information back. "The Oort Cloud is made up of leftover planetesimals which are rocky and icy objects, Ice dwarfs are small icy planetary bodies that orbit beyond Neptune, and Kupier's belt is like the asteroid belt but consists of frozen bits of methane, ammonia and water."

[Kaiia Blackthorn]

"A very concise definition," Kaiia said. "Another two points to Hufflepuff. It's believed that the Oort cloud is the region where comets orginate. It is thought to extend almost a lightyear from the sun. Who can tell us what a lightyear is?"

[Caeleb Dracus]

"It's how far light from the sun travels in one earth year... well 365 and a quarter days. It's about 10 trillion kilometers." Caeleb said casually. "But it means travelling in a vacuum, not in an atmosphere." he added as an afterthought.

[Kaiia Blackthorn]

"That's right. It applies to any light, not just from the sun. To give you an idea of the scale, light from the sun takes 8 minutes to reach earth." She banished the diagram, and conjured something new, a nebulous shape that looked a little like a cloud. "This is roughly what the solar system looked like when it first began to form. It started as a vast, cloud of dust and gas. It was very cold - around -230^oC, and it was much larger than the current size of the solar system. One of the key features was that it was spinning, albeit slowly. Why was this important?"

[Caeleb Dracus]

Caeleb thought about the question, frowning.
"Well..." he began holding his hands up and twirling them. "...I suppose...if it was spinning then it would have created a sort of vortex pulling everything into the centre..." he trailed off trying to work out what he was trying to say. "Is that why it got smaller?" he asked, not really sure where he was going with this.