Planets of the solar system: Big ones.

“Albert, that big planet ahead looks blue, is that water?”

Those pretty clouds may contain a bit of water but there is a lot of other poisonous stuff like methane.

“What's that?”

A gas that comes from rotting things and cow dung, amongst other things.

“Oh how did it get out here then?”

It was here when the planets were first made from the gas and dust from blown up stars and bits left over the from big bang.

“Still it's a pretty impressive compared to poor Pluto. What's that planet called?”

That's Neptune, the first real planet we've met in this solar system, but the astronomers have made such a mess with their new definition of a planet that Neptune might not be a planet either.

“But it's huge!”

That's right four times bigger than Earth but one of the definitions of a planet is that it has to have cleared its orbit.

“What does that mean?”

It is supposed to mean that it should be the only thing in its orbit. Any decent sized planet's gravity with make any smaller mini-planet in the same orbit crash into it or get captured as a moon. The problem is that as Pluto cross Neptune's orbit, Neptune seems to fail that test.

“So it's not a planet?”

Oh, it's definitely a planet, scientists just need to get their words straight on defining a planet.

“They're meant to be the smart ones aren't they?”

Scientists are usually smart but often foolish and you can nearly always find something they can't agree on. Even something that seems quite simple can get scientists vexed, but Neptune has been here far long than scientists have been on Earth and Neptune will still be here when that definition is long forgotten.

“So tell me something interesting about Neptune. Is it outside that Goldilocks zone you were talking about last time?”

Oh definitely not, at almost three billion miles from the Sun it's always cold out here. One of Neptune's 11 moons, Triton, is famous for being one of the coldest places in the solar system at -240oC and having a crinkled surface like a cantaloupe melon. Apart from the poisonous atmosphere on Neptune there is also the problem with the weather which seems to be pretty awful too. One the space probes that have been sent to explore out here found a hurricane bigger than the whole Earth and the winds get up to 700 miles per hour.

“So there is not chance of any life down there?”

Well, even though the sun doesn't warm up the surface, the inside of Neptune might be quite hot and people have even suggested there might be vast oceans of water deep down. If there was any life in those seas it would have to be very different to us, the combination of gravity and heat from the centre would make the ocean more like a pressure cooker with a temperature of several thousand degrees centigrade.

“What's the strangest thing about Neptune?”

The wildest idea, possibly in the whole solar system, came from scientists in America in 1999. They recreated the high pressure and high temperature atmosphere of Neptune in a laboratory and found that the methane gas could be turned in diamond dust.

“Diamonds? Are you serious?”

Methane contains atoms of carbon, and diamonds are a form of pure compressed carbon so it makes sense. So the scientists, Benedetti and Raymond Jeanloz, suggested that it might be literally raining diamonds on Neptune and on Uranus too which that is big lump of a planet over there.

“Uranus looks a bit like Neptune, just a bit greener.”

They are almost like twins and made of the same stuff with lots of methane. Uranus most definitely wins the prize for the moons with the best collections of names. If you think what poets on earth achieved with just one moon called "the moon", imagine how lyrically they would have waxed with five moons called Ariel, Umbriel, Titania, Oberon and Miranda.

“Do all these big planets out here have loads of moons? They all seem to have loads.”

The Earth is unusual with only have one moon, these big planets also have rings but the rings on Uranus and Neptune are pretty small.

“Now that planet has really nice rings.”

That’s Saturn, it’s mostly made up of gases, but has sixty moons at the last count, and the most amazing set of rings that are more than 170,000 miles across.

“Impressive. What are they made of?"

They're made up of lumps of ice, dust and rocks. For all their size they are very thin, only a few miles thick or even less in places.

“Where do they come from?”

Well, they may be collections of small rocks that never made it to be moon sized, or they might have once been moons that were destroyed by colliding with each other or by the gravity of their planet pulling them apart. Now how about this for a strange fact. Although Saturn is the second largest planet in the solar system and weighs almost one hundred times more than the Earth (a mere 50,000 billion billion tonnes), it would float if you could find a big enough bath to float it in.

"How can a planet that weighs 50,000 billion billion tonnes float?"

All it means is that, like ice or wood or anything else that floats, the density of Saturn is less than water. An iceberg can be very heavy, but as long it weighs less than an equal volume of water it will float.

"And Saturn weighs less than an equal volume of water?"

Precisely. Archimedes, another of the ancient Greeks and one of their best mathematicians, worked all this out and it is still called the Archimedes Principle.

“How could he have worked out that Saturn would float? I thought they didn't know what the planets really were?”

That's right but he worked out the principle that applies to all floating or sinking things. It all started when he was asked to work out if a goldsmith had cheated King Hieron II of Syracuse when making a crown. When the idea came to him in the bath, he is supposed to have run naked down the street shouting Eureka – or I’ve got it. Once the cheating goldsmith had been sorted out, Archimedes developed the idea to explain how things float. Unlike some other ideas dating from ancient Greece, the Archimedes Principle has stood the test of time and is as valid today as when Archimedes leapt out of his bath dripping with water and enthusiasm.

“Now that is one massive planet over there.”

This next one is Jupiter is huge, the biggest planet in the solar system

“What’s that big red spot?”

That swirling red area, reasonably enough called the ‘Red Spot’, is a storm that is more than twice the size of the earth and it’s been there for at least 100 years. So you earthlings shouldn’t complain too much about your weather.

In some ways Jupiter is like a mini solar system. The planet itself is very similar to the sun being mostly hydrogen (about 85%) and most of the rest being helium. It is encircled by four large moons and twelve medium sized ones. In total Jupiter has over 60 moons if you count the very small ones too. The only small thing about Jupiter is its ring; a rather puny affair compared with Saturn or even Uranus.

"If Jupiter is big and made of the same stuff as the sun why doesn't it glow?"

Jupiter is almost as big as the smallest star but it doesn't have enough gravity to make the centre hot enough for nuclear reactions to start. In stars the gravitational forces compress the gases in the centre raising the temperature to the required 10 million degrees or so. Jupiter hasn't got enough gravity to do that.

“What would have happened if Jupiter had been big enough to become a star?”

In many solar systems that's exactly what did happen, there are lots of stars that have companion stars and they rotate around each other. If that had happened in our solar system, I doubt the Earth would be such as a nice place to live in terms of temperature. It might get a bit hot with two suns but it would make for interesting sunsets.

“Like in Star Wars.”

When did the star wars happen, while I was dead?

“Don't worry Albert, it's just a film.”