Bye, bye Pluto.
We have only known about it for seventy-six years, it's smaller than the moon and it's one of the coldest places in the solar system. Pluto was back in the news this summer because astronomers all over the world disagree about the word 'planet'.
Before 1930, the sequence went like this: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Each of these is so big that its gravity pulls it into the shape of a ball (in other words, a sphere). They all go around the sun, rather than another object in space. That is one way to answer the question "What is a planet?" That, however, is not the only definition. There is a huge piece of rock about the size of France called Ceres. It goes around the sun and it's roughly the shape of a sphere. Its orbit is between Mars and Jupiter and if you know where to look, you can see it with a telescope. Is it a planet or an asteroid? Is it something else altogether? You see, if Pluto is a planet, then perhaps this should be. Then we will have to rewrite the school books. If Pluto isn't, we'll have to as well.
This discussion isn't new. Not everyone agreed that Pluto was a planet when it was discovered. It's just that now we know some more about the objects further away. There's a bigger one called 'Planet X'. The more popular name is Xena. There's another one with the name of 2003 UB313. They haven't chosen a better name for it yet. Does that mean they are planets? If so, what about all the other things we find further out? Just how many planets are there in our solar system?
Well, there's one more item on the checklist. After a fierce and controversial discussion, a planet must 'have cleared its orbit of other objects'. What that means is that there should not be anything else in its way as it goes round the sun. To be a planet, any small objects should either not be there or should have already crashed into its surface. Pluto fails this test because it crosses the orbit of Neptune. Problem solved? Not exactly. Some astronomers say that this doesn't work for all the other planets either. After all, Haley's comet crosses the orbit of the Earth every 76 years and there are all sorts of small objects that do the same thing.
But what's the alternative? We could just add every new object to the list of planets and rewrite the textbooks every few years. That's not a popular idea any more than taking Pluto off the list of planets. Pluto is now a 'minor planet' so it still has more status than an asteroid. It's still there in space and it's still there in school classrooms. Astronomers though, are not likely to stop arguing about it, whatever we call it.
Glossary
a checklist lista kontrolna
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See if you can find the five words by reading their definitions. You need to solve the first one before you can solve the next one.
1) This is the gas that goes around a planet. Ours is the only one in the solar system that humans can breath.
2) This is an instrument for measuring the pressure of 1).
3) This is the liquid that you can find in 2).
4) This is the type of substance that 3) belongs to.
5) This is the most common type of 4) in the human body.
T M W E M Y Q U I Q E P Y U X
C N F J R I K H T U K L X O L
I N E L F M N X P F Y D S X K
O G A X C K R F I O I D D S E
U V V M C A G R B Q I S J O B
I J A G D N L Y Z A V I M Y R
Z Y M M K C G C P A O G E E Y
K M C K E C O V I P N J T M A
P G O M G R P B U U H E A W E
S R K V H T C D Z C M Z L T M
B W W C M Z U U V O B K H F L
A T M O S P H E R E L O K A L
S Y S W G B G A V Y Z I P R Y
F T V X J P B Z V O B L V N T
X A M N D C F K F U M X A Q I
KEY
1) atmosphere 2) barometer 3) mercury 4) metal 5) calcium
Did you know?
Pluto is named after the Greek god of the underworld.
We can see more a more detailed image of the moon from earth than of Pluto from the Hubble space telescope.
Pluto spins at the same rate as its moon orbits. If you were on the surface of Pluto, you could only see the moon from one side.