The Planets

September 16, 2019

We have been spending this summer, the best time of the year, viewing the most interesting planets in the Night Skies of Tenerife, so today we take a closer look at the Planets.

4.6 Billion years ago, after the formation of our sun there was just a great swirling cloud of dust and gas.  The leftovers from the formation of our sun.  Over tens of millions of years this started to stick together and form the first rocks.  Eventually gravity assembled the rocks to form planetary embryos that in time formed the four closest planets to the sun.

Today the closest planet to the sun is Mercury, enduring the full power of the sun.  Further out is Venus chocked by a thick atmosphere, then Venus´s neighbour, Earth, and furthest of the rocky planets is Mars, a cold desert world.  Together they form the only rocky, terrestrial planets of the solar system and of those four one is unique.

There is no other place in our solar system like earth.  Which, when you think about it is strange.  As all the atoms of carbon, nitrogen, oxygen and Iron were all present when the cloud collapsed that formed our solar system and the planets and moons formed.  Yet the Earth is a lone oasis of life in an otherwise desolate solar system.

Mercury is a small tortured world.  More than any other planet it has endure the unflinching glare of the sun for billions of years.  Mercury is a planet of mysteries.  It is in a quite elliptical orbit which means at times it can be up to 70 million kilometres from the sun or at other times as close as 46million.  That means that due to its very thin atmosphere, temperatures can rise to over 400◦C on the surface and at night drop to below -100C.  Its also locked into what’s called a spin orbit resonance.    This means the planet spins exactly 3 times on its axis for every 2 orbits of the sun.  That in turn means that its day is twice as long as its year.  Meaning it would actually be possible to walk at a speed of around 2miles per hour and keep the midday sun on your back or to be in eternal twilight.


The most boring to my favorite.

I remember waiting for weeks for Mars to arrive on the Tenerife horizon.  Every night getting more excited.  I had seen Jupiter 500million miles away and Saturn 1 billion miles away and figured that mars only being 54 million away would look amazing through our 12” Dob.  How wrong was I?  as the giants faded to the horizon in the west and I showed Mars all winter, I could only make a joke of it and say it was in my opinion, the most boring planet to look at through a telescope.  Albeit it was still exciting to be looking at a planet but even in opposition to Earth apart from the faint hint of polar ice caps, it really was a let-down.

Further out we have the Gas giants of Jupiter and Saturn.  My two favorite planets to view.  Jupiter shining so brightly reflecting the suns light even before the sun sets, not that Craig our trainee astronomer can ever see it until it is getting very dark.

Jupiter is half a billion miles away on average.  It’s the oldest and biggest planet in the system.   So big in fact that it is 2.5 times more massive than all the other planets combined.  You might think that a planet so large would have a day much longer than ours here on earth.  You would be wrong.  A day on Jupiter is only 9 hours long.  It spins so fast that its magnetic field is warped like a plectrum for a guitar and helps protect the earth from asteroids heading our way.  It occasionally throws the odd one at us too though, hence there being no dinosaurs anymore.  When you view Jupiter’s marble effect stripes and can see the four Galilean moons, you feel content that all the books and google didn’t lie!

My favorite planet to view through the telescope is Saturn.  Shining as a bright looking star in the sky the Dob brings it to life and it burst light through the lens showing in all its glory the rings.  Sitting twice as far away as Jupiter at 1 billion miles, it’s a real WOW moment and we love wows on our stargazing tours.  The rings were formed by the gravity of the planet ripping apart an ice moon and spreading the debris out around the planet.  Although the planet is a bout 4.8 billion years old, its rings were formed in the time of the dinosaurs some 2.5 million years ago.

Further on to the outer most edge of our solar system.

Past Saturn we stop talking in millions and start thinking in Billions of miles.  Uranus and Neptune being the Ice giants so far away from our sun.  I’m yet to see either of these with the dob but wouldn’t think I would see much more than a faint looking blueish star for Neptune.  That is a shame as I would love to see it.  The other week with a private tour we did view Pluto for the first time.  OK so not considered a planet anymore since it hasn’t cleared its own orbit around the sun, but still, well worth a look in my eyes just so I can say I have seen it.

So when thinking of Pluto lets look at why its not a planet anymore.  In August 2006 the International Astronomical Union (IAU) downgraded the status of Pluto to that of  “dwarf planet.”  This means that from now on only the rocky worlds of the inner Solar System and the gas giants of the outer system will be designated as planets. The “inner Solar System” is the region of space that is smaller than the radius of Jupiter’s orbit around the sun. It contains the asteroid belt as well as the terrestrial planets, Mercury, Venus, Earth, and Mars. The “gas giants” of course are Jupiter & Saturn, & the icy planets of Neptune, and Uranus.  So now we have eight planets instead of the nine we used to have.

What is a Dwarf Planet?

A “dwarf planet,” as defined by the IAU, is a celestial body in direct orbit of the Sun that is massive enough that its shape is controlled by gravitational forces rather than mechanical forces (and is thus ellipsoid in shape), but has not cleared its neighboring region of other objects.

So, the three criteria of the IAU for a full-sized planet are:

  1. It is in orbit around the Sun.
  2. It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
  3. It has “cleared the neighborhood” around its orbit.

Pluto meets only two of these criteria, losing out on the third. In all the billions of years it has lived there, it has not managed to clear its neighborhood. You may wonder what that means, “not clearing its neighboring region of other objects?”  Sounds like a minesweeper in space!  This means that the planet has become gravitationally dominant — there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence, in its vicinity in space.

So any large body that does not meet these criteria is now classed as a “dwarf planet,” and that includes Pluto, which shares its orbital neighborhood with Kuiper belt objects such as the plutinos.

Well if a chat about the planets hasn´t inspired the inner child in you to book a tour right now I don’t know what will?  Come take a look for yourselves and join us on Tenerife’s best stargazing tour! (Best Thing to Do in Tenerife at Night 2019).

Until the next blog this is Kieran signing off.


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