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A question about planets


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#1    Jopaan

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Posted 30 March 2007 - 03:43 AM

Realistically, is it possible for there to be actual planets beyond the kuiper belt? Because for a few hundred years after we discovered Uranus, we figured that there would be no more planets after it. Then we found Neptune. I know this is probably a stupid question with a simple answer, but I'm just speculating. Thank you original.gif


#2    Waspie_Dwarf

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Posted 30 March 2007 - 05:11 AM

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Realistically, is it possible for there to be actual planets beyond the kuiper belt?


It's possible. It is, however, unlikely that there are any large planets not too far beyond the Kuiper belt.


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Because for a few hundred years after we discovered Uranus, we figured that there would be no more planets after it. Then we found Neptune.


This is wrong, and it is the reason it is wrong that explains why it is unlikely that there are further large planets to be found at a reaonable distance.

Uranus was discovered in 1781. Almost as soon as it was discovered it was noticed that it was not following it's predicted orbital path. It was realiesd that there was another, as yet, undiscovered planet beyond Uranus. Two mathematicians, the Frenchman Le Verrier and the Englishman Adams independently calculated a position for this planet. Johann Galle at the Berlin Observatory, using Le Verrier's calculations, discovered Neptune in 1846. (so there was only 65 years between them and certainly not a few hundred).

It is probable that a large planet would show itself in the same way. Maybe observations of the Kuiper belt objects will lead us to finding such a world and there are those who believe it exists. However there is no real evidence for it.

It is likely that there are other objects orbiting the sun further out than the Kuiper belt. The Kuiper belt was theorised long before it was discovered. Again it was calculated mathematically. The origins of one family of short period comets seemed to be traceable to a definite region of space, what we now call the Kuiper belt. However, even earlier than Kuipers suggestion, the astronomr Jan Oort had calculated that longer period comets came from a region even further out, now known as the Oort cloud.

The Oort cloud is around 1 light year away so the sun's influence is great enough that planets could be orbiting a long way out.

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#3    Jopaan

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Posted 30 March 2007 - 05:28 AM

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so there was only 65 years between them and certainly not a few hundred.

Oop, sorry. I wasn't thinking clearly.
Thanks anyways.

Edited by Jopaan, 30 March 2007 - 05:28 AM.


#4    Waspie_Dwarf

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Posted 30 March 2007 - 05:47 AM

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Oop, sorry. I wasn't thinking clearly.


I tried thinking once. It gave me a terible headache. I've tried to avoid it ever since.

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#5    Legatus Legionis

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Posted 30 March 2007 - 02:53 PM

Maybe , but there a big chance that the planet/ Minor Planet Object might actually not exist, because maybe the gravity of the sun was weak the it cannot permanently orbit around the sun and thus found it's way in deep space.


#6    BigDaddy_GFS

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Posted 05 April 2007 - 03:38 PM

Personally, I think there are two questions that must be answered.

1) What's the true definition of a planet?

We've all heard the debate over Pluto. Planet or planetoid?


2) Where do we draw the border for our solar system?

Just because we see an object beyond Pluto, does that mean it's really part of our system?  Maybe it's just passing through.

Edited by BigDaddy_GFS, 05 April 2007 - 03:43 PM.

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#7    Waspie_Dwarf

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Posted 05 April 2007 - 05:33 PM

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Personally, I think there are two questions that must be answered.

1) What's the true definition of a planet?

We've all heard the debate over Pluto. Planet or planetoid?


This is a difficult one because there is no easy way to define the difference. Any suggested method needs a certain degree of arbitrary limits. Several suggestions were put forward including that to be considered a planet an object should have sufficient mass to become nearly spherical. This would have included Pluto. It would also have included the largest asteroid Ceres (and possibly others) and many of the Kuiper Belt Objects (KBO). Of the Kuiper belt objects Eris (which is larger than Pluto), Pluto, 2005 FY9, Sedna, Orcus, Quaoar and Varuna would all be planets. However 2003 EL61, which is the 4th largest KBO so far found (and so fitting between 2005 FY9 and Sedna in that list) would not be a planet as it has a highly elongated shape.

This begs the question how spherical does an object have to be to be called a planet? Through a telescope Jupiter can be seen to be oblate. Its high rotational speed causes it to bulge at the equator and flatten at the poles to such an extent that you don't need instruments to measure this, you can see it.

Another suggestion (and one that was adopted) was that a planet should be the dominant object in its neighbourhood. This dominance was defined as having swept clean its orbit of smaller objects. This would exclude all the asteroids and KBOs as even the largest would not fit the description of "dominating".

This throws up another question, how much of it's orbit does it have to have swept clean? Sharing an orbit with Jupiter 60o in front and 60o behind are a group of asteroids called the Trojans. Does this mean that Jupiter does not dominate it's neighbourhood and isn't a planet.

The truth is there is no real scientific way to separate a small planet and a large asteroid or dwarf planet (the new family of bodies that include Pluto, Ceres and other large KBOs and asteroids). Nor is there a real scientific way to determine the boundary between a large asteroid and a small dwarf planet.

Even with the recent definitions that the IAU agreed on they didn't answer all the questions they were asked to. One problem they will have to settle in the future is the classification of Pluto's largest moon, Charon. A moon does not truly orbit a planet; they actually both orbit a common centre of mass called a barycentre. In reality when the planet is large and the moon is small this barycentre is so close to the actual centre of the planet that it can be ignored. Even in the case of the Earth and Moon (the Moon is the largest satellite in relation to it's planet except Pluto-Charon) the barycentre lies deep under the surface of the Earth. This is not the case with Pluto-Charon. The barycentre lies outside of Pluto. Some argue that this means that Charon should not be considered a moon at all but that it should be a dwarf planet in it's own right. This would make Pluto-Charon a true double dwarf planet system.

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2) Where do we draw the border for our solar system?

Just because we see an object beyond Pluto, does that mean it's really part of our system?  Maybe it's just passing through.


The first part of this is difficult to answer. Some astronomers will tell you that the solar system ends at the heliopause. This is the point at which the solar wind (particles coming from the Sun) is equalled by the interstellar wind (particles coming from other stars). They will argue that beyond this you are in interstellar space. This heliopause is thought to be about 100 Astronomical Units (AU) (1 AU is the distance between the Earth and the Sun or about 93 million miles, 150 million km).

However the suns gravitational influence extends further. As I have said earlier it is theorised that the Oort cloud could be as much as 1 light year away (1ly = 63,240 AU).

The reality is that there is no sharp edge to the solar system; the sun’s influence just fades with distance.

This second part is far easier to answer. Once such an object was discovered it would be a relatively simple job to calculate the objects orbit. It might take a few years as it takes several observations over a period of time to do. What often happens is that it is found that the object has been observed several times in the past but it's true identity not realised. If such pre-discovery images are found the orbit can be calculated almost straight away. Once the orbit is calculated it would be obvious whether the object was gravitationally bound to the sun or not.


"Space is big. Really big. You just won't believe how vastly, hugely, mind-boggingly big it is. I mean, you may think it's a long way down the street to the chemist, but that's just peanuts to space." - The Hitch-Hikers Guide to the Galaxy - Douglas Adams 1952 - 2001

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