What Happens if We Remove a Planet from the Solar System?

[MathematicalPhysicist]

let's say we remove one of the planets from it's orbit around the sun (let's remove it far away from the solar system) what would (if any)
happen to the orbits of the other planets around the sun?

my bet nothing.

 

[Phobos]

I think there's an element of chaos to the system. Let's say we remove Jupiter. The effect over our life time would probably be zero (except of course for the Jovian moons which would immediately freak out). But the further you look into the future, the more effect there would be. My WAG is that the solar system configuration would be "significantly different" in few billion years.

I'm basing this on the fact that predicting orbits become more uncertain the further in the future you are trying to look...due to all the minor gravitational nudges over time that eventually add up to something significant. This is more noticeable for the minor bodies such as comets & asteroids for which uncertainties seem to stack up on timescales of thousands of years (or maybe even hundreds of years for such things near Earth orbit). Planets, being more massive, are a bit harder to mess with.

 

[Nereid]

First of all, the 'removing' would quite likely be very disruptive, and how it happened would matter a lot. Let's assume the planet with the short straw just 'disappeared'.

Next, the question of the long-term stability of the solar system has been around for a long time, and looked at deeply from an analytic perspective (before computers) then with increasing precision through digital modelling. The conclusion, if I recall correctly, is that the 8 main planets are more or less OK (some uncertainty about Pluto-Charon, e.g. how much impact does the full EKB have?)

Then there's our galactic environment. In our journey around the Milky Way centre, we will certainly encounter giant molecular clouds, and occassionally come within < 1 light-year of another star. Does this matter, re the stability of the solar system? If all there is is the nine planets and asteroids (and probably the EKB), probably not. But what about the Oort cloud?

Take out Jupiter, and within a few thousand (tens of thousand?) years the Earth (and many other planets) would likely suffer a very nasty bombardment by asteroids. The Trojans would be 'liberated', the resonance gaps would all change (and new, weaker ones due to Saturn would be created?), and where would the Galilean moons go?

My WAG: if the Titus-Bode 'law' isn't just a co-incidence, then there'd be a quite significant lot of orbital readjustments, irrespective of asteroid, liberated moons, and Oort cloud object bombardments.

 

[NEOclassic]

True motion of the Sun

Originally posted by Nereid
Then there's our galactic environment. In our journey around the Milky Way centre, we will certainly encounter giant molecular clouds, and occassionally come within < 1 light-year of another star.

Hi Nereid,
"journey around the [axis of rotation of the galaxy]center" reflects the concept of a true Trekkie. The true motion of the SS is a radial vector from the axis of rotation where the SS was created 6.02 E 9 Earth years ago. The analogy of the fireworks item, the "pin wheel", helps to understand the physical behavior. Each little sparkle of light might eventially fall to the ground but its direction is more or less radial relative to the axis of spin; there is no physical reason why a spark should ever follow a circular path. If that is not understood let me give you another analogy:
Suppose there's a few inches of snow on the ground and also that you have an AK-47 that shoots arrows. Now suppose that you fire a "burp" aiming to begin toward north and rapidly rotating your aim toward the south -- now go out and survey the arrows - don't be surprised that each arrow and its trajectory path in the snow, are on a line (radius)pointing back to the point where you were standing. Cheers, Jim

 

[Mentat]

Interesting question, LoopQuantumGravity. I wonder though: If you were to remove one of the planets it would have a noticable gravitational affect on the sun, wouldn't it? After all, Relativity dictates that it's not just the planet orbiting the sun, but the sun orbiting the planet. Now, since you will have affected the position and movement of the sun, by removing one of its satellites, you will also affect all of the other planets, since the sun's gravity is (obviously) affecting all of them.

What I'm basically saying is that the removal of a planet would jar the sun in the opposite direction, at least somewhat, and thus there would be an affect on anything that is in a gravitational relationship (for lack of better terms) with the sun.

 

[Nereid]

Mentat said:

the removal of a planet would jar the sun in the opposite direction, at least somewhat, and thus there would be an affect on anything that is in a gravitational relationship (for lack of better terms) with the sun.

Mentat, can you say more about the 'affect' [sic] please? Perhaps give some quantatitive examples too?

 

[Janus]

Originally posted by Mentat


What I'm basically saying is that the removal of a planet would jar the sun in the opposite direction, at least somewhat, and thus there would be an affect on anything that is in a gravitational relationship (for lack of better terms) with the sun.

Well basically, the effect Jupiter has on the Sun is that they both orbit their common center of gravity. This common center of gravity is about 82,000 km above the Sun's surface. Since the Sun Orbits this point in the same time as Jupiter, the Sun's orbital velocity would be about 1.3 m/s.

If Jupiter were to suddenly vanish, the effect wold be like letting go of a sling shot; the Sun would traveling off in a straight line at 1.3 m/s.

The other planet already are effected by this orbital velocity so they would just follow along.

There might be a slight distrubance as the Cog of the system suddenly shifts 777,000 km. But this is pretty small compared to the orbital distances involved or even the present variations in orbital distances due to eccentricities.

 

[LURCH]

Originally posted by Janus
This common center of gravity is about 82,000 km above the Sun's surface. Since the Sun Orbits this point in the same time as Jupiter, the Sun's orbital velocity would be about 1.3 m/s.

If Jupiter were to suddenly vanish, the effect wold be like letting go of a sling shot; the Sun would traveling off in a straight line at 1.3 m/s.

Think you dropped a couple of decimals, there. The center of gravity between the Sun and Jupiter is deep inside the Sun, very near the center. And the Sun's orbital velocity is about 13 m/s (rather than 1.30). But these are minutia (and typos, I suspect), the principle of the statement is true, the Sun's course will be altered.

Also, the other planets would have to be effected, because some of the planest in our system were only discovered because of their gravitational effect on others. So the removal of one would have to change the orbits of the others to a degree detectable by instruments, if not noticable to the naked eye.

 

[Janus]

Originally posted by LURCH
Think you dropped a couple of decimals, there. The center of gravity between the Sun and Jupiter is deep inside the Sun, very near the center. And the Sun's orbital velocity is about 13 m/s (rather than 1.30). But these are minutia (and typos, I suspect), the principle of the statement is true, the Sun's course will be altered.

The formula for finding the distance of the Sun-Jupiter Barycenter form the center of the sun is :

D = d_jup-sun M_jup/(M_jup+M_sun)

This gives

D = 778.33e27Km ( 1.9e27kg)/(1.9.e27kg + 1.99e30kg)

= 742,420 km

The sun's radius is 695,000km, so the

Barycenter is 47420 km above the Sun's surface. (my earlier figure is off somewhat because I used rougher number's for the jupiter-sun mass/ratio which put my sun center-barycenter distance about 5% high. )

Though you're right, I did drop a decimal on the velocity, it should be about 12.5 m/s.

Also, the other planets would have to be effected, because some of the planest in our system were only discovered because of their gravitational effect on others. So the removal of one would have to change the orbits of the others to a degree detectable by instruments, if not noticable to the naked eye.

In this case, I was only talking about the isolated effect the loss of Jupiter would have on the Sun and its motion, and what that would in turn have on the other planets. The loss of the perturbations Jupiter causes on the other planet's is another issue.

I don't think that anyone is arguing that the loss of Jupiter would have no effect on the rest of the system.

 

[Jeebus]

You are right in thinking of this question in terms of inertia, the
tendency of an object to remain in uniform (i.e. constant speed, straight line) motion unless it is acted upon by an external force. If the Sun were to disappear suddenly (not that I can imagine a physical means by which that might happen), the gravitational force it exerts on the Earth and the other planets would disappear, as well. Each of the planets would fly off in straight lines, tangent to their old orbits, with the speeds they had when they were orbiting. Each planet would also continue to spin about its own axis. People and objects would still be held to the Earth by the Earth's gravity. That gravity has nothing to do with the Earth's axial spin, however. In fact, the axial spin slightly _decreases_ the effective downward force felt by people and things at the surface of the Earth. So, if the Sun were magically to disappear, the Earth would continue moving, but in a straight line rather than a circle. The really alarming change would be the devastating loss of energy - no Sun warming the planet would have catastrophic consequences for life (not just human, but all life) on Earth. That's why I am glad that I can think of no way in which your scenario could come about!

But I'll let the Physics majors handle this one.

 

[theEVIL1]

You bet nothing??

Read (or if you are in HS) re-read classical physics especially the chapters on Kepler's laws. Then you will have the obvious answer to your pondering.

 

[Mentat]

Originally posted by Janus
Well basically, the effect Jupiter has on the Sun is that they both orbit their common center of gravity. This common center of gravity is about 82,000 km above the Sun's surface. Since the Sun Orbits this point in the same time as Jupiter, the Sun's orbital velocity would be about 1.3 m/s.

If Jupiter were to suddenly vanish, the effect wold be like letting go of a sling shot; the Sun would traveling off in a straight line at 1.3 m/s.

The other planet already are effected by this orbital velocity so they would just follow along.

There might be a slight distrubance as the Cog of the system suddenly shifts 777,000 km. But this is pretty small compared to the orbital distances involved or even the present variations in orbital distances due to eccentricities.

Well, that last point was what I was really wondering about. How negligible are the effect of a 777,000 km (didn't know that figure before, but knew it was pretty big) shift on the other orbiting bodies?

It's not just that they are already affected by this orbital situation (between Jupiter and the Sun), but also that the sudden disturbance of the shift in the Sun's path on the it's gravitation field should itself be noticed by the planets, shouldn't it?

 

[Mentat]

Originally posted by Nereid
Mentat said:
Mentat, can you say more about the 'affect' [sic] please? Perhaps give some quantatitive examples too?

See Janus' later posts. He covered my point, but apparently the effect is rather more negligible than I had thought.

 

[NEOclassic]

Welcome to PF

Originally posted by theEVIL1
Read (or if you are in HS) re-read classical physics especially the chapters on Kepler's laws. Then you will have the obvious answer to your pondering.

Hi EVIL,
Briefly, Kepler's three laws are:
1. Orbits are ellipses and the Sun is at one of the foci.
2. A line from a planet sweeps over equal areas in equal times.
3. A planet's orbital period squared is proportional to orbital radius cubed.
It is noteworthy that the latter two "laws" apply equally well to circular orbits. The three halves law is obvious when the double integration of f=ma is considered. The trajectory of the inner surface of a collapsing imploded spherical shell follows the 3/2 - rule explicitly.
The first law is the only one that is validated by actual data in showing that the orbits are ellipses; however, Kepler's "thought experiment" that the Sun is at a focus is only proximate - besides, mathematically speaking, the ellipse would degenerated into a parabola.

Go back to the primary hypothesis of this string. Hasn't it been assumed that a planet goes in a circle and to guarantee momentum conservation, the Sun also goes in its own circle in the same direction and that there is a virtual pivot point somewhere between the two bodies such that the two circles are concentric? If the Sun's mass were centered at an elliptic focus, shouldn't the axis of Solar spin and that of the concentric circles be coincidental? Cheers, Jim

 

[NEOclassic]

More about Jupiter's orbital eccentricity

Originally posted by Janus
Well basically, the effect Jupiter has on the Sun is that they both orbit their common center of gravity. This common center of gravity is about 82,000 km above the Sun's surface. Since the Sun Orbits this point in the same time as Jupiter, the Sun's orbital velocity would be about 1.3 m/s.

If Jupiter were to suddenly vanish, the effect wold be like letting go of a sling shot; the Sun would traveling off in a straight line at 1.3 m/s.

Hi Janus,
Thank you for the modeling that shows quite lucidly that there is some doubt that the Sun is placed at a focus point of the elliptic orbitals of the planets. That causes some wonder concerning the force model that causes the ellipse orbit. Consider my following thought experiment:

The Solar System is already moving on a radial straight line (relative to the galaxy’s axis of rotation) at an average velocity of about 10^-6 light-years per Earth year. I would suspect that the Sun’s mass dominates the system’s trajectory. Instead of the Sun moving, the fulcrum of equilibrium is itself static; i.e., (the center of the Sun and the fulcrum remain permanently on the major axis of the orbit) and that despite the rotational flow of solar matter that is of no consequence. This “thought model” postulates that the aphelion (of the Jupiter orbit) occurs when the fulcrum is between the Sun’s center and Jupiter; later (5.93 Earth years after aphelion) when Jupiter has orbited 180 degrees, it will be at perihelion with the Sun’s center being between Jupiter and the fulcrum.

This modeling would validate the elliptic shape of the orbit without the need for the Sun to be postulated at an elliptic focus. That there is a fulcrum that is not at the Sun’s center has already established the Sun as not being at an elliptic focus.

Thanks for your audience, Jim

 

[Loren Booda]

SWAG: The effect of removing a planet from our nonlinear solar system might be calculated in terms of the relation of its mass upon existing chaotic attractors. If an attractor relies soley or heavily upon the mass of the planet, a major planetary disappearence would cause most Sol satelites to truly "wander," unlikely reachieving stable orbits.

My guess relies on our planetary solar system having 10! Lagrangian points, and a similar number of attractors. Removing one planet would eliminate 10 Lagrange points, or change only 1/362,880 of the total. Attractors likewise would be affected by the overall change in solar system mass. Jupiter and Mercury comprise 1/1000 and 3/20,000,000 solar system masses. Assuming a direct relation between mass and attractor effect, Jupiter would displace the equivalent of ~3,600 attractors, and Mercury likely none.

An initial Jovian shift of overall Earth orbit by 1/1000 (less than the distance to the Moon) seems paltry, but cumulatively in an unstable nonlinear system could butterfly us beyond the sun's grasp.

 

[Nereid]

Suggestion - let's try it!

What does everyone think of doing an experiment on this?

If we could find someone who has a digital orrery (and who knows how to drive it!), we could ask them to take out a planet and see what happens.

It'd probably be a good idea to agree on where we're starting from, what we plan to do, and what we want to look for, before we start the actual experiment.

 

[marcus]

Originally posted by Nereid
What does everyone think of doing an experiment on this?

it is a REALLY exciting suggestion

 

[marcus]

Originally posted by Nereid

It'd probably be a good idea to agree on where we're starting from, what we plan to do, and what we want to look for, before we start the actual experiment.

do you mean we should agree on
what planet (including or not including its satellites)
to remove from the solar system as it was on (say) january 1
of some year

and we should make some hypotheses about what might happen so we can look to see if such-and-such does or doesn't?

Is this what one needs to agree on:
A. which planet to magically remove
B. from the status of the system at which date
C. what effects to look for in the output of the orrery?

I would like very much to be included, if you are willing. But if
the group is limited only to those who were here earlier I shall
be happy to withdraw. Must say this is interesting.

 

[NEOclassic]

Originally posted by marcus
do you mean we should agree on
what planet (including or not including its satellites)
to remove from the solar system as it was on (say) january 1
of some year

and we should make some hypotheses about what might happen so we can look to see if such-and-such does or doesn't?

Is this what one needs to agree on:
A. which planet to magically remove
B. from the status of the system at which date
C. what effects to look for in the output of the orrery?

I would like very much to be included, if you are willing. But if
the group is limited only to those who were here earlier I shall
be happy to withdraw. Must say this is interesting.

Hello Marcus,
You have my invitation to join this string - anyone who is familiar with the original "planetarium" proposed by the Earl of Orrery some time near 1700 A.D. is likely serious about role of "thought experimentation" in physics areas too large or too small for real experiments. In 1660 a group of scientists (epistemologically driven)in England grouped together and were shortly named the "Royal Society" by King Charles II. By 1700 the Earl was in his mid-twenties and I believe he had become a member of that society.

Let me suggest that Mercury and/or Pluto could likely be of greatest interest because the extraction of either of these planets would have very small, if any influence on Jupiter's orbit, for example.

Thanks for your audience, Jim.

 

[MathematicalPhysicist]

Originally posted by NEOclassic
Hello Marcus,
You have my invitation to join this string - anyone who is familiar with the original "planetarium" proposed by the Earl of Orrery some time near 1700 A.D. is likely serious about role of "thought experimentation" in physics areas too large or too small for real experiments. In 1660 a group of scientists (epistemologically driven)in England grouped together and were shortly named the "Royal Society" by King Charles II. By 1700 the Earl was in his mid-twenties and I believe he had become a member of that society.

Let me suggest that Mercury and/or Pluto could likely be of greatest interest because the extraction of either of these planets would have very small, if any influence on Jupiter's orbit, for example.

Thanks for your audience, Jim.

but the extraction of mercury could change the orbit of Earth and make it closer to the sun (which would make things much hotter).

 

[Nereid]

Originally posted by marcus
do you mean we should agree on
what planet (including or not including its satellites)
to remove from the solar system as it was on (say) january 1
of some year

and we should make some hypotheses about what might happen so we can look to see if such-and-such does or doesn't?

Is this what one needs to agree on:
A. which planet to magically remove
B. from the status of the system at which date
C. what effects to look for in the output of the orrery?

I would like very much to be included, if you are willing. But if
the group is limited only to those who were here earlier I shall
be happy to withdraw. Must say this is interesting.

marcus, you are very welcome to join.

Personally, I welcome anyone interested to join, the more the merrier. I would also like to have Phobos, as Mentor, join too.

If this proves to be useful and good fun, I'd like to see if we could do more of these.

 

[marcus]

Originally posted by Nereid
marcus, you are very welcome to join.

thanks so much, it is a really good idea
I am waiting (actually with some impatience) for Loop
and Mentat and others who started the discussion
to suggest WHICH planet to take out. Also didnt
Phobos or someone already suggest what kind of
behaviors one might look for because different
resonances to adjust to, so they (whoever it was)
could formulate that in a clear way and then we could
have a kind of mark one preliminary specification of
the problem (subject to revision) and we could proceed
to inquire if anyone has a computermodel of the solar
system (orrery). Getting a real buz from this and eager
for LQG/Mentat and others to specify the experiment!

 

[NEOclassic]

Originally posted by marcus
do you mean we should agree on
what planet (including or not including its satellites)
to remove from the solar system as it was on (say) january 1
of some year

Hi Marcus,
Your choice of January 1 as the moment of some event might well have been a matter of random convenience, but it could also have been based on the centruies ago premise that the Earth's perihelion be made to occur on January 1. Actually, possibly because of precession, the second of perihelion is currently at some time, say on the 4th of Jan. Cheers,Jim

 

[Nereid]

digital orreries

There seem to be at least two which may be suitable:
Planet's Orbits (latest version is 1.6)
Orrery, by Photodesk

There's also a Linux-based program, called Hitchhiker 2000.

ORSA (Orbital Reconstruction, Simulation and Analysis) would surely do the trick too (thank you selfAdjoint), but may have a very steep learning curve.

Does anyone own (or know someone who owns) any of these?

Does anyone know of any others which may be suitable?

Without a program, we can't do anything.

 

[Rader]

missing planet

According to Keplers law, the missing plantet between Jupiter and Mars that is supposed to be there is nothing but a ring of asteriods. Is the mean orbital path of this asteroid belt the Roch limit in respect to Jupiter? Knowing the mean orbital path of the asteroid belt and the diameter of Jupiter. What would have been the diameter of that planet?

 

[Nereid]

Originally posted by Rader
According to Keplers law, the missing plantet between Jupiter and Mars that is supposed to be there is nothing but a ring of asteriods. Is the mean orbital path of this asteroid belt the Roch limit in respect to Jupiter? Knowing the mean orbital path of the asteroid belt and the diameter of Jupiter. What would have been the diameter of that planet?

Titus-Bode law perhaps (I'm not familiar with a law with Kepler's name on it that relates to the spacing of planetary orbits)?

The asteroid belt has nothing to do with Jupiter's Roche limit; the Jovian satellites are more impacted by that, especially the small inner ones (e.g. Amalthea).

The baleful influence of Jupiter on the formation of a planet at approx the distance from the Sun of the asteroids had to do with disruption of formation. The idea is that planets form by the hierarchical accretion of planetisimals; the proto-Jupiter formed early and was huge (the proto-Sun didn't heat up the planetisimals at Jupiter's distance enough to boil off the volatile ices). There are lots of Jovian resonance orbits in the asteroid belt - orbits whose periods of revolution are a simple fraction Jupiter's. Planetisimals at or near these orbits will have their orbits severely disrupted by Jupiter - they would not be able to coalesce to form a larger planet. Where did they go? Most ended up forming part of other planets; some were flung out of the solar system entirely; a tiny fraction remain today as asteroids.

The process continues today, albeit at a much slower rate.

 

[Rader]

Originally posted by Nereid
Titus-Bode law perhaps (I'm not familiar with a law with Kepler's name on it that relates to the spacing of planetary orbits)?

Keplers third law dictates the distance planets would be found. There seems to have been once, one between Mars and Jupiter. The outer planets were discovered by using the formula of Keplers to find there orbit.
http://home.cvc.org/science/kepler.htm
Square of any planet's orbital period (sidereal) is proportional to cube of its mean distance (semi-major axis) from Sun.

The asteroid belt has nothing to do with Jupiter's Roche limit; the Jovian satellites are more impacted by that, especially the small inner ones (e.g. Amalthea).

I will google this Titus-Bode law. http://www-thphys.physics.ox.ac.uk/users/Astrophysics/guides/solsystem/titusbode.shtml your right it does.
My thought was that maybe if this theoretical planet got knocked out of orbit by some other object and moved into the Roch limit of Jupiter, that that could be the reason for its dissapearance.

The baleful influence of Jupiter on the formation of a planet at approx the distance from the Sun of the asteroids had to do with disruption of formation. The idea is that planets form by the hierarchical accretion of planetisimals; the proto-Jupiter formed early and was huge (the proto-Sun didn't heat up the planetisimals at Jupiter's distance enough to boil off the volatile ices). There are lots of Jovian resonance orbits in the asteroid belt - orbits whose periods of revolution are a simple fraction Jupiter's. Planetisimals at or near these orbits will have their orbits severely disrupted by Jupiter - they would not be able to coalesce to form a larger planet. Where did they go? Most ended up forming part of other planets; some were flung out of the solar system entirely; a tiny fraction remain today as asteroids.

Yes there is two, maybe more theories. One there never was a planet, because of the gravitational forces of Jupiter. The other there was one, and for some reason that planet fell into the Roche limit of Jupiter and was destroyed. Lots of the strange orbits of the asteroids are do to the effects of gravity from other bodies in space over time.

The process continues today, albeit at a much slower rate.

Its funny how Caos theory works to create beauty.

 

[Nereid]

If Kepler's third is: "Square of any planet's orbital period (sidereal) is proportional to cube of its mean distance (semi-major axis) from Sun", how does that 'dictate the distance planets would be found'?

AFAIK, this law is simply a result of Newton's law of gravitation, for two objects in orbit around their mutual centre of gravity, where one mass is very much greater than the other, and the orbits are determined only be gravity. In fact, isn't this an exercise given to undergrad physics students?

 

[selfAdjoint]

The Kepler third does not dictate where the planets are found, if by that you mean the distances from the Sun at which they are found, or the distribution of their semi-major axes. That is a matter for accident, or if you prefer, initial conditions.

There is an approximate logarithmic law for the axes which is thought to reflect magnetohydrodynamics in the condensing presolar cloud.

 

[Rader]

Kepler +Titus-Bode law

Just to clarify,I have read that William Hershal used Keplers law to search the sky, in the orbit that it should be found. Kepler lived 1571-1630, Bode 1747-1826, Willliam Hershal 1738-1822. Bode's law had no theoretical justification when it was first introduced; it did, however, agree with the soon-to-be-discovered planet Uranus' orbit (19.2 au actual; 19.7 au predicted). Similarly, it predicted a missing planet between Mars and Jupiter. Then did Hershal and Bode work together to find Uranus? I have not found this. It has been said that Herschal searched the skies using what was know of Keplers law.

 

[selfAdjoint]

I do not know If Herschell used Bode's law, if he did it could only have been as an aid. The way Herschell found Uransus was by analyzing the unexplained perturbations in the motions of Saturn and Jupiter. These planets were found to be sometimes a little ahead and sometimes a litle behind their schedules as computed from Newton's theory.

Herschell assumed these perturbations were due to an undiscovered planet's gravity acting on them. He worked out the forces and was able to locate the planet by crossing two lines of force (actually I'll bet there were more than two!). Once he had a single position, he could use Kepler's third law to figure the orbital period of the new planet, and then he could revise his calculations with old data interpreted in light of that period, and so compute the orbit of the planet he proposed to name Georgium, and which some wanted to call Herschellium, but which was finally named for the Greek sky god Uranus (latinization of Ouranos).

I don't believe Herschell and Bode, who lived in different countries, ever worked together.

 

[Nereid]

AFAIK, Herschel found Uranus by accident, not as the result of a dedicated search (Neptune, however, was discovered not far from where it was predicted, using Newton's laws and the observed deviations of Uranus' orbit from what it should have been if there were only 6 planets).

At the time the first asteroids were discovered (Ceres, in 1801), it was noted that its orbit was about where the Titus-Bode law said there should be a planet, and at first it was thought that Ceres might be that planet.

 

[Rader]

the lucky find

Originally posted by Nereid
AFAIK, Herschel found Uranus by accident, not as the result of a dedicated search (Neptune, however, was discovered not far from where it was predicted, using Newton's laws and the observed deviations of Uranus' orbit from what it should have been if there were only 6 planets).

At the time the first asteroids were discovered (Ceres, in 1801), it was noted that its orbit was about where the Titus-Bode law said there should be a planet, and at first it was thought that Ceres might be that planet.

Methodical plate inspecion was Herschels method, like they use to search comets.

It is often said that Herschel found Uranus accidentally but this was disputed by Herschel himself. As he correctly pointed out, he was using the finest telescope in the world, in a systematic survey of the sky when he came upon the seventh planet. He was certain in his own mind that he would have eventually discovered it given his methodical search. As it was the planet had been sighted and noted in star charts no less than 20 times in the prior 90 years. Shortly after discovering Uranus Herschel found Uranus' two largest Moons Oberon and Titania.
http://www.frostydrew.org/observatory/columns/2002/may.htm

 

[Nereid]

Rader wrote: Methodical plate inspecion was Herschels method, like they use to search comets.

According to the information in the site below, the first photographic plate of a star wasn't taken until 1850, long after Herschel death.

http://www.astro.virginia.edu/~afs5z/photography.html

As to how specific the goals of Herschel's observing were, the material you cite certainly has an authentic ring to it. However, I wonder to what extent he was searching for a seventh planet - and had good reasons for why he searched where he did - as opposed to a general interest in the sky, and was systematically searching the sky for anything interesting?