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Learning objectives

By the end of this section, you will be able to:

  • Compare the orbital characteristics of Pluto with those of the planets
  • Describe information about Pluto’s surface deduced from the New Horizons images
  • Note some distinguishing characteristics of Pluto’s large moon Charon

Pluto is not a moon, but we discuss it here because its size and composition are similar to many moons in the outer solar system. Our understanding of Pluto (and its large moon Charon ) have changed dramatically as a result of the New Horizons flyby in 2015.

Is pluto a planet?

Pluto was discovered through a careful, systematic search, unlike Neptune , whose position was calculated from gravitational theory. Nevertheless, the history of the search for Pluto began with indications that Uranus had slight departures from its predicted orbit, departures that could be due to the gravitation of an undiscovered “Planet X.” Early in the twentieth century, several astronomers, most notably Percival Lowell , then at the peak of his fame as an advocate of intelligent life on Mars, became interested in searching for this ninth planet.

Lowell and his contemporaries based their calculations primarily on tiny unexplained irregularities in the motion of Uranus. Lowell’s computations indicated two possible locations for a perturbing Planet X; the more likely of the two was in the constellation Gemini. He predicted a mass for the planet intermediate between the masses of Earth and Neptune (his calculations gave about 6 Earth masses). Other astronomers, however, obtained other solutions from the tiny orbital irregularities, even including one model that indicated two planets beyond Neptune.

At his Arizona observatory, Lowell searched without success for the unknown planet from 1906 until his death in 1916, and the search was not renewed until 1929. In February 1930, a young observing assistant named Clyde Tombaugh (see the Clyde Tombaugh: From the Farm to Fame feature box), comparing photographs he made on January 23 and 29 of that year, found a faint object whose motion appeared to be about right for a planet far beyond the orbit of Neptune ( [link] ). The new planet was named for Pluto, the Roman god of the underworld, who dwelt in remote darkness, just like the new planet. The choice of this name, among hundreds suggested, was helped by the fact that the first two letters were Percival Lowell’s initials.

Pluto’s motion.

An image demonstrating the motion of Pluto. On the left Pluto’s location in the night sky on January 23, and on the right Pluto’s location has moved on January 29.
Portions of the two photographs by which Clyde Tombaugh discovered Pluto in 1930. The left one was taken on January 23 and the right on January 29. Note that Pluto, indicated by an arrow, has moved among the stars during those six nights. If we hadn’t put an arrow next to it, though, you probably would never have spotted the dot that moved. (credit: modification of work by the Lowell Observatory Archives)

Although the discovery of Pluto appeared initially to be a vindication of gravitational theory similar to the earlier triumph of Adams and Le Verrier in predicting the position of Neptune, we now know that Lowell’s calculations were wrong. When its mass and size were finally measured, it was found that Pluto could not possibly have exerted any measurable pull on either Uranus or Neptune. Astronomers are now convinced that the reported small anomalies in the motions of Uranus are not, and never were, real.

Questions & Answers

what is comets , astroids ,
Sumit Reply
hi, I am yamini, I am in class 8 but very much interested in astronomy and go to NASA, what are the subjects in which I can master and lead to NASA.
yamini Reply
Hello Yamini, Im Ken and I'm avery intrested in joining the NASA too. Is nice to meet you.
Ken
very*
Ken
nice to meet u ken
yamini
What I know about the subjects is that you have to be a master on science and math, also if you know about aviation is better too
Ken
I read that the Russian language is very important, is not a requirement but it's like an extra point!
Ken
yeah like physics, chemistry and maths, they are my most favorite.
yamini
than you can go free of cost
Arush
Yes, That's right!
Ken
how
yamini
Oh! Free of cost?
Ken
hello yamini nice to meet you
Burak
what is time
Abdul Reply
Time is relative
mrunal
pls elaborate
sakshi
the clear defination.I know that.
Abdul
In planet mars there the life exits or not and is there water there
Eshwarsa Reply
see till now nothing can be found as u know that the curiosity rover has struck in mars
Maya
It has been proven that there are water molecules on Mars but not enough that most lifeforms could thrive upon.
Ariana
There are huge amounts of water in the ice caps and under the surface. The surface and chemistry indicate that Mars had cosiderable amounts of water on its surface in the past.
Julius
what is your opinion about the theory of Vedas about modern physics..
Manish Reply
i think in some ways vedas are also correct but not everytime
Maya
I agree
sakshi
hmm even I agree
Samuel
Is there any patened theory about time relativitg in growth and development?
donot Reply
some astronomer's says that there is no alien exist but why search for extra terrestrial intelligence center is established
Eshwarsa Reply
No One Knows That For Absolute Fact, The Universe Is Too Huge To Have Any Type Of Idea About What Exist In The Far Reaches Of Our Universe.....
Adam
Check Out The Drake Equation.....
Adam
their should be aliens as like ours there would be another planet
Maya
which could have existed life on it
Maya
adam i want to ask a question
Maya
can kepler 1st law be applied on all the planets of the universe
Maya
hello, anyone home?
Denise
sjskskfhjkkktewqqw and try?
Lanika Reply
what is this ?
Samuel
hi I am Samuel from India mumbai
Samuel
nice to meet you
Samuel
thats my question, what is this?
penzias and wilson's a discovery of the cosmic microwave background is a nice example of scientific serendipity-something that is found by chance but turns out to have a positive outcome
Jacqueline Reply
how should I make my carrier in astronomy
Ayush Reply
I think that Newton's third law is not appropriate if any also thinks like this please reply me
Ayush
Can you explain your reasoning
Huh
why u think so
joseph
yes Ayush u are right
Yoganshu
I think when we apply force to a object it start moving but , a/c to Newton's third law every action has equal and opposite reaction,so object should also exert equal force on us and it should not move due to balanced force
Ayush
if I am not right then reply me
Ayush
no
Zack
because of friction that opposes that force and help us to move ahead
Manish
but this is not satisfied as third law say another thing
Ayush
you are telling why object moves
Ayush
you have to think a/c to third law
Ayush
its because of its mass
Maya
because it is applying equal and opposite force but also our mass is also less in comparison to the object
Maya
which is why we cant move the object but it can make move us
Maya
manish is too correct in his place because we need to apply force which would overcome the frictional force
Maya
My dear friends, can u plz tell me that among u guys who are in the field of cosmology
Madhav Reply
😢I am not there in cosmology
Samuel
Just A Science Fan.....
Adam
Adam even I am 😥😅😅😂😂
Samuel
I am also not in cosmology but I am just a fan or we can say science and part of NASA is my dream
Yoganshu
yoganshu Arya same here
Samuel
you are from which country
Yoganshu
hi yoganshu
Samuel
India
Samuel
which state
Samuel
I am also from India
Yoganshu
from delhi
Yoganshu
and u...?
Yoganshu
I am from Maharashtra
Samuel
from which state?
Yoganshu
You are a ASTRONOMER ...
Yoganshu
or a scientist..
Yoganshu
or just a member
Yoganshu
What is time...? not about Newton= time is constant..... that all scientists openions n point of view I m knowing. . what can be the Perfect Definition of Time
Madhav Reply
time is what clock reads
Ayush
Who is the best astronomer of India at present time
Gian Reply
Jayant Narlikar, Proponent Of Steady State Cosmology.....
Adam
What is the real colour of sun rays
Gian Reply
white.. so white it becomes violet.. so violet it become ultraviolet
Tom
white and red and yellow
Bianca
Vibgyor
Samuel
the real colour of sunlight is White
Madhav
the Sun's has a variety of waves all throughout the elextromagnetic spectrum.
Jacie
we only see it as a few bc of how some of them get redshifted (? can that term be applied for something so local?) by some particles in our upper atmosphere
Jacie
Vibgyor will be when, the white light will pass through the clouds ( prism ) then Refraction phenomenon leads us to 7 colours splitting from a single colour "White " light
Madhav
so the sun rays r of White colour
Madhav
Taurus in astronomy and horoscope?
Yasser Reply

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Source:  OpenStax, Astronomy. OpenStax CNX. Apr 12, 2017 Download for free at http://cnx.org/content/col11992/1.13
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