The Discovery of Pluto

Although the discovery of Neptune rang the death knell for one proposed law of physics, it cemented the status of Newton's law of gravity as paramount.    In addition, the method by which Neptune was discovered clearly suggested that the same method could be used to predict the existence and location of the next planet.  All we have to do is observe the location of Neptune over a period of years, note the discrepancies between the observed and predicted positions, and use those discrepancies to determine the mass and location of the 9th planet.

In 1848, J. Babinet made the first such prediction of a planet with a mass 12 times that of the Earth.  Note that at this time, Neptune, which has an orbital period of 165 years, had only moved about 4 degrees in the sky, out of one 360 orbit, and even today has not yet completed one orbit around the Sun since it was discovered!

Percival Lowell, a rich gentlemen from Boston and a self-taught astronomer, built himself an observatory in Flagstaff, AZ, dedicated to observing Mars and proving that Martians existed. (But that is another story).  He also used the Adams/Leverrier method to predict that a planet of 7 earth masses exists out there beyond Neptune.  He searched in vain for this planet until his death in 1916.

In 1929, the director of Lowell Observatory decided to hire someone to take up the search for the predicted planet.  He hired Clyde Tombaugh, a self taught amateur from Kansas.  The method he used to search for this planet was straightforward: take two images of same part of sky, 2-3 days apart.  Use a "blink comparator" to compare images (up to 400,000 stars per image) on each 14" x 17" photographic plate.  Look for "moving" star.  After about a year, on Feb 18, 1930, he found Pluto, about 6 degrees from Lowell's predicted position.

What is Pluto?

Pluto is an object whose primary orbit is around the Sun.  But what size is it? Too big, too small, or just right?

At the moment of discovery, Pluto was about 10 times fainter than Lowell had predicted.  Assuming brightness is correlated with size, and size correlated with mass, it was clear that Pluto's mass must be ~10 times smaller than had been predicted (i.e., less than one Earth Mass).  This is certainly small enough and apparently big enough to meet our criteria for being a planet. Therefore, we can safely conclude that Pluto is a planet.  Of course, this really makes Pluto the 10th planet, if we count at least Ceres, from among the asteroids.  But who's counting?

For a long time, we knew very little about Pluto.  In fact, at a minimum distance of more than 4 billion km from Earth, we still know very little other than the shape, size and tilt of Pluto's orbit.

What about Pluto's mass?  Initial estimates were based on predictions from how massive the predicted Pluto must be in order to perturb Neptune's orbit such that it would produce noticable discrepancies in the position of Neptune.  These estimates had dropped from about 12 Earth masses to 7 Earth masses by the time Pluto was discovered.

Year EstimatedMass of Pluto
1848 12 Mearth
~1900 7 Mearth
1930 1 Mearth
1960s 0.1 Mearth
1970s 0.003 Mearth
1978 0.0021 Mearth

After the discovery, Pluto's mass was based on guesses about how big it was and what it was made out of.  A big object reflects more sunlight than a small object; a dark object (carbon rich rock; dirty ice) reflects less sunlight than a bright object.  Initial estimates of Pluto's mass, post discovery, began at 1 Mearth.  By the 1960s, the estimated mass had dropped to about 0.1 Mearth and by the early 1970s to 0.003 Mearth.  Finally, in 1978, the matter was settled: Christy and Harrington discovered Pluto's moon, Charon.  From straightforward observations of the orbital period of the moon and the planet-moon separation, one can determine the mass of the moon and planet from Kepler's and Newton's laws.

The answer: Mpluto = 0.0021 Mearth.  The diameter of Pluto is 2390 km.  Charon has a mass equal to 1/6 that of Pluto and a diameter = 1186 km.  It's a good thing that Charon was discovered, because, had it not it appears that Pluto's mass would have become negative by now.

Does Pluto's known mass raise any questions in your mind about it's discovery?

If not, it should.  Pluto was discovered because Percival Lowell, among others was convinced that Neptune was continually in the wrong place.  Why? Some massive object, more distant that Neptune, was tugging on it, competing with the Sun gravitationally for control over Neptune.  This was how Neptune was discovered.  And Neptune has a mass about 18 times that of the Earth.  It is a big object, as it must be to exert that kind of influence on Uranus despite the enormous distances between those two planets.

Cleary, by the same logic, the planet-to-be-discovered that is perturbing Neptune must be a big planet in order to exert effective gravitational tugs on Neptune from great distances.

But wait! Pluto, we now know, has a mass 500 times smaller than the Earth's, 9000 times smaller than that of Neptune.  At best, Pluto's effect on Neptune would be 9000 times smaller than Neptune's effect on Uranus. How could Pluto possibly mess with Neptune's orbit? It can't. Must we therefore conclude that there is yet another large planet, Planet X, that is really responsible for the so-called perturbations in Neptune's orbit and that is hiding out there waiting to be discovered?

In 1995, Miles Standish, of the Jet Propulsion Laboratory, the acknowledged NASA guru of planetary positions and orbits, took on the task of figuring this out.  He used all the data collected by all NASA spacecraft to refine the masses and orbits of all the known objects in the solar system.  He used this information to work backwards and predict where Neptune should have been at each time it was observed in the historical record.   He found that all the historical observations of Neptune were fine, accurate to well within the observational errors.  From the historical observations, one would not be led to predict that another planet existed.  Therefore, the theoretical work that led to the prediction of the existence of Pluto was wrong! There never should have been any such prediction.  A previous generation of astronomers misread and misinterpreted their data and found evidence for what they wanted to see, not for what the data demonstrated.

Percivial Lowell never should have been looking for Pluto and Clyde Tombaugh never should have found Pluto.  But Clyde Tombaugh did find Pluto.  And he found it within 6 degrees of where Lowell predicted it should be but about 10 times fainter than predicted.

Six degrees?  too faint?

Remember Neptune's discovery? It was found on the first night by Johann Galle within two minutes of arc of the predicted position.  And Neptune was about as big and bright as it was predicted to be in order to perturb the orbit of Uranus.  Pluto was ~two hundred times further from the predicted position than was Neptune.  Yet it was found, relatively close to the predicted position, given the huge swath of sky that exists to look in.

Should this give us pause to wonder: Why did Clyde Tombaugh find Pluto in the same general part of the sky that Percival Lowell said he should be searching when there was absolutely no reason, on the basis of the data, to have ever predicted the existence of Pluto?

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