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What Was Star K, the Mysterious Missing Planet of the 19th Century?

Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.

Like many good mysteries, it is best to establish the characters. In this true story, I will first examine the background on the individual responsible for the hype over *K, then we shall dive into what this object could have been...


James Ferguson was already a renowned astronomer when he entered into the *K mystery. Before that though, he had found the first asteroid from a 9.6 refracting telescope: 31 Euphrosome, on September 1, 1854. He followed this up with teo more asteroids: 50 Virginia on October 4, 1857 and 60 Echo on September 14, 1860. Clearly, this guy could spot celestial objects. Others noticed his skill, also. Twice (in 1854 and 1860) he received the Lalande Prize of the French Academy of Sciences. He would retire with over 90 papers published and was considered very reliable which is what makes this case so bizarre and something that likely haunted him for years after the object disappeared (Baum 43).


The story of *K began on April 12, 1849 when Annibale de Gasparis, the assistant astronomer at Specola di Capodimonte in Naples, found a new 10th magnitude object in the sky. By comparing its motion to stars around it, the mystery object was found to have retrograde motion and a general heading along the equator.

On May 11th, Fabri Scarpellini announces that after more observations the object is in fact an asteroid and is named 10 Hygiea. Our buddy Ferguson decided to do some follow-up observations through the summer and fall of 1850 using a filar to measure the angular distance with nearby stars as a reference (43, 45).

What is this filar, you may ask? It was a micrometer located “at the common focus of the objective lens” an was a great tool for finding the accurate angular distance between objects. Inside the box was 2 wires on parallel frames which moved towards each other or away, depending on the twist of a screw. One would calibrate the filar for one revolution based on the angular distance seen at that magnification and go on from there. This tool would play a role to come and possibly explain what happened (46-7)

What is It?

With all these observations in tow, the discovery of 10 Hygeia was published in The Astronomical Journal on January 18, 1851. Amongst the readers was John Russell Hind from the Royal Astronomical Society, whose big interest was on planets beyond Uranus. Remember, this was in the aftermath of the Neptune drama, so planets were all the rage. And because Hind was searching, he was familiar with many regions of the sky in hopes of seeing some deviations. One of those patches was the area that 10 Hygiea happened to have been in when Ferguson was making his filar measurements, so Hind took a look. Upon examining that area of the sky, he found that 8 out of the 22 stars Ferguson had used were not in Hind’s stellar catalogue. Upon cross refencing with others, Hind was able to account for all but one, a 9.10 magnitude star labeled *K. Nowhere in his maps could said star be found, so was there a mistake on someone’s part? (47-9)

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After all, 10 Hygeia had been found during a point where it had been travelling in front of a dense star field. But Ferguson had mentioned in his report that he had not included stars he felt had been incorrectly measured. Besides, Hind knew of Ferguson’s quality in his work and therefore did not doubt him. Hind could only conclude it was a variable star or even perhaps a new asteroid. The latter possibility reached Lt. Maury, the chief of Ferguson, through William Bond, a mutual friend of both Maury and Ferguson who also happened to be the director of the Harvard College Astronomical Observatory (49-50).

Once Maury found out, he sent Ferguson on the hunt for *K. The first night of observation was conducted on August 29, 1851 using a large refractor but nothing was spotted. Maury decided to have the 1850 data examined in hopes of spotting it and back tracking its position. They found that the right ascension would have changed but not the declination. Using this, a possible candidate for *K was spotted in October 16-22, 1850 data but interspersed. Hind hears about this and feels *K must be a planet and so writes to William Gram, the secretary of the US Navy, about it. With the recent Neptune debacle, fresh in everyone’s minds, Gram wastes no time in prompting Hind to get on the case. Therefore, Ferguson was again tasked with hunting down *K (50-1).

By November of 1851, a positive identification still eluded Ferguson. Thus, Hind decided to seek some advice and writes to his friend Benjamin Apthorp Gould about *K. Ben feels that based on the right ascension change, the object cannot be a mistake and that the motion means it cannot be between Mars and Jupiter. Using Kepler’s Laws, he arrives as a distance of 137 AU and a period of 1600 years. Of course, Ben knows such a visible object doesn’t turn off and on with such rapidity and he mentions the intrigue behind such a phenomenon (52-3, 38).

Time goes by and still nothing was found. Ferguson grew concerned for if *K was real then it was heading into the Sagittarius constellation which is star dense and hard to get photo plates for. Not much had been documented and so discerning an object amongst a sea of unknowns would be a challenge. Despite this, he presses on and starts to photograph the area on August 29, 1851. He continued on until December 11, when he finally gave up in frustration (54, 38).

So what happened here? We clearly have a respectable astronomer unable to locate an object that was repeatedly viewed but not consistently present. Christian Heinrich Friedrich, a later astronomer of the 19th century, felt the filar was to blame. You see, three different wires are present in it for different levels of calibration, with the number one wire closest to the main screw. Through some calculations of stars near the *K sightings, Friedrich was able to show that a majority of Ferguson’s observations was actually using the first wire when he had recorded using the second. With this in mind, Friedrich converted the data of Ferguson and was able to show that *K was in fact Lalande No. 36613, a normal star. Any moment when the star disappeared was due to misreading the wire value. Many astronomers made sure to emphasize that this was a minor mistake of Ferguson and that no disgrace should befall him (62-3).

© 2017 Leonard Kelley


Leonard Kelley (author) on May 10, 2017:

Thanks, Coffeequeeen! I love mysteries of astronomy and I don't hear people talking too much about this one. Here is to hoping this may change that!

Louise Powles from Norfolk, England on May 10, 2017:

Space and astronomy has always interested me. I've never heard of this before, so reading your article has been really interesting. I've learned something today, thankyou. =)

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