History of Pluto

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Lowell Observatory and the community of Flagstaff are known as the Home of Pluto because of so many Pluto-related discoveries made here. Explore upcoming Pluto-related tours to plan an in-depth dive into our history.

Pluto and Lowell Observatory

Lawrence Lowell/Pluto Discovery Telescope

Early Searches for a 9th Planet

Since Neptune’s discovery in 1846, several astronomers had suggested the existence of yet another planet based on apparent irregularities in the orbit of Uranus. Percival Lowell began expressing his own views about this theoretical planet in his 1903 book, The Solar System, based on a series of lectures he had presented the previous year at MIT.

These first passages by Lowell were brief but marked the beginnings of what would turn into his passionate quest to find the planet he eventually referred to as “X”. He would not live to see his vision realized, but he laid the groundwork for Clyde Tombaugh’s eventual discovery in 1930.

The first phase of Percival Lowell’s hunt for Planet X ran from 1905 to 1909. He referred to the effort as “The Invariable Plane Search.” Lowell hired a team of computers—in those days, computers were people who carried out mathematical calculations–led by head computer Elizabeth Williams. They calculated likely locations for the new planet based on the observed perturbations of Uranus. Lowell then photographed these areas with his 24” Clark Refractor. When he realized this instrument was inadequate for the search because of its small field of view, he purchased a new five-inch Brashear telescope to continue the work.
In 1910 Lowell escalated his search after his former colleague-turned rival William Pickering published an orbit and positions of a theoretical trans-Neptunian planet. Lowell now saw his search as a race to beat Pickering and other would-be planet discoverers to the punch. From 1910 through his death in 1916, he spent significant hours on this second phase of his quest.

Lowell redoubled his mathematical efforts by incorporating the latest technology, including a Millionaire calculating machine that is still on display at Lowell Observatory. He also borrowed a nine-inch telescope from Swarthmore College’s Sproul Observatory, a vast improvement over the five-inch Brashear. With new calculations and improved equipment, Lowell estimated locations of Planet X and published his findings in the 1915 Lowell Observatory publication, Memoir on a Trans-Neptunian Planet. Unfortunately, Lowell died the following year, before he had a chance to complete a photographic search in the targeted area of the sky.

Adapted from Pluto and Lowell Observatory, Kevin Schindler and Will Grundy, 2018.

Clyde Tombaugh Discovers Pluto

Eleven years after Lowell’s death, the final phase of Lowell Observatory’s search for Planet X commenced when Roger Lowell Putnam, Percival’s nephew, became the observatory’s sole trustee. One of his top priorities was to resume the search for Planet X and prove Lowell’s predictions.

Abbott Lawrence Lowell, Percival’s younger brother and President of Harvard University, donated $10,000 for the construction of a new telescope, a thirteen-inch photographic instrument known as an astrograph. To operate this telescope, director V.M. Slipher hired a young man from Kansas of “the self-made type,” Clyde Tombaugh.

Tombaugh arrived in Flagstaff in 1929 and soon took over the systematic search for Planet X, examining the area of sky Lowell had indicated in his Memoir on a Trans-Neptunian Planet. The 13-inch telescope was ideal for the search, and Tombaugh had the patience and attention to detail necessary for the work.

At night, Tombaugh captured images of the sky on glass photographic plates. He then examined pairs of these plates that captured the same portion of the sky but were taken a few days apart. His job was to use a machine called a blink comparator to look at every image from one plate to the other and see if it changed position. Most did not, but those that did represented a possible planet that changed position relative to the background stars. Later in life, Tombaugh estimated he spent 7,000 hours at the blink comparator eyepiece during his 14-year career at Lowell Observatory, an average of 500 hours per year or 63 full working days.

Tuesday, February 18, 1930, started out as a typical day for Tombaugh. He woke up at about 7:00 a.m. and drove down Mars Hill to downtown Flagstaff, where he ate breakfast and picked up the observatory’s mail.

Tombaugh returned to the observatory and by 9:00 a.m. was in the blink comparator room, settling in for another tedious day of staring through the comparator’s microscope eyepiece. The plates he examined on this day captured a portion of the constellation Gemini. He spent three hours at the comparator that morning.
At noon, Tombaugh drove downtown for lunch and returned by 1:00 p.m. for another lengthy session of blinking. At about 4:00 p.m., he saw a faint image popping in and out of view. He had been doing this blinking business long enough to know when he had an obvious planet suspect. This definitely was one. With mounting excitement, he spent the next 45 minutes taking measurements and checking a backup set of plates taken with a 5-inch telescope mounted to the main 13-inch. Sure enough, the images were also on these plates, in the exact expected position.

Tombaugh called in astronomer Carl Lampland from across the hall, then hurried to observatory director V.M. Slipher’s office. With as much composure as he could muster, Tombaugh announced, “I have found your Planet X.”

Adapted from Pluto and Lowell Observatory, Kevin Schindler and Will Grundy, 2018.

Studying Pluto Through the Years

Following Pluto’s discovery, observations continued for a while at Lowell Observatory, chiefly by Earl Slipher and Carl Lampland. But the technology of the day severely limited what they could learn. Pluto was never more than a speck on the emulsion of the glass photographic plates used for imaging the sky in that era, and it was too faint for the detailed spectroscopic studies that could have revealed its surface composition or the presence of its atmosphere. Slipher observed Pluto by eye using Lowell Observatory’s 24-inch Clark refracting telescope, in the hopes of resolving its disk and determining its diameter, but even on nights of excellent seeing he was unable to make out a disk and concluded that Pluto could be no larger than half an arcsecond across, corresponding to an upper limit of seven thousand miles on its diameter. Carl Lampland photographed Pluto through various color filters and was the first scientist to report its yellowish color, but he was unable to detect any patterns of variation in its brightness or color due to rotation. Lampland continued to record photographic plates of Pluto using the 42-inch telescope at Lowell Observatory until his death in 1951, and the resulting collection of glass plates eventually went on to play a role in navigating New Horizons to its successful encounter with Pluto 

But after 1951, the staff of Lowell Observatory does not appear to have done any further Pluto research for several decades.

It was Lowell staff’s opportunistic pursuit of occultations that finally brought the observatory back into the business of studying Pluto, when a stellar occultation visible from Australia and New Zealand was predicted for 1988. An occultation occurs when two celestial bodies become aligned such that one of them blocks, or occults, the view of another.

The 1988 event provided a key breakthrough in Pluto studies, proving the existence of Pluto’s atmosphere and revealing its thermal structure.

Future Lowell scientists Ted Dunham and Amanda Bosh joined a team that studied the event aboard an airborne observatory. On the ground, some groups observed from well-placed, fixed observatories, while others used portable systems. Lowell’s Bob Millis and Ralph Nye set up in northeastern Australia. Their observations, coupled with those of the other teams, revealed that the occulted star’s light dimmed gradually, which indicated an atmosphere was present.

Pluto occultation studies have continued at Lowell through the present day, involving numerous additional staff members over the years.

Meanwhile, astronomers at Lowell also used the Hubble Space Telescope to study Pluto. Several observations starting in the mid-1990s led Lowell’s Marc Buie to create the first albedo maps of Pluto, which showed the changes in reflectance of the planet’s surface.

Buie later collaborated with Will Grundy, who arrived at Lowell in 1997 and since then has carried the torch of Pluto research at Lowell, using both observational and laboratory techniques to characterize the composition of Pluto and other icy bodies.

Adapted from Pluto and Lowell Observatory, Kevin Schindler and Will Grundy, 2018.

Lawrence Lowell (Pluto Discovery) Telescope Restoration

After Clyde Tombaugh discovered Pluto with the 13” Lawrence Lowell Telescope, he continued searching for other planets until 1942, covering about 75% of the sky. The telescope was subsequently used to study asteroids and comets and search for small natural satellites of Earth and the Moon.

Later, astronomers found the telescope to be a perfect tool for one of the most important yet least known projects at Lowell, a proper motion survey of stars that compared the apparent versus actual movement of stars over time. The telescope was moved to Lowell’s Anderson Mesa dark sky site in 1970 and used for this survey until 1980.

Scientists again used the telescope for asteroid studies until it was returned to its original dome on the main Lowell campus on August 11, 1993. The telescope and its dome were put on display so visitors could experience firsthand the instrument used to discover Pluto.

More than two decades and a million visitors later, the facility was in need of refurbishment.
In 2016, after 87 years of constant use, first for research and later education, the telescope’s lens assembly, mount, and other components were simply worn out.

The refurbishment included testing and fixing the telescope’s drive motor and other electronic components; restoring plate holders and control knobs; cleaning plate holders, control knobs, optics, and other parts; and removing, cleaning, and reinstalling the telescope itself. The restoration team was the same one that fixed up Lowell’s magnificent 24” Clark Refractor. It consisted of five Lowell staff members with expertise in telescope maintenance and restoration, instrumentation, machining, and woodworking. The work started in 2017 and was finished the following year.

New Horizons Reveals the World of Pluto

The face of Pluto changed forever with New Horizons’ flyby in 2015. It would now grow from the dot that Clyde Tombaugh saw when he discovered the Planet 85 years earlier, to a world with active geology and awe-inspiring surface features.

Many members of the New Horizons Science had Lowell Observatory ties, particularly Will Grundy, who headed the mission’s surface composition team.

The first data set to be transmitted back after closest approach was colloquially referred to as the New York Times dataset. It contained a few choice observations from each of New Horizons’ scientific instruments, highly compressed in order to get them down as quickly as possible. The compression produced strange-looking jpeg artifacts, but the data enabled the science team to release a spectacular series of images and new discoveries over the days following the encounter, culminating in another press conference on July 17. Images from the NYT dataset lived up to the ambition of their nickname, appearing above the fold on the front page of the New York Times on July 16.

After the encounter phase, the science team dispersed and the pace relaxed, but only a little. The calendar was dictated by the slow communication rate from the edge of the solar system. Owing to the low power of the spacecraft’s radio transmission (24 watts maximum) and small size of its antenna (seven feet), the highest possible data rate was only about 2,000 bits (2 kbit) per second. This is extremely slow, as anyone old enough to remember connecting to the Internet over a telephone line using a modem will recall. Typical modem speeds were much higher at 24, 36 or 56 kbit/sec. At this rate, which Lowell astronomer Gerard van Belle characterized as a “tweet per second,” it took about a year and a half for New Horizons to transmit all of the encounter data home, though of course the science team prioritized the order so that the most important observations were sent first. There was plenty of work to be done as the encounter data trickled in. For an entire year, the team put out weekly image and news releases. The process to develop these communications was modeled on the daily cycle of meetings during the encounter phase but now conducted via telecons instead of in person and spread over the course of a week, instead of repeating every day. The team was also busy writing scientific papers, including five that appeared in a special issue of Science in late 2016 and dozens more that came out in a special issue of Icarus in early 2017. There were also presentations to create and present at a series of scientific meetings, colloquium talks whenever anyone visited another institution, plus countless public talks. Considerable data processing work was needed, too, since all of the data had to be delivered to the public Planetary Data System (PDS) archive, along with higher-level products such as maps and image mosaics. Many of the papers and data products had delivery deadlines, meaning that the quality of the work had to be tailored to the available time rather than doing the best that can be done and taking as long as that requires, a very different mode from how individual scientists usually operate. Thus, the early papers were quickly superseded by subsequent ones, and those were, in turn, eclipsed by later papers.

New Horizons Pluto Was a Success

Pluto was the first of its class of small icy planets to be explored. Through New Horizons, a new generation across the globe experienced the thrill of planetary discovery for the first time.

Here are of some of New Horizons’ most remarkable Pluto findings, according to Principal Investigator and Lowell Advisory Board member Alan Stern:

  • The complexity of Pluto and its satellites is far beyond what we expected.
  • The degree of current activity on Pluto’s surface and the youth of some surfaces on Pluto are simply astounding.
  • Pluto’s atmospheric hazes and lower-than-predicted atmospheric escape rate upended all of the pre-flyby models.
  • Charon’s enormous equatorial extensional tectonic belt hints at the freezing of a former water ice ocean inside Charon in the distant past. Other evidence found by New Horizons indicates Pluto could well have an internal water-ice ocean today.
  • All of Pluto’s moons that can be age-dated by surface craters have the same, ancient age—adding weight to the theory that they were formed together in a single collision between Pluto and another planet in the Kuiper Belt long ago.
  • Charon’s dark, red polar cap is unprecedented in the solar system and may be the result of atmospheric gases that escaped Pluto and then accreted on Charon’s surface.
  • Pluto’s vast 1,000-kilometer-wide heart-shaped nitrogen glacier (informally called Sputnik Planum) that New Horizons discovered is the largest known glacier in the solar system.
  • Pluto shows evidence of vast changes in atmospheric pressure and, possibly, past presence of running or standing liquid volatiles on its surface – something only seen elsewhere on Earth, Mars and Saturn’s moon Titan in our solar system.
  • The lack of additional Pluto satellites beyond what was discovered before New Horizons was unexpected.
  • Pluto’s atmosphere is blue. Who knew?

Adapted from Pluto and Lowell Observatory, Kevin Schindler and Will Grundy, 2018.

Pluto Timeline

Highlights of Lowell’s Pluto Legacy

1902 Percival Lowell Suggests Existence of a 9th Planet

In the course of studying Mars and attempting to prove the presence of intelligent life there, Percival Lowell familiarized himself with the rest of the known planets and the solar system in general. His research led him to suggest,  in 1902, the existence of a ninth planet based initially on the perceived relationship between the orbits of planets and meteor showers, as well as with comets. He published these ideas the following year and, while his “evidence’ for a new planet changed, his certainty of its existence solidified.

1905 Lowell’s First Search Begins

By 1905, Percival Lowell felt so certain of the existence of a ninth planet that he commenced a search. It was a fairly quiet effort, and not very organized. To some degree it was more a matter of hunting and pecking than a dedicated search. He involved a few staff members who experimented with several different telescopes and related equipment. Targeting areas of the sky where Lowell believed the planet lurked, his team captured images on glass photographic plates and then examined them for the suspect. This initial search lasted until 1910.

1910 Lowell’s Second Search Begins

Spurred by the news that an astronomer at Harvard College Observatory, William Pickering (who had previously helped Lowell establish his observatory) was also looking for a planet, Lowell put his search efforts into high gear. He acquired more appropriate telescopes and purchased a device called a blink comparator (see below). He also brought on several “computers”, mathematically astute people who generated calculations to help Lowell figure out where he should look for his “Planet X”. Lowell continued this two-pronged, mathematical/observational approach for several years but died in 1916 before hitting pay dirt.

1911 Lowell Purchases Blink Comparator

Perhaps the most critical instrument Lowell acquired during these early searches was a blink comparator machine. This specialized stereo microscope held the photographic plates side-by-side. A mechanical shutter allowed the observer to alternately see first one image and then the other while looking through a microscope eyepiece. The instrument was originally designed to hold 6” x 7” plates, but Lowell staff found they could more proficiently examine the sky using 14” x 17” plates. Lampland modified the apparatus accordingly by designing slip frames allowing for a quarter of the large plates to be examined at a time.

Nineteen years after the purchase of the blink comparator, Clyde Tombaugh used it to discover Pluto.

1930 Clyde Tombaugh Discovers Pluto

On February 18, 1930, Clyde Tombaugh discovered Pluto. Thus kicked off a hectic time for the entire observatory staff, as they worked furiously to gather as much data as possible about the new planet. Tombaugh took more plates with the 13” astrograph and also photographed the new object with the 24″ Clark, while Carl Lampland imaged it with the 42” in hopes of seeing  the body as a disk (he came up empty). The observations were still critical, because Slipher wanted to not only confirm that the body was orbiting beyond Neptune, but also determine its orbit. This was only possible by measuring its movement over as many days as possible. Tombaugh stopped blinking so Lampland could use the blink comparator for these position measurements; in fact, Tombaugh didn’t resume blinking until May 26. None of the Lowell staff—the Slipher brothers, Lampland, or Tombaugh—had any experience calculating orbits, so they discreetly contacted several colleagues from other observatories to help. 

By mid-March, the scientists hadn’t yet worked out an orbit but otherwise felt they had enough evidence to justify announcing this object as a new planet. The question now became, “When should we tell the world?” Observatory staff chose March 13 because that would have been Percival Lowell’s 75th birthday, a fitting tribute to the man whose inspiration led to the discovery of this new planet. Furthermore, William Herschel discovered Uranus on March 13, 1781.

On the evening of March 12, Lowell director V.M. Slipher sent a telegram to the Harvard College Observatory. From this, officials created Harvard College Observatory Announcement Card 108, which officially announced the new planet’s discovery when circulated the following day.

1978 Jim Christy Discovers Charon

Since Pluto’s discovery in 1930, astronomers searched for possible moons. They came up empty-handed until 1978, when Jim Christy discovered Charon.

Christy had ties in Flagstaff, having worked at the U.S. Naval Observatory Flagstaff Station (NOFS). This facility was established in 1955 to serve as the Naval Observatory’s dark-sky site for optical and near-infrared astronomy. Christy began working there in 1962, spending much of his time photographing and precisely measuring the positions of double stars while studying astronomy at the University of Arizona in Tucson. Occasionally for his research, he used Lowell Observatory’s 24” Clark Refractor. Christy earned his bachelor of science degree in astronomy in 1965 and continued working at NOFS until a transfer took him to the Naval Observatory’s headquarters in Washington, D.C. In the summer of 1978, he was splitting his time between moving into a new home and continuing a project he had started the previous year that involved measuring the position of Pluto in order to refine its orbit.

To make these calculations, Christy requested astronomers at his old stomping grounds in Flagstaff to take several images of Pluto using NOFS’s sixty-one-inch telescope. Named the Kaj Strand Telescope, after a Danish astronomer who served as scientific director of NOFS from 1963 to 1977, this instrument saw first light in 1964 and is the largest telescope operated by the Naval Observatory. Located four miles west of Flagstaff, its iconic dome is a landmark to travelers along Interstate 40 near the community of Bellemont.

NOFS astronomer Anthony Hewitt took the requested Pluto images on April 13 and May 12, 1978, capturing them, like Clyde Tombaugh had in his discovery of Pluto nearly a half century earlier, on photographic glass plates. They were then sent to Christy in Washington, D.C., and on June 22, he began examining these and other Pluto plates from 1965, 1970 and 1971. Christy used a high-precision measuring machine called Starscan that projected an image—at thirty times magnification—onto a three-foot screen.

Many of the plates seemed to be of poor quality because Pluto appeared asymmetrical, with a bulge in the north– south orientation. What really got Christy scratching his head, however, was the fact that the background stars didn’t exhibit the same lopsidedness. Elongation of all the bodies—Pluto and the stars—would have been easily explained as defective images, but with only Pluto being distorted, the answer had to be something else.

In the foreword to the book Out of the Darkness: The Planet Pluto by Clyde Tombaugh and Patrick Moore, Christy wrote, “The elongation was fainter than the core of Pluto’s image, which made it appear unlike any image caused by motion of Pluto in its orbit. I thought about the possibility of a flare of some sort being emitted by Pluto, but at the instant I realized that the 12 May elongation was north of Pluto and that the 13 April elongation was to the south, the concept—moon—jumped into my thoughts.”

The next day, Christy detected the bulge on enough of the plates to determine that it migrated around Pluto over a period of 6.39 days. His colleague James Harrington made some orbital calculations that confirmed this observation. Thus, not only did Christy have a new moon, but he also knew its orbit. 

As the person who discovered Pluto’s companion, Christy had the honor of naming it. He initially favored Oz (from The Wizard of Oz) and Char-on (his wife’s nickname plus “on” to make it an object, like “electron”), but these didn’t hold with the convention of naming moons after mythological characters. However, in scanning through his dictionary late one night, he was astounded to see that Charon was a real name, the mythological boatman who ferried the souls of the dead across the River Styx to the underworld, which was the domain of Pluto. Christy had his name, one that followed appropriate scientific naming rules while honoring his wife. Charlene likes to joke, “Many husbands promise their wives the moon, but mine delivered.”

1988 Pluto’s Atmosphere Discovered

For years, astronomers suggested the presence of an atmosphere around Pluto but lacked proof.  Finally, in 1985, a team of scientists opened the door for gathering such evidence when they predicted a June 9, 1988 occultation involving Pluto and a 13th magnitude star in Virgo.

The blocking of a star’s light by a closer celestial body can help astronomers learn a lot about the occulting body, such as its diameter and presence of a ring system (the Uranian rings were discovered using this method in 1977) and atmosphere. In a typical occultation, starlight will abruptly disappear as the occulting body moves in front of it. However, if an atmosphere is present, the starlight will dim more gradually.

The majority of the occultation would occur over the ocean so Elliot’s MIT team, which included current SOFIA team leader Ted Dunham, decided to observe it using the Kuiper Airborne Observatory (KAO). The KAO consisted of a 36-inch telescope mounted in a Lockheed C-141A Starlifter aircraft.  Flying at an altitude up to 48,000 feet, KAO operated from 1974-1995 and was the immediate predecessor of SOFIA.

Meanwhile, other scientists dragged telescopes to seven ground-based observing sites, three in New Zealand and four in Australia.  

Thanks to excellent preparation and a dose of luck that included ideal weather conditions, observers at the various stations detected a slow dimming of light as Pluto passed in front of the distant star, proving the existence of Pluto’s atmosphere. Today we know this thin veil of gas consists of nitrogen, methane, and carbon monoxide gases which originate from surface ices that have sublimated.

2015 New Horizons Flies By Pluto

During the third week of July 2015, Lowell Observatory’s Will Grundy played a prominent role in the celebrated fly-by of Pluto by the New Horizons spacecraft. Grundy led the mission’s surface composition team and balanced his time during this frenetic week between analyzing incoming data and explaining mission highlights to the two hundred media representatives who flocked to the mission’s operations center at the Applied Physics Laboratory in Laurel, Maryland.

As New Horizons gathered so-called fail-safe data just before closest approach (in the event the spacecraft was destroyed by impacting a previously undetected moon while approaching Pluto, the mission wouldn’t have been a total loss), Grundy could really start looking at some exciting data, though this would serve merely as a warm-up for the closest approach data that will stream back to Earth over the next year and a half.

From this fail-safe data, Grundy and his team created preliminary but nonetheless breathtaking false-color images of Pluto and Charon showcasing variations in surface material and features of each body. Pluto, for instance, contains an abundance of methane ice but its appearance varies from the north ice cap to the Tombaugh Regio region (the light-colored, heart-shaped region of Pluto).

Charon exhibited a six-mile-deep gash just visible on the moon’s limb and a spectacular series of canyons and cliffs spanning 600 miles.
Pluto’s image was just as remarkable, revealing a startling range of mountains up to 11,000 feet in height. The presence of these mountains, along with an unexpected crater-free surface, has had scientists scratching their heads and will certainly lead to new theories on the development and behavior of the Pluto system and icy worlds in general.

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