Happy
International Pluto Day! Ninety-five years ago today, on February 18, 1930,
24-year-old Clyde Tombaugh discovered Planet Pluto while blinking between
photographic plates of the sky taken on two separate nights in January of that
year.
The goal was to
find anything that moved against the background stars between nights. That
object would not be a star but a planet within our solar system.
The announcement
of Pluto’s discovery was not made until a month later, on March 13, 1930 by the
Lowell Observatory, which chose this date because it was the birthday of its
late founder Percival Lowell as well as the anniversary of the discovery of
Uranus in 1781.
Incredibly,
Tombaugh was not even credited in initial announcements of the discovery. He
was described as a “junior astronomer” and not named, as the observatory wanted
to focus the credit on Percival Lowell, who had predicted the existence of a
planet beyond Neptune but had not actually found it.
Ironically, given
that Lowell died despairing about never having discovered the planet, Pluto
does appear on what are called “precovery” images he took around 1905 and 1906.
It is not unusual for scientists to search earlier images after the discovery
of a celestial object to determine whether that object appeared in a photo but
remained unrecognized.
That was the case
with Lowell and Pluto. He had photographed Pluto, but he didn’t recognize it
for what it was, likely because he was looking for a giant planet like Uranus
or Neptune rather than a small one, which Pluto turned out to be.
Celebrated annually at the Lowell Observatory to this day with an I Heart Pluto
festival, Pluto’s discovery remains in many ways a triumph of underdogs.
Tombaugh had only a high school diploma when he made the discovery. While searching
for the planet, he was told by a professional astronomer that he was wasting
his time, that if there were any more planets to be found, they would already
have been found.
When Lowell Observatory
director Vesto Slipher published an article about the discovery in Science
News-Letter, he credited Lowell without even mentioning Tombaugh.
Similarly, in the same publication, Harvard astronomer Harlow Shapley and Yale
astronomer Frank Schlesinger wrote, “to Professor Lowell…belongs the credit.”
A certain elitism
among astronomers of the time led them to have trouble accepting that a “kid”
without an advanced degree had discovered a planet. Citizen science was not yet
a “thing,” and the notion that an ordinary person could make the type of
discovery for which those with PhDs worked hard for many years appeared to make
those scholars uncomfortable.
After falling into obscurity for decades as the planet the least was known
about, Pluto began to regain popularity during the 1960s and 1970s. Its largest
moon, Charon, really a binary partner, was found in 1978.
Tombaugh himself
described feeling like an outsider, someone who did not quite measure up to
others in his field. This is evident from his sense of genuine surprise at his
burgeoning popularity following the 50th anniversary celebrations of Pluto’s
discovery, in 1980. Biographer David
Levy quotes Tombaugh’s ironic reaction. “I misjudged the attitude of
astronomers. I thought I was a nobody. I thought they had contempt for me”
For almost a
century, Pluto and its discoverer have shared this underdog quality, from some,
a sense that they weren’t good enough and didn’t measure up, but from others, the
sense of being a folk hero and a rock star.
Both were seriously underestimated—Tombaugh as a professional astronomer
and Pluto as a planet.
Pluto may be
smaller than most solar system planets and have an eccentric orbit, but those
things do not preclude it from being a planet. They just make it a different type
of planet. In the last 30 years, exoplanets have been found with orbits around
their stars that are far more eccentric than Pluto’s path around the Sun. Some
of these exoplanets are giant, Jupiter-like worlds.
New Horizons
surprised scientists and ordinary people worldwide, who had expected to see a
dead world but instead were treated to images of one that is geologically
active and has some planetary processes seen elsewhere in the solar system only
on Earth and Mars.
Yet even now,
misinformation about Pluto is rampant. An article about the I Heart Pluto
celebration stated that Pluto’s “reclassification” was done because objects
larger than Pluto were discovered in the outer solar system. This is not true
and is based on information that was proven wrong close to 15 years ago.
Eris was initially
thought to be larger than Pluto, but a team of astronomers led by Bruno Sicardy
observed it occult (pass in front of) a star in 2010 and found Eris to be marginally
smaller than Pluto yet slightly more massive. It is disappointing that so many
years later, the initial error about Eris being larger is still being spread in
the mainstream media.
With time,
Tombaugh has been recognized and fully credited for his discovery. Similarly,
as time passes, I believe Pluto will be recognized for and credited as the
planet it is. It may be different from the solar system’s four terrestrials and
four jovians, but it is far from the only planet of its kind. And it is only a
matter of time before similar small planets are discovered orbiting other
stars.
Meanwhile, the
same persistence and perseverance that led to Pluto’s discovery will motivate
those of us who fight for its rightful recognition as a planet. Dwarf planets
are planets too!
