NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker
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This week marked the 90th anniversary of the discovery of Pluto.
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It’s considered a dwarf planet — a designation that continues to spur vigorous debate.
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Pluto also continues to fascinate astronomers because of its small size, distance from Earth, unusual orbit, and synchronous moons.
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NASA’s New Horizons mission launched in 2016 to explore Pluto and other icy objects in the Kuiper Belt.
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Ninety years ago this week, a young astronomer named Clyde Tombaugh discovered a small, icy object beyond the reaches of the known solar system. It was Pluto.
Astronomers had suspected Pluto’s existence for about 15 years before that — astronomer Percival Lowell had observed deviations in Neptune and Uranus’ orbits that suggested the gravitational pull of another body.
Lowell ultimately died without finding that mysterious “Planet X,” as he called it. Tombaugh continued his work.
When he discovered Pluto, Planet X became real — for a time. But in 2003, a world larger than Pluto was found in the Kuiper Belt — the region beyond Neptune that’s thought to contain trillions of other icy bodies. That eventually prompted the International Astronomical Union to reclassify Pluto as a dwarf planet.
Many planetary scientists still think it’s a planet, though.
“How can an adjective in front of a noun not describe the noun?” Alan Stern, the principal investigator of NASA’s New Horizons probe, said in an interview in 2017. “There are dwarf stars but they’re still considered stars.”
In 2016, the New Horizons mission launched to explore Pluto, its moons, and the Kuiper Belt. The probe has sent back unprecedented images of Pluto and other distant space objects — here are some of our best images of the dwarf planet, and what its discovery taught us about the solar system.
When Tombaugh discovered Pluto (the first frame of the animation below is a digital zoom-in on the image he captured), he was using 1930’s most cutting-edge technology: a “blink comparator.”
That machine compared two images of the sky taken by a telescope a month apart. Tombaugh noted that from one image to the next, a small object shifted by a few millimeters — a sign of an orbiting world.
Pluto orbits the sun at a distance of about 3.6 billion miles (5.8 billion kilometers) on average — about 40 times as far as Earth’s orbit.
NASA
Source: NASA.
Pluto is 5.5 times smaller than Earth, at 1,400 miles (2,380 kilometers) wide. That’s about half as wide as the US, or two-thirds the width of our moon.
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
Source: NASA.
After Tombaugh’s discovery, astronomers learned that Pluto’s orbit is more elliptical than the eight other planets’ paths and not on the same plane as theirs.
Multiple Hubble Space Telescope images were combined to create these views of Pluto’s surface as it spins. They indicate that Pluto’s surface composition might change over time,
NASA/ESA/SRI (M. Buie)
In 1978, US Naval Observatory astronomer James Christy discovered a “small bump” on Pluto’s side. He realized the bump was an object orbiting Pluto: a moon.
The image that Pluto’s large moon Charon was discovered in in June 1978.
U.S. Naval Observatory
Source: NASA.
The large moon is called Charon, and it’s unusual because the time it takes to orbit Pluto is the same amount of time it takes Pluto to rotate, so the moon always hovers over the same spot.
.S. Naval Observatory; NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
They were the first known “double planet” — two bodies that orbit a common center of gravity — to be discovered. Pluto also has four other moons.
In 1992, astronomers David Jewitt and Janet Luu discovered the first Kuiper Belt object aside from Pluto, called 1992 QB1. It was evidence that more objects lurk in the distant solar system.
European Southern Observatory
The discovery suggested Pluto was probably not the only “planet” beyond Neptune.
In the 1990s, NASA discovered more worlds similar to Pluto in the Kuiper Belt. Astronomers think many of the objects there are early remnants of the formation of the solar system.
Then in 2003, astronomers found a Kuiper Belt object that became known as Eris. It’s larger than Pluto, so gave rise to discussions about what it really means to be a planet.
An image from NASA’s Hubble Space Telescope of moons orbiting Pluto.
NASA
In 2006, the International Astronomical Union (IAU) downgraded Pluto from a planet to a dwarf planet. Eris was designated a dwarf planet, too.
The IAU set three criteria to define a full-sized planet:
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The planet is in orbit around the sun.
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It has sufficient mass to assume “hydrostatic equilibrium,” or essentially, a round shape.
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It has “cleared the neighborhood” around its orbit.
The IAU says Pluto “has not cleared its neighboring region of other objects,” since it shares its orbital neighborhood with other Kuiper Belt objects.
“Just so you know, in my view, Pluto is a planet,” NASA administrator Jim Bridenstine said during a tour of the aerospace lab at the University of Colorado Boulder in August 2019.
NASA’s New Horizons spacecraft looked back toward the sun and captured this view of the ice plains and icy mountains along Pluto’s horizon on July 14, 2015.
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
Source: Business Insider.
NASA’s New Horizons mission launched in 2006 to study Pluto, its moons, and other distant objects.
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker
The nuclear-powered spacecraft flew by Pluto in July 2015.
It was the first spacecraft to visit the dwarf planet.
NASA/JHUAPL/SwRI
By the time the probe reached Pluto, it had traveled farther from Earth and been in space longer than any deep-space spacecraft before it (more than nine years).
Charon’s size and proximity to Pluto were two of the main reasons astronomers wanted to send New Horizons there.
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
“The importance of the discovery of Charon really cannot be underestimated,” Stern said in a NASA announcement. “We on the New Horizons team owe a big debt of gratitude to Jim Christy for his landmark discovery.”
The mission sent back new data about Pluto’s surface and moons, and indicated that the system was more complex than astronomers had realized.
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
New Horizons’ data hinted that Pluto could have an internal water-ice ocean.
New Horizons is still in space collecting data.
Pluto’s receding crescent as seen by NASA’s New Horizons probe when the spacecraft was 120,000 miles (200,000 kilometers) from Pluto, March 24, 2017.
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
It flew by the farthest object ever visited, called “Arrokoth,” on January 1, 2019.
Its images revealed Arrokoth to be an icy, snowman-shaped object.
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko
“We’ve never seen anything like this anywhere in the solar system,” Stern said. “It is sending the planetary science community back to the drawing board to understand how planetesimals — the building blocks of the planets — form.”
The animation of Arrokoth below was released last week, compiled from the data New Horizons gathered during its flyby.
A brief animation moves between two New Horizons views of Arrokoth from the spacecraft’s New Year’s 2019 flyby.
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Roman Tkachenko
New Horizons is slated to continue exploring the Kuiper Belt through 2021, and potentially fly by another object there if it encounters a worthy candidate.
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