Everyone's favorite celestial outcast may no longer be an official planet, but that doesn't mean we're any less fascinated by it.
Pluto was discovered in 1930 by American astronomer Clyde Tombaugh, but after spending several decades as our solar system's ninth planet, its classification came under scrutiny in 1992 when scientists found objects of similar (and in some cases, greater) size to Pluto in the nearby Kuiper belt. After years of heated debate, Pluto was officially demoted from "planet" to "dwarf planet" in 2006 when the International Astronomical Union formerly defined the difference between these two types of celestial bodies.
A planet was a celestial body that:
- Is in orbit around the sun.
- Is round or nearly round.
- Has "cleared the neighborhood" around its orbit (be gravitationally dominant in its area).
Sadly, Pluto was demoted thanks to the third definition. Although there has been resistance to this relabeling by a handful of scientists, the most passionate cries came from the public. After all, everyone wants to root for the underdog!
Even, planetary scientist Philip Metzger argued in 2018 that Pluto should be reinstated as a planet. In his study, a team of scientists combed through 200 years of literature and only found one example from 1802 that used "clearing the neighborhood" as a requirement, which is based on a since-disproven rationale.
"It's a sloppy definition," Metzger told New Atlas. "They didn't say what they meant by clearing their orbit. If you take that literally, then there are no planets, because no planet clears its orbit."
The ostensible "snub" remains a comical sore spot for many people, which is why the Internet exploded with countless Pluto jokes and memes as NASA began releasing images from the first-ever fly-by of the dwarf planet in 2015.
The spacecraft that conducted the mission, New Horizons, was launched from Cape Canaveral on Jan. 19, 2006 — almost exactly seven months before the IAU convened in Prague to determine the fate of Pluto.
Since then, the probe has endured a long and lonely journey to reach Pluto and the nearby Kuiper belt in hopes of better understanding how these celestial bodies and our solar system at large were formed.
While there are plenty of sarcastic Pluto jokes to go around, there's also plenty of genuine, unbridled enthusiasm in response to the data yielded from the fly-by. To express this excitement, NASA even released a series of mosaic images of Pluto and its moon, Charon, that is composed of response to the #PlutoTime social media campaign:
"The 'Pluto Time' concept and widget was developed by the New Horizons science team so that people could experience the approximate sunlight level on Pluto at noon — generally around dawn or dusk on Earth," NASA explains in a press release. "The mosaics include not only dim skies on Earth, but famous landmarks, selfies, and even family pets."
The mosaic images are based on these magnificent, color-enhanced portraits of the dwarf planet and its moon:
"Pluto’s surface sports a remarkable range of subtle colors, enhanced in this view to a rainbow of pale blues, yellows, oranges, and deep reds. Many landforms have their own distinct colors, telling a complex geological and climatological story that scientists have only just begun to decode," writes NASA editor Tricia Talbert.
On the other hand, Charon's color palette is a bit more subdued apart from the northern polar region (known informally as "Mordor Macula"), which is tinted with a rusty red color.
Continue below to see more New Horizons imagery released by NASA.
Methane ice and dunes
This image is one of the highest-resolution images captured by the New Horizons spacecraft and features mountains that end along the shoreline of the Sputnik Planum, a nitrogen-based ice plain. The entire area in this image is about 50 miles wide.
In 2018, researchers further studied the appearance of dune-like patterns on the Sputnik Planum. Instead of running parallel like sand dunes on Earth, these patterns run perpendicular to the wind. They believe the dunes could be formed by grain-sized solid methane ice that "have been lofted into the atmosphere by the melting of surrounding nitrogen ice or blown down from nearby mountains."
Frigid mountains and icy plains
The Tartarus Dorsa Mountains (top left) are characterized by intricate patterns of blue gray ridges giving way to reddish valleys. The image at top right is one of the sharpest images of Pluto's surface to date — spanning across about 75 miles, we can see a group of reddish ice mountains flanked by textured plains in all directions. Finally, the image at the bottom shows a stunningly textured 3-D perspective of Pluto's mountains that was captured about 15 minutes after New Horizons' closest approach.
Pluto's blue sky
This dramatic, eclipse-like photo shows us the high-altitude blue haze of Pluto's thin atmosphere, which is believed to be quite similar to Saturn's moon, Titan.
"The source of both hazes likely involves sunlight-initiated chemical reactions of nitrogen and methane, leading to relatively small, soot-like particles (called tholins) that grow as they settle toward the surface," NASA writer Sarah Loff explains. "This image was generated by software that combines information from blue, red and near-infrared images to replicate the color a human eye would perceive as closely as possible."
A spinning world beneath you
In the time-lapse video above, Pluto's crater-filled landscape rushes by under the New Horizons spacecraft. The time-lapse stills represent only a fraction of the images taken of Pluto's surface and collected by New Horizons.
The heart of Pluto
When NASA's images of Pluto first came out, one of the most extraordinary findings was the heart-shaped expanse (known informally as the Tombaugh Regio) that stretches about 990 miles across the planet's surface. In the color-enhanced photo above, we see the western lobe of the "heart," which has been informally referred to as the Sputnik Planum.
Pluto's heart is made of ice — huge glaciers consisting of of nitrogen, methane and carbon dioxide. But how did it form? Researchers from the Université Pierre et Marie Curie in France recently published a study in the journal Nature offering an answer. "We developed a thermal model of Pluto's surface to understand the mechanisms of condensation/sublimation of its ice at a global scale," lead researcher Tanguy Bertrand told ResearchGate. The thermal model showed that underneath the ice is a deep basin about 2.5 miles deep. This basin traps the cold and collects the ice into its trenches.
"[W]e discovered that the heart shape is to a large degree created by highly volatile nitrogen ice that unavoidably accumulates in the basin and forms a permanent reservoir of ice, as observed by New Horizons," Bertrand told ResearchGate.
The five moons of Pluto
Charon wasn't the only one of Pluto's moons to get some love from New Horizons! While Nix and Hyrda stretch about 25 miles in diameter, Kerberos and Styx average only about 6-7 miles.
Editor's note: This story has been updated since it was originally published in October 2015.