Could Pluto ever be a planet again?
New measurements from Tuesday's New Horizons flyby show that Pluto is bigger than expected – it's one-fifth of Earth’s diameter. Is that enough to make it requalify as a planet?
Pluto and Charon display striking color and brightness contrast in this composite image from July 11, showing high-resolution black-and-white LORRI images colorized with Ralph data collected from the last rotation of Pluto. Color data being returned by the spacecraft now will update these images, bringing color contrast into sharper focus.
NASA-JHUAPL-SWRI
Right before Tuesday's historic flyby mission of Pluto, NASA received updated measurements of the dwarf planet and its moon, Charon. It turns out the icy, dust-colored world is “somewhat larger than many prior estimates,” according to a mission update issued Monday.
“Recent measurements obtained by New Horizons indicate that Pluto has a diameter of 2370 km, 18.5% that of Earth's, while Charon has a diameter of 1208 km, 9.5% that of Earth's,” according to NASA’s New Horizons website.
"The size of Pluto has been debated since its discovery in 1930. We are excited to finally lay this question to rest," said mission scientist Bill McKinnon of Washington University in a NASA press release.
Clyde Tombaugh discovered Pluto on February 18, 1930, and it was classified as a planet that same year.
Pluto remained a planet for more than seven decades. But in a highly contentious 2006 decision, the International Astronomical Union (IAU) voted to change the official status of Pluto from that of a planet to the new “dwarf planet” category. Other planets included in the list of dwarf planets include Ceres, Eris, Makemake, and Haumea, although it is possible that more than 100 dwarf planets exist. In this vote, the IAU distinguished planets from “dwarf planets” and “Small Solar System Bodies,” and Pluto became the prototype for “Trans-Neptunian Objects” – a new category of space objects.
But now that Pluto is bigger than we thought, can it go back to being a planet?
Probably not. In its 2006 vote to reclassify Pluto, the IAU came up with its first-ever definition of 'planet,' which includes three criteria. First, it must orbit the sun, second it must be more or less round, and third, it must "clear the neighborhood" around its orbit. Pluto meets the first two, but falls short of the third, crossing the orbit of Neptune and those of other objects in the Kuiper belt, where it is located.
But not every scientist supported Pluto's reclassification.
In a 2011 interview with SPACE.com, Alan Stern, a planetary scientist at the Southwest Research Institute in Boulder, Colo., reasserted his opinion that the IAU reclassification of Pluto was “absurd.”
He specifically objected to the stipulation that Pluto had to clear the neighborhood of its orbital trajectory, saying “In no other branch of science am I familiar with something that absurd. "We're going to call it a cow, except when it's in a herd." A river is a river, independent of whether there are other rivers nearby. In science, we call things what they are based on their attributes, not what they're next to.”
“Our understanding about the universe and our place in it has changed over time. New information can cause us to rethink what we know and reevaluate how we classify objects in order to better understand them. New ideas and perspectives can come from questioning a theory or seeing where a classification breaks down,” according to NASA’s information about planet classification.
The mission plans to determine the size of the moons around Pluto in the upcoming portion of their mission.