In 2006, Pluto was demoted from a full-fledged planet to a dwarf planet. The reason: Pluto was not special anymore.
Astronomers discovered many other Pluto objects in Pluto's section of the solar system, the Kuiper Belt. If Pluto was a planet, why was not Eris? Or Haumea or Makemake?
As Vox explained in 2015, the Kuiper Belt is likely to contain dozens of other Pluto objects yet to be discovered. That, along with the fact that Pluto is tiny, concluded the case: Pluto should be known as a dwarf planet.
But then something unexpected happened: Pluto became incredibly fascinating. When the space probe New Horizons flew past Pluto in 2015, it became clear that at the end of the solar system it was not just a boring sphere of rock and ice. It was a geographically dynamic world. Its mostly smooth surface indicates that its crust has constantly reshaped and wiped out impact craters. Astronomers even speculate that under Pluto's heart-shaped pelvis there is a dynamic, muddy sea.
"When we see [a world] like Pluto with its many known features – icebergs, nitrogen glaciers, blue skies with smog layers – we and our colleagues naturally find the word" planet "describing it," write David Grinspoon and Alan Stern , Authors of a new book about the New Horizons mission, in the Washington Post. (Stern was the principal scientific researcher on New Horizon's mission.) Grinspoon is an astrobiologist and science journalist.)
You're not alone. There is a small grassroots initiative among scientists to expand the definition of what a planet is.
"If you do not call a round world" Planet, "it simply falls away from the mental radar of humans," Kirby Runyon, a planetary geomorphologist at Johns Hopkins University, told me in an interview in 2017. "There is a psychological power to the word" planet "that makes it clear to people that it is an important place in space."
Runyon was part of the team that analyzed the geological data from New Horizons during the flyby. "I was overwhelmed by how beautiful and geologically diverse Pluto and its Charon satellite are," he says. As soon as he saw this side of Pluto, Runyon was bothered that it was no longer a full-fledged planet.
So, along with five colleagues from New Horizons (including Stern) from various institutions, Runyon recently proposed a new definition of a planet that recognizes that large and small space objects have amazing geological features. And they have rekindled the debate over Pluto, which some other planetary scientists say would only be left alone.
Runyon's new definition of a planet would mean that there are hundreds of planets in our solar system
The solar system is full of fascinating moons and dwarf planets that receive no attention because they do not conform to the official definition of the International Astronomical Union, which states that a planet:
- is one Object orbiting the Sun (and not another planet's satellite)
- Is essentially spherical
- Has "cleared" its orbit (ie, it does not divide its orbit with another significant space object)
To The objects that do not suit them include Enceladus, a moon of Saturn that spits huge clouds of steam into space. Jupiter's Ganymede is the ninth-largest object in the solar system. Titan, another moon of Saturn, is the only moon with a dense atmosphere. Nor does the definition take into account the large number of planets astronomers discover outside the solar system. In their post article, Stern and Grinspoon argue that the official definition is "that essentially all planets in the universe are not planets".
Like-minded planetary scientists think the IAU definition, especially the third part of it, is vague and unhelpful. About Christmas 2016, when Runyon was back with his parents in Michigan, he woke up one morning and wrote a new, more comprehensive definition of what a planet is.
"It was just passion, it just bubbled up in me," he says.
Runyon presented it together with several prominent scientific co-authors at the Lunar and Planetary Science Conference in March 2017. Here it is:
A planet is a substellar mass body that has never undergone nuclear fusion and has enough intrinsic gravity to assume a spherical shape … regardless of its orbit parameters.
Or simpler: "A simple paraphrase of our planetary definition – particularly suitable for elementary students could be 'round objects in space that are smaller than stars," write Runyon and his co-authors.
That definition would mean that the moon a planet is all round moon in the solar system would be planets Pluto would be a planet Charon would encircle it
This debate on Pluto's planetary status is unlikely to end soon
In March 2017, Neil deGrasse Tyson responded Runyon's proposal on The Late Show with Stephen Colbert Tyson, a longtime proponent of "Pluto is not a planet," said Pluto sometimes traversed Neptune's orbit, and that "This is not a kind of behavior for a planet , No! "Other planetary scientists, such as Mike Brown, who discovered some objects in the Kuiper belt that displaced Pluto from the planet list, have also not lagged behind." Nobody wants the moon to be a planet, "Brown told the CBC his Twitter handle is still "@ plutokiller .")
But Runyon and his co-authors do not call The IAU wants to take its definition and hopes to grassroots among planetary scientists and science teachers initiate to begin with.
So how would you describe a round moon under this new definition? "They are planets" Runyon says. " And you can mix and match adjectives. Enceladus could rightly be classified as an icy dwarf satellite planet.
Is not that confusing? At elementary school, kids are taught about the Eight Planets in the Solar System, and sometimes that can be difficult.
"Having 110 or more planets should not be considered confusion," he says When he thinks about planets in the new way, he'll help the students understand the science behind them better. "If you've memorized the PE In the new definition, children would understand the intrinsic scientific properties that make a planet a planet make and name them.
I still think it might be confusing.
But Runyon brings another potential benefit of teaching kids more planets, and that's more to my credit: it will fuel their sense of wonder.
"One thing I really want is that [educators] writers and illustrators of children's books about space become aware of this definition," he says. "So kids can show the room so they can see how many places in space they can imagine to bring a spaceship ashore."