At the turn of the century Alan Stern began planning what would become NASA's New Horizons mission to Pluto. The spacecraft launched in January 2006, only to see the International Astronomical Union demote Pluto to a dwarf planet that August.
At the time, a defiant Stern said the New Horizons team continued to view Pluto as a full-fledged planet, despite what the "official" astronomical community might have to say about it.
Now, a little more than a decade later, with Pluto in its rear view, the New Horizons team is still not dropping the matter. At the NASA-sponsored Lunar and Planetary Science Conference in Texas this week, they presented a short paper (PDF) that would radically redefine what constitutes a planet. The new definition would add not only Pluto, but also more than a hundred other objects in the solar system -- including a number of moons -- to the roster of planets.
This latest bit of agitation from Stern and his team once again has stirred up a science and semantics debate that is simultaneously meaningless and central to how we view the universe.
"Almost all planetary scientists have moved on," Jean-Luc Margot, chair of the Earth, planetary and space sciences department at UCLA, told me via email. "There are a few people, primarily and perhaps not coincidentally associated with the New Horizons mission to Pluto, who apparently cannot accept a universe where Pluto is not classified as a planet."
Margot published a paper in 2015 proposing a refinement to the official IAU definition that would make it easier to classify planets beyond our solar system as well.
The definition adopted in 2006 that kicked Pluto out of the planetary club lays out three criteria for membership: a planet must orbit the sun, be massive enough that its own gravity has forced it into a nearly round shape and have cleared the neighborhood around its orbit.
That last bit is what disqualifies Pluto, because it hangs around in the Kuiper Belt with all kinds of other icy objects.
Kirby Runyon, a member of the New Horizons team and lead author of the new short paper, says that what's happening in Pluto's orbit doesn't change what can be observed about the body itself.
Pluto "has everything going on on its surface that you associate with a planet," Runyon said in a news release last week. "There's nothing non-planet about it."
Runyon and his colleagues argue that a planet shouldn't be defined by its surroundings, but instead as "a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters."
They say the definition could be stated even more simply as "round objects in space that are smaller than stars."
This definition would give us at least 110 official planets in our solar system, including our own moon and more far-flung moons of the gas giants like Europa and Enceladus. The notion has sent smoke sailing out of the ears of a number of planetary scientists this week.
Carolyn Porco, who currently leads the imaging team for the Cassini spacecraft, told me via email: "It would serve no purpose at all, except to make the people who study small-bodies feel more important, to hijack the word planet which, as it is, has consistently all throughout history (except for Pluto) had the implicit (if not explicit) definition of a body that has cleared out its orbital corridor."
Porco was also part of the Voyager mission and an associate member of the New Horizons team. She notes things happening above and below the surface of various worlds, which Runyon prioritizes, does depend on what's happening in their orbital corridors. Subsurface oceans on places like Europa, for example, are possible by virtue of the fact they are moons orbiting a planet and not planets orbiting a sun.
The points of contention when it comes to defining a planet are nearly endless, which explains why the debate continues to persist. For Runyon and the other current New Horizons team members, the goal seems to be less about a rigorous debate over a system of scientific classification than the best way to generate public interest in space, including the New Horizons mission.
Runyon thinks the very word planet carries a certain "psychological weight" and more planets in the solar system would mean more public interest.
"I want the public to fall in love with planetary exploration as I have," Runyon said. "It drives home the point of continued exploration."
But Porco and numerous other scientists, like Columbia University's David Kipping in the video below, make the case that a word already exists to describe what Runyon's broader definition seems to be aiming for: worlds. In fact, I've already used it in this story in that exact sense of the word.
So why can't we just leave planets as they are, try to get the public excited about the wider array of "worlds" out there and hope that the New Horizons folks eventually make peace with Pluto as a dwarf planet?
"The bottom line is that neither the IAU definition nor the new definition make any sense," Larry Lebofsky, who focuses on science education for the Planetary Science Institute, said in an email. "The first is too narrow and the second is too broad."
He thinks getting a better classification scheme is especially important for science education because it helps us put the vastness of the universe in order and understand it. He suggests a scheme of five or more criteria required to classify a body as a planet, so long as it's useful enough to encourage further understanding.
"This way, when you observe an object, and it has one or two characteristics that do not fit into your classification, you are in a position to ask 'why?' and this can lead to new areas of research."
Even if a better process for defining a planet could encourage new research and exploration, the debate is likely to rage on so long as Pluto remains a martyr to the process.
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