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Pluto is still a planet, according to NASA Administrator Jim Bridenstine. The announcement puts him in adept company with noted homo astronomer Jerry Smith (above correct) who staked out a like position on our solar system'south smallest and nearly distant dwarf planet. Smith was unavailable for comment, but Bridenstine was willing to speak out on the controversial topic anyway. While attending a Beginning robotics event in Colorado, Bridenstine stuck upward for the diminutive orb.

"Just so y'all know, in my view, Pluto is a planet. And you tin can write that the NASA Administrator declared Pluto a planet in one case once more. I'm sticking by that, it's the manner I learned it, and I'k committed to it," Bridenstine said.

Bridenstine is, of course, wrong. Bridenstine is, of course, right. How you read the situation depends on which sources of authority you credit and how much you lot intendance about things like being aligned with the opinions of scientists and astronomers. But before we discuss that epistemological point, let's talk virtually planets. Why isn't Pluto a planet any longer?

The Problem With Planets

Once upon a time, the number of planets considered to exist part of the solar system was far higher than it is today. Improvements to telescope design throughout the 1800s soon meant that astronomers were swimming in "planets," including objects similar Vesta, Ceres, and Juno. At the fourth dimension of their discoveries, all of these asteroids were considered to be planets. The discovery of 10 Hygea was hailed in the 1850 Annual of Scientific Discovery, which declares that the solar system is now comprised of 18 planets — more than than double the number we recognize today.

As for Pluto, it has a Kuiper Belt problem. In 1992, scientists discovered 15760 Albion and with information technology, confirmed the existence of the Kuiper Chugalug. It was the first trans-Neptunian object (TNO) to be discovered after Pluto and Charon, but it was far from the last. In that location are now more than two,000 TNOs known to exist, and while Pluto bears little resemblance to any of the inner planets, its erratic orbit and full general characteristics fit the template of a TNO perfectly. (Fun fact: Neptune's moon Triton is a very similar world to Pluto, as far as its composition and geology and is regarded every bit a captured TNO that likely wreaked havoc on whatever moon system Neptune possessed earlier it was captured past the gas giant).

Pluto

Pluto, as imaged past New Horizons.

Many of these TNOs are close to Pluto in size and shape. To take them into business relationship, nosotros either needed to once more drastically aggrandize the number of planets in the solar system or define the word in a way that would exclude objects like Sedna, Qaoar, Eris, Makemake, and Haumea. In 2006, the International Astronomers' Marriage decided to formalize the definition of a planet. It considered a number of proposals, including some that would have recognized the dwarf planet Ceres and even potentially 4 Vesta as a planet.

Ultimately, the IAU decided that a planet had three distinguishing characteristics:

ane. It is in orbit around the Lord's day.
2. Information technology has sufficient mass to be in hydrostatic equilibrium (it must be rounded by the effects of its ain gravity).
3. Information technology must have "cleared the neighborhood" around its orbit.

Clearing the neighborhood means that the planet is gravitationally dominant in its ain local arrangement. The Moon is much larger than a typical satellite for a planet Earth's size should be — that's one of the reasons nosotros call back it formed in an unusual way — but Globe even so completely dominates the Earth-Moon gravitational system.

Point #2 disqualifies an object like iv Vesta from being a planet because Vesta is not (quite) in hydrostatic equilibrium. Point #three knocks out objects like Ceres and Pluto. Ceres is in the midst of the asteroid belt, while Pluto's barycenter with its moon, Charon, is outside of Pluto itself. Charon does non orbit Pluto — Charon and Pluto orbit a common point in space, above both of their surfaces.

Therefore, according to the IAU, Pluto is not a planet because information technology does not meet the third qualification.

But Bridenstine's declaration that Pluto is a planet because that'due south what he was taught is a mutual way for people to understand this situation equally well. One common cerebral fallacy that humans autumn prey to is anchoring bias. It's our tendency to recollect the kickoff piece of data we learned most a thing, whether that information is truthful or not, and information technology "anchors" our perceptions of after data that we are presented with. Just because we don't classify Pluto as a planet any longer doesn't mean Pluto doesn't "feel" similar a planet, for lack of a improve phrase.

I sometimes wonder if the unabridged result would accept been less controversial if scientists had communicated that the number of planets was going to take to be changed, no matter what. People who get unhappy about Pluto not beingness a planet often fixate on the number of planets in World's solar system, as though nine were a better number than eight. One wonders how they would take reacted to discovering that instead of nine, the new appropriate number was well over 20. If Pluto and the TNOs are planets, then objects similar Ceres would too have a potent claim to the title too. 1 suspects the defenders of Nine Planet Theory would be just as unhappy with that world as they are today, with 1 critical difference: By declaring there are but 8 planets, astronomers avoided request everyone to memorize a dozen new names.

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