Over time, even facts we consider steadfast truths can change. People used to think doctors could forgo washing their hands before surgery. Knowledge is ever evolving.

The seven ideas below probably changed since your school days. Re-educate yourself.

THEN: Pluto is a planet
NOW: Pluto isn't a planet

We've known since the late 1800s that a ninth planet, after Uranus, potentially existed. In 1906, Percival Lowell, the founder of the Lowell Observatory in Flagstaff, Arizona, even began a research project intended to locate the mysterious 'Planet X.'

Then in 1930, a 23-year-old newbie at the facility found it. The discoverer, Clyde Tombaugh, had been tasked with systematically comparing photographs of the sky taken weeks apart to search for any moving objects.

He eventually saw one and submitted his finding to the Harvard College Observatory. After an 11-year-old English girl named the new planet (for the Roman god of the underworld), we started including Pluto as a planet in our solar system.

But in 2003, an astronomer found a larger object beyond Pluto – which he named Eris, according to NASA. The new information caused a bunch of other astronomers to question what really makes a planet a planet, and they decided, based on size and location, that Pluto just didn't make the cut.

Neither did Eris, actually. Pluto was demoted to a dwarf planet.

Needless to say, elementary schools kids were pretty bummed.

But there may be hope. Researchers have recently been debating whether to make Pluto a planet again.

THEN: Diamond is the hardest substance
NOW: Ultrahard nanotwinned cubic boron nitride is the hardest substance

We've known about two substances harder than a diamond since 2009: wurtzite boron nitride and lonsdaleite, according to Scientific American. The first resists indentation with 18 percent more fortitude than a diamond, and the second – a whopping 58 percent.

Unfortunately, both substances are rather unusual and unstable in nature. In fact, the study's authors, published in the journal Physical Review Letters, only calculated the new substances' hardness, instead of actually testing it using a tangible specimen. That makes the discovery a bit theoretical.

But another contender was published in the January 2013 issue of the journal Nature. In the simplest terms, researchers compressed boron nitride particles to form 'ultrahard nanotwinned cubic boron nitride.'

They simply re-organised the particles like an onion, or a flaky rose, or those little Russian dolls that fit inside one another, as the team explained to Wired.

As a result, expect women everywhere to start asking for ultrahard nanotwinned cubic boron nitride engagement rings. Because those really are forever.

THEN: Witches in Salem were burned at the stake
NOW: They were actually hanged

Even if you didn't read Arthur Miller's The Crucible in high school, you probably learned somewhere that the townspeople of Salem burned witches at the stake.

But that never happened, according to Richard Trask, a town archivist for Danvers (formerly known as Salem Village.) He also chaired the Salem Village Witchcraft Tercentennial Committee from 1990 to 1992 and wrote a book detailing the time period called Salem Village Witch Hysteria.

At the time of the trials, New England still followed English law, which listed witchcraft as a felony punishable by hanging – not burning at the stake, Trask said. In Europe, however, the church labelled witchcraft heresy and did tie up suspected practitioners and light them on fire. You can see where the confusion started.

THEN: Israelite slaves built the pyramids
NOW: Egyptians workers built the pyramids themselves

Even movies like 'The Prince Of Egypt' perpetuate the idea that slaves built the pyramids. Although many think the Bible tells us they did, the book doesn't mention the story specifically.

This popular myth reportedly stems from comments made by former Israeli Prime Minister Menachem Begin when visiting Egypt in 1977, according to Amihai Mazar, professor at the Hebrew University of Jerusalem.

"No Jews built the pyramids because Jews didn't exist at the period when the pyramids were built," Mazar told the AP.

Recent archaeological finds actually show that Egyptians built the pyramids themselves. Workers were recruited from poor families in the north and south but were highly respected, earning crypts near the pyramids and even proper preparation for burial.

Slaves wouldn't have been treated so honorably.

THEN: Folding a piece of paper more than seven times is mathematically impossible
NOW: The record stands at 13

Whether in art class or science, this rumour definitely spread among the masses. But Britney Gallivan, a California high school student, didn't bite.

She, with some volunteers, bought a giant, US$85 roll of toilet paper and proceeded to blow everyone's mind by folding it a surprising 11 times. She realised everyone else who tried had been alternating folding directions, and even developed an equation, based on the thickness and width of the specific paper, explaining why you shouldn't.

Gallivan was a keynote speaker at the 2006 National Council of Teachers of Mathematics convention. She graduated from the University of California, Berkeley with a degree in Environmental Science in 2007. And since then, she's appeared on MythBusters.

In 2012, students at St. Mark's School in Southborough, Massachusetts, broke Gallivan's record, folding paper 13 times.

THEN: The Great Wall of China is the only man-made structure visible from space
NOW: Many man-made places are visible from space

Technically, this wasn't ever a solid 'truth' – just a fact third-graders ubiquitously included in their class reports and diorama presentations. In fact, rumours that you can see the landmark, not only from a spaceship, but all the way from the moon, date back as far as 1938.

In 2003 though, the first Chinese astronaut finally shattered the myth.

The party responsible, a man named Yang Liwei, admitted he couldn't see the Great Wall from space, according to NASA.

Other photos surfaced here and there. The consensus became that you can, indeed, catch glimpses of the Wall but only under the right conditions (snow on the structure) or with a zoom-capable camera. You can also see the lights of large cities – and major roadways, bridges, airports, dams, and reservoirs.

The moon factoid, however, is totally wrong.

"The only thing you can see from the Moon is a beautiful sphere, mostly white, some blue and patches of yellow, and every once in a while some green vegetation," Apollo 12 astronaut Alan Bean told NASA. "No man-made object is visible at this scale."

To further clarify, people probably mean these structures are visible from satellites orbiting Earth – but that's not actual space.

THEN: Five (or three) kingdoms of classification exist
NOW: There might be as many as eight kingdoms

Depending when you grew up, your middle school science teacher probably lectured about three main kingdoms of life – animals, plants, and bacteria (monera) – or five, including fungi and protists, too.

Either way, we've expanded our classification of life since then.

The more species we find and analyse, the more complex labelling life becomes. In addition to the five kingdoms above, we now know of archaea, previously thrown under monera. Archaea superficially look like other one-celled organisms called eubacteria, but they're completely different.

Even larger systems exist which further divide eubacteria into two more kingdoms or separate chromista from all the other protists.

In the US, however, we stick with six: plants, animals, protists, fungi, archaebacteria, and eubacteria.

This article was originally published by Business Insider.

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