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

THEN: Pluto is a planet.

This is a mock-up, created by NASA, of Pluto and Eris. NASA

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, Az., 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.

Needless to say, elementary schools kids were pretty bummed.

Even though astronomer Mike Brown, author of "How I Killed Pluto and Why It Had It Coming," started the controversy, meme-ified astrophysicist Neil deGrasse Tyson defended him on The Verge.

"If Neptune were analogized with a Chevy Impala in mass, then how big is pluto compared to that? Pluto would be a matchbox car sitting on the curb," Tyson said.

THEN: Diamond is the hardest substance.

A pyramidal diamond embedded in the working surface of a Vickers hardness tester. Wikimedia Commons

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% more fortitude than a diamond, and the second — a whopping 58%.

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-organized 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, women everywhere starting asking for ultrahard nanotwinned cubic boron nitride engagement rings. Because those really are forever.

THEN: Witches in Salem were burned at the stake.

A famous illustration of a courtroom scene during the Salem Witch Trials. The woman on the floor is most likely Mary Walcott. Wikimedia Commons

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 labeled 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.

The pyramids at Giza, named Khufu and Khafra. Robert Johnson/Business Insider

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, end of story.

THEN: A Brontosaurus is a dinosaur.

The "Brontosaurus" at the Carnegie Museum in Pittsburgh. Wikimedia Commons

NOW: A Brontosaurus is a combination of Apatosaurus and Camarasaurus.

When you remember learning of a giant plant-eating dinosaur with a very long neck, you're probably thinking of the Brontosaurus. But technically, that brand of beast doesn't exist. The misconception all started with a period in paleontology known as "The Bone Wars," curator of the Carnegie Museum of Natural History in Pittsburgh Matt Lamanna told NPR. Sounds dramatic, right? Well, the scenario kind of was.

Two paleontologists, Othniel Charles Marsh and Edward Drinker Cope, were engaged in a bitter rivalry for evidence of dinosaurs.

"There are stories of either Cope or Marsh telling their fossil collectors to smash skeletons that were still in the ground, just so the other guy couldn't get them," Lamanna said.

The two burned a lot of cash seeking to one-up each other in the fame department.

In 1877, in the heat of the conflict, Marsh discovered a partial skeleton, unfortunately lacking a head, which he named Apatosaurus. He used the skull of another dinosaur, a Camarasaurus, to finish the replica.

But when Marsh discovered another skeleton two years later, he named it a Brontosaurus, when in reality, he had just found a more complete Apatosaurus.

Scientists spotted the mistake in 1903, but the Carnegie Museum didn't replace the head on the skeleton until 1979. And many today still don't know the truth.

THEN: Our evolutionary link to other primates is missing.

Ida, the fossil considered the "missing link." AP Photo/Michael Probst NOW: We found "Ida."

Meet "Ida," a critical "link" (formerly a missing one) in the study of evolution from apes to humans. She even has her own website: Revealing the Link.

In 2009, paleontologist Jorn Hurum and his team discovered this 47-million-year-old fossil in Germany in essentially perfect condition.

Ida, University of Michigan paleontologist Philip Gingerich told National Geographic, bridges the evolutionary split between higher primates such as monkeys, apes, and humans and their more distant relatives like lemurs.

Ida, scientifically classified Darwinius masillae, is a lemur-like skeleton with primate-like features, such as opposable thumbs, no claws, and fairly short limbs. Her discovery fills a large gap in the evolutionary timeline.

THEN: Folding a piece of paper more than seven times is mathematically impossible.

Britney Gallivan folded a roll like this, but much larger, more than 7 times. Flickr/Jane Waterbury

NOW: The record stands at 11.

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

She, with some volunteers, bought a giant, $85 roll of toilet paper and proceeded to blow everyone's mind by folding it a surprising 11 times. She realized 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.

[h/t Listverse]

THEN: The Great Wall Of China is the only man-made structure visible from space.

This photo of central Inner Mongolia, about 200 miles north of Beijing, was taken on Nov. 24, 2004, from the International Space Station. The Great Wall isn't very visible, if at all. NASA

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, rumors 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 and bridges and airports and 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.

Fungi, one of the original kingdoms of living classification. Flickr/Tony Higsett



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 analyze, the more complex labeling 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 U.S., however, we stick with six: plants, animals, protists, fungi, archaebacteria, and eubacteria.

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