Poor Pluto. For 76 years, it occupied the coveted spot as the ninth planet in our solar system, the farthest from our sun. A mysterious celestial body whose diameter is about the distance between New York and Houston, with an average temperature of -387°F. The lord of five moons. The only planet named by an 11-year-old girl. Pluto had a lot going for it.
But in 2006, it was knocked off its planetary pedestal by astronomers at the International Astronomical Union. So no, Pluto doesn’t count as one of the planets in our solar system. But it wasn’t as cut-and-dried a decision as you might imagine. Let’s take a look at that controversy along with one misconception about each of the still-recognized planets, adapted from an episode of Misconceptions on YouTube.
1. Misconception: All astronomers demoted Pluto’s planetary status.
According to the International Astronomical Union, a celestial body has to meet three criteria to be considered a fully recognized planet in our solar system. One, it has to orbit the sun, which Pluto definitely does. Two, it has to be roughly spherical—check. And three, it has to be the most gravitationally dominant object in its orbit, clearing the area of similarly sized objects (with a few agreed-upon exceptions). That’s where Pluto fails the test. It still shares some turf with other large objects in the Kuiper Belt, which means it hasn’t cleared its orbital area.
The union voted to recategorize Pluto as a dwarf planet. But the move was super controversial. Less than 5 percent of the world’s astronomers took part in the vote, and many others disagreed with the International Astronomical Union’s criteria. There was a major public outcry. Adults who had grown up learning about the nine familiar planets were confused and upset. What should my very educated mother just serve us now?
Many respected scientists remain convinced that Pluto should be a planet due to its size, shape, and orbit, no matter what the eggheads at the IAU think. People have … strong opinions about Pluto.
2. Misconception: Mercury is the hottest planet.
Tiny, rocky Mercury is the closest planet to the sun and the smallest in the whole solar system, not counting Pluto. It has an oval-shaped orbit that brings it as near as 29 million miles to the sun and up to 43 million miles away from it. But even at that relative proximity, it’s still not the hottest planet.
Mercury turns on its axis pretty slowly. Each full day/night cycle on Mercury takes 176 Earth days. Its average temperature during a long daytime can reach a toasty 800°F. Its nighttime average plunges to -290°F, which is far too cold to sustain life, as far as we know.
The main reason for the big swing in temps is Mercury’s lack of atmosphere. On Earth, the atmosphere protects us from solar radiation and insulates the planet from extreme temperature variations. Mercury has none of these perks. Instead, it has an exosphere of oxygen, sodium, hydrogen, and other atoms that have been scoured off Mercury’s surface by blasts of solar wind and meteoroid impacts. With no cozy atmospheric blanket to trap heat, Mercury loses the warmth it absorbs during its days.
3. Misconception: Venus is Earth’s “sister” planet.
People often think of Earth and Venus as twinsies. They’re roughly the same size and density. They’re both rocky planets featuring valleys, mountains, and volcanoes. Unlike Mercury, both Earth and Venus have atmospheres that trap gasses and heat.
But … that’s about where the similarities end. Venus differs from Earth in some major ways, like in its toxic cocktail of atmospheric compounds, which retain tons of heat to make Venus the hottest planet in the solar system. Its surface temp can reach a scorching 900°F. Instead of Earth’s breathable blend of oxygen and nitrogen, Venus’s “air” is primarily carbon dioxide, the same stuff causing global warming on our own planet. Instead of our clouds of harmless water, Venus’s clouds are made of deadly sulfuric acid. The planet’s incredibly dense atmosphere exerts the same amount of pressure as Earth’s oceans at a depth of half a mile.
Plus, Earth supports life, while nothing living has been found on Venus. But that doesn’t mean scientists aren’t looking. While the surface of Venus is way too hot to accommodate life as we know it, the planet’s atmosphere actually thins and cools as altitude increases. Thirty miles above the surface of Venus, temperatures vary between 86°F and 158°F, and the pressure relents to about the same level found on the Earth’s surface. That leads some scientists to consider the possibility of extremophile bacteria, like those found in hydrothermal vents or ultra-acidic volcanoes on Earth, living in Venus’s poisonous clouds. Scientists have even detected some mysterious black streaks at the tops of the clouds, which may be caused by iron-rich deposits—or perhaps by an as-yet-undiscovered microbial form with a chemical makeup that can withstand Venus’s toxic atmosphere. But for now, that’s only a speculative theory.
3. Misconception: Earth is a sphere.
Just because Earth isn’t perfectly round does not mean it’s actually flat. That idea got debunked even earlier than 1492, when Columbus supposedly proved that you couldn’t sail off the edge of the planet.
Scientific evidence does show that Earth is not a perfectly round sphere. According to the National Oceanic and Atmospheric Administration, Earth is an “irregularly shaped ellipsoid.” It’s about 26 miles wider in diameter at the equator than it is at the poles, due to the centrifugal force generated by Earth spinning on its axis. Basically, it’s fatter around the middle.
But that’s not the whole story: Earth’s shape is constantly changing topographically. Floods erode land. Volcanoes create new land. Earthquakes can alter the land in dramatic ways. And climate change is having a huge impact on our planet’s geophysics. When Earth’s mass shifts from one place to another, it affects the planet’s gravitational forces. Here’s an example: Greenland is losing mass rapidly because its ice sheet is melting. The remaining ice sheet doesn’t press down on the bedrock of Greenland as much, because it weighs less, so the ground is actually rising. A 2020 paper reported that at one place in Greenland, the land has risen roughly 10 feet above relative sea level since 1900—in other words, a large, scary amount.
At the same time, since Greenland is losing mass, it’s also losing gravitational force. The ocean volume that used to “hug” Greenland’s shores is drifting to the tropics instead, increasing that region’s gravitational force. You can see it in the rising seas around islands in the south Pacific Ocean, which leads to flooding and other problems for residents.
4. Misconception: People have landed on Mars.
In some ways, Mars is more of a sibling planet to Earth than Venus is. No other planet besides Earth has been scrutinized as much as Mars, from the work of ancient Egyptian astronomers to today’s robotic rovers and orbiters. Space agencies and private companies are focusing on Mars as a potential abode for humanity if—or when—Earth becomes uninhabitable. We’ve collected samples of Mars’s crust, studied its atmosphere and seasons, taken photos of its eerily rusty landscape, and mapped its surface. We’ve discovered organic molecules buried deep in billion-year-old Martian rocks. We’ve even flown a helicopter there (not to Mars, though—the trip started and ended on the Red Planet.)
One thing we haven’t done? Actually gone to Mars. No human has ever landed set foot there, though according to Caltech planetary scientist Bethany Ehlmann, one of the most common misconceptions is that astronauts have done so. In fact, we’ve gained our vast knowledge about Mars by observing it from afar or by monitoring it with sensors on the planet that beam information back to Earth.
So, if we’re sending all these awesome robots to Mars, could sending actual people really be that difficult? As it turns out, yes. There are a few reasons why.
One, it’s really far. When Earth and Mars are at the nearest points in their orbits to each other, a phenomenon called “close approach,” Mars is still at least 34 million miles away—and that’s the absolute theoretical minimum, given fluctuations over time. In 2022, the close approach actually left us over 50 million miles away. Because the planets’ orbits are elliptical and the distance between them varies, close approach doesn’t happen very often—usually about every 26 months. That means the optimal window for successfully launching a spacecraft to Mars is limited to roughly once every two years.
Two, we don’t have the technology to keep an astronaut alive during the journey. Any trip to Mars currently requires a minimum of 21 months due to the positioning of the planets during the journey: Beginning with the launch window near close approach, it would take nine months to fly from Earth to Mars. Then the crew would need to spend three to four months waiting for the planets to align for the nine-month return trip. The crew would need to bring every single thing for sustaining life with them: food, water, clothing, oxygen, and fuel, plus scientific instruments, building materials, spare parts, medical supplies … the list goes on. And while we might be able to drop off some of these supplies on Mars, there’s still the whole getting there thing. Today’s spacecrafts don’t have the ability to carry all that weight for so long.
Three, nobody knows exactly how to land a crewed spacecraft in Mars’s thin atmosphere or how astronauts would fare on the Red Planet. Astronaut Scott Kelly experienced several physiological changes after spending one year aboard the International Space Station, and it’s likely that any Martian voyager would experience worse. Would the long-term exposure to the Martian atmosphere cause medical problems? Would the relative lack of gravity cause muscles to turn to Jell-O? Would one’s heart and organs continue to function over such long periods? Maybe? We just don’t know.
For all these reasons and more, humans have never been to Mars, but research into the possibility continues.
5. Misconception: You can fly a spaceship through Jupiter.
Jupiter is the larger of what astronomers call the “gas giants,” which also include Saturn. Unlike the four rocky planets already mentioned, the gas giants’ mass is primarily, well, what we think of as gases, such as hydrogen and helium, along with some liquids. But that doesn’t mean you can zoom through it.
Jupiter’s mass is more than twice that of all other planets put together. Gravity holds all of the swirling matter in a rapidly rotating ball. Any spacecraft attempting a fly-though would first meet the three layers of Jupiter’s cloud system, which is about 44 miles thick and contains a mix of frozen ammonia and water ice. The clouds collide with warmer gasses rising from Jupiter’s interior, creating powerful winds and storms. The Great Red Spot is a monster storm with winds topping 400 miles per hour. Try getting a command module through that.
But, let’s say you make it past the cloud layer and enter Jupiter’s inner atmosphere. Crushing pressure and skyrocketing temperatures turn gases into liquids and form what NASA calls the solar system’s largest ocean. The 25,000-mile-deep pool of liquid metallic hydrogen can conduct electricity. It also uses the planet’s fast rotation to generate an immensely strong magnetic field. It’s no Sea of Tranquility.
Finally, you wouldn’t even get close to Jupiter’s mysterious core. This roiling mix of iron and silicates, which could be either a stew of loose matter or a solid form, may reach 90,000°F.
Pretty much everything we’ve learned about Jupiter we’ve gleaned by observing it from afar, and the one craft we deliberately crashed into it—Galileo, in 2003—didn’t emerge from the other side. As NASA says, the probe “penetrated 124 miles into Jupiter’s violent atmosphere before it was crushed, melted, and/or vaporized by the intense pressure and temperature.” The bottom line: Your puny spaceship, on its way through Jupiter, isn’t gonna make it.
6. Misconception: Saturn’s rings are solid.
Saturn is the second-largest planet in the solar system as well as the smaller gas giant, but its rings instantly set it apart from its planetary neighbors. Galileo observed Saturn’s bright rings through a telescope in 1610, the same year he discovered Jupiter’s four largest moons. The Dutch astronomer Christiaan Huygens formally described the rings in 1655. Over the following four centuries, though, astronomers struggled to understand the rings’ composition and origins, mainly because Saturn is so far from Earth—an average of 900 million miles away.
That began to change in 1979, when NASA sent the first spacecraft to Saturn. By that time, scientists had observed Saturn’s main rings and dubbed them A, B, and C.
Fainter rings were dubbed D and E. (No points for creativity there.) The space probe Pioneer 11, Earth’s first visitor to Saturn, discovered another ring along with other clues to their makeup. Then, Voyager 1 and 2 captured images showing that the main rings are actually thousands of thin ringlets.
In 1997, NASA launched the Cassini orbiter on a mission to Saturn. It took about seven years to get there. Once it arrived, it began orbiting the planet—the first spacecraft to ever do that—and beaming back incredibly detailed images of the planet’s rings.
They do resemble a solid grooved plane surrounding the planet, like a close-up of Voyager’s Golden Record. But each ring is composed of countless fragments of water ice and rock, the remains of asteroids, comets, or moons that broke up when they came into contact with Saturn’s gravity. Some of the pieces are as tiny as grains of sand; some are the size of mountains, and the rest fall somewhere in between. And like a vinyl LP, the rings are super flat: the main rings are 170,000 miles in diameter, but only 30 feet to a half-mile in height. They vary in density and shape, and some rings intertwine with their neighbors.
But that wasn’t all—Cassini also captured never-before-seen details about Saturn’s largest moons. It deployed the European Space Agency’s Huygens probe to the surface of Titan, revealing topography carved into mountains and valleys. It also discovered liquid water on Enceladus, ejected from geyser-like formations possibly fed by relatively warm pools of water below the moon’s surface. It even discovered another ring around Saturn.
So, Saturn’s rings are anything but solid. And even though Cassini ran low on fuel and was intentionally flown into Saturn itself in 2017 (RIP), astronomers are still making discoveries from its 20-year mission.
7. Misconception: Uranus doesn’t stink.
Let’s get the jokes out of the way right now. The planet Uranus—generally pronounced “YUR-a-nus” in scientific circles—really got the short end of the stick when astronomer William Herschel discovered it in 1781. Herschel initially wanted to name it Georgium Sidus after the reigning monarch, George III. But that wasn’t too popular outside of Great Britain, so astronomers agreed on Uranus instead. The name honors the Greek god of the sky.
Voyager 2 conducted our only fly-by of Uranus back in 1986. From the data collected by Voyager 2 and a variety of telescopes, we’ve learned a few key facts. It’s one of the least-dense planets in the solar system and one of two ice giants (the other one is Neptune). It has several faint rings and 27 moons. Uranus’s mass is mostly water and ammonia surrounding a tiny rocky core that can reach 9000°F.
Uranus’s atmosphere is a different story: It’s a big gassy blanket of hydrogen, helium, and methane. The same gas that cows expel also gives Uranus its aquamarine-blue color. On top of that, the planet supports clouds of hydrogen sulfide, which is responsible for its, uh, signature scent. It seems a little on the nose, but Uranus really does smell like farts.
In 2018, a team led by Oxford University researcher Patrick Irwin confirmed the long-debated composition of these odiferous clouds by analyzing the way they refracted sunlight. The data suggested the clouds were made up of molecules of hydrogen sulfide, the same compound that makes rotten eggs stink, which seemed to pervade the planet’s cold and windy upper atmosphere.
Fortunately, the scientists didn’t have to actually go there and take a sniff for themselves. As Irwin noted, “suffocation and exposure in the -200°C [-328°F] atmosphere made of mostly hydrogen, helium, and methane would take its toll long before the smell.”
8. Misconception: Neptune and Uranus are the same color.
We don’t know a lot about Uranus, but we know even less about Neptune, the outermost planet. Voyager 2 flew by Neptune on its 1980s mission. We know that the planet has faint rings, several moons, and the windiest atmosphere in the solar system, with gusts blasting up to 1200 miles per hour.
Like Uranus, Neptune has an atmosphere composed of hydrogen, helium, and a smidgen of methane, which makes it appear blue. But Neptune is a deeper cobalt blue, contrasting with Uranus’s light turquoise blue. Scientists have wondered what the differentiating factor could be, since their size, mass, and chemical composition are so similar.
Using the same telescopes that helped them uncover Uranus’s smell, the Oxford researchers discovered that Uranus is enveloped in a much thicker obscuring layer of methane haze, which appears whitish. It might be the result of a long-ago impact that quieted the activity in Uranus’s lower atmosphere. Neptune’s haze layer is thinner and may have precipitated like snow, revealing its deeper blue hue.
On the other hand, NASA’s new James Webb Space Telescope gave us an entirely new view of Neptune last September. Whereas Voyager 2 pictured Neptune as a dark blue dot, Webb’s infrared telescope showed it as a dazzling white orb circled with glowing rings. Seven of Neptune’s 14 known moons appear. Triton, its largest and strangest moon, shines like a bright star thanks to its reflective frozen nitrogen-covered surface. With Webb’s ability to capture unprecedented detail, we will surely learn more about Neptune in the years to come.
This article was updated in 2023.