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Why are planets always round?

The stars, planets, and many moons are extremely round. Why don’t they take other shapes?

KEY TAKEAWAYS

  • In our Solar System, all of the planets, many moons and smaller objects, and the Sun are all round. 
  • Above a size of approximately ~400 kilometers in radius, practically all rocky bodies are round; above ~200 kilometers in radius, most icy bodies are, too. 
  • There are no irregular objects out of hydrostatic equilibrium above a certain size, and physics can explain why.

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For more than 2,000 years, humanity has known that our planet, Earth, is round in shape. Just as the Moon and Sun appear round, so does not only Earth, but every planet in our Solar System. Even non-planets get in on the “round” action as well. Earth’s moon, Jupiter’s four largest moons, four of Saturn’s five largest, Uranus’s five largest and Neptune’s largest moons are all round, as well as the asteroid Ceres and numerous Kuiper belt and Oort cloud objects. Some smaller objects as little as ~200 km in radius are round, while Neptune’s Proteus and Saturn’s Iapetus, significantly larger, are not. Why is this? Why are other shapes not possible for the largest objects of all? That’s the question of Sgt. Randy Pennington, who wrote in:

“[Someone] asked me, ‘okay, so we’ve gone to space and traveled throughout the Solar System, and every planet that we’ve measured is round. But why?’ And I knew the planets were round, but I don’t know why. What would happen if a planet were shaped like a cube, or a pyramid, and why aren’t there any? But I know someone who’ll know… so why, Ethan, why are all the planets always round?”

It’s true: Every planet is round, and some are even rounder than others. Moreover, the stars are also always round, many moons and even some asteroids and Kuiper belt objects are round. Here’s the science of what’s going on.

planets
Under a size cutoff of 10,000 kilometers, objects appear to be round, pulled into hydrostatic equilibrium via their gravity and rotation, combined. However, once you go to planetary radii below ~800 kilometers, hydrostatic equilibrium, or even roundness, are no longer certainties. (Credit: Emily Lakdawalla; data from NASA/JPL, JHUAPL/SwRI, SSI, and UCLA/MPS/DLR/IDA)

The first thing to recognize is that normal matter can clump together in any amount. Individual atoms and even subatomic particles, like atomic nuclei or free electrons, exist in great abundances in stellar systems, as well as in interstellar space. Atoms also link up to form molecules, which can exist freely or as parts of other systems, and molecules themselves can clump together in amounts both great and small.

While there are nuclear and electromagnetic forces at play, both of which can easily overwhelm any other forces, when you get large amounts of mass together, it’s actually the weakest force of all that wins: gravity. If you get enough normal matter together in one place — regardless of the type, phase, origin, or nature of matter that you have — it will contract until it’s a single, gravitationally bound object.

When these objects are small, they tend to form minuscule, dustball-like structures. These grain-like particles aren’t actually held together via gravity, but rather via electrostatic forces. Simply bringing them close to the Sun, where they’re exposed to things like solar radiation and the solar wind, is enough to destroy them. If you want something more robust, you have to look to larger masses, enabling the force of gravity to become more dominant.

A schematic view of the strange peanut-shaped asteroid Itokawa. Itokawa is an example of a rubble-pile asteroid, but determinations of its density have revealed that it is likely a result of a merger between two bodies that have different compositions. It cannot pull itself into a round shape. (Credit: ESO, JAXA)

Take the above-pictured asteroid, for example: Itokawa. Itokawa is large enough to be its own gravitationally bound structure, weighing in at about ~30 million tons. It’s only a few hundred meters across on a side, but that’s enough to illustrate, at least at this scale, what gravitation can and cannot do. When you’ve accumulated more than a “grain” of matter but not more than a few million tons, here’s what you wind up with.

  • A “rubble pile” body. Instead of being one solid object, you get what looks like a collection of many different grains and pebbles, all held together through their mutual gravitation.
  • An object that isn’t differentiated. If you have a lot of mass together, you get a differentiation of your layers, where the densest materials sink to the center, forming a core, while the less-dense materials like a mantle or crust “float” atop them. Itokawa, and other objects of comparable masses and sizes, can’t do that.
  • A composition that shows the merger of different bodies. This one isn’t necessary, but it does happen frequently, and Itokawa is a spectacular example of it: the two portions of the “peanut” that composes Itokawa have dramatically different densities, indicating that these were once two separate objects that have now, gravitationally, merged together.

All told, these objects can hold themselves together gravitationally, but aren’t round.

Comet 67P/Churyumov-Gerasimenko was imaged many times by the ESA’s Rosetta mission, where its irregular shape, volatile and outgassing surface, and cometary activity were all observed. The comet itself would have to be much larger and more massive to ever approach a “round” shape. (Credit: ESA/Rosetta/MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

Why don’t these small objects become round? It’s because the forces between atoms and molecules — governed by electrons and the electromagnetic force — are stronger than the force of gravity at this scale. Gravitation is always attractive, and pulls every particle of matter toward the center of mass of the objects of which they’re part. But there are also forces between atoms and molecules that determine their shape and configuration.

Ice crystals form in lattices; silicate rocks can form amorphously; dust particles can get compacted into soils or even solid shapes; etc. When a gravitational force is applied to a large body or collection of bodies, it exerts a pressure: a force over an area. If the pressure is great enough, it will override whatever initial conditions or shapes an object possesses to start with, and compel it to reshape itself into a more energetically stable configuration.

In the case of self-gravitating bodies, overcoming whatever random initial shape and configuration you start with is the first obstacle you face, and just how much mass is required depends on what your object is made out of. You can form a cube, a pyramid, or whatever potato-esque shape nature can dream up, but if you’re too massive, and the force of gravity is too large, you won’t maintain it, and will instead be pulled into a round shape.

This selection of asteroids and comets visited by spacecraft spans many orders of magnitude in size, from sub-kilometer bodies to objects more than 100 km on a side. However, none of these objects have enough mass to be pulled into a round shape. Gravitation can hold them together, but cannot reshape them. (Credit: Planetary Society – Emily Lakdawalla)

If you’re below about 1018 kilograms (a quadrillion tons or so), you’ll be below about 100 kilometers in radius, and that is always too small, or low in mass, to pull yourself into a round shape. Itokawa falls short of this threshold by a factor of many millions, as do most of the known asteroids.

However, if you can accumulate enough material to rise above this mass and size threshold, you’ve got a chance at rough “roundness.”

Saturn’s moon Mimas, for example, is slightly under 200 kilometers in radius, but is undoubtedly rounded. In fact, it’s the smallest astronomical body presently known that’s in a round shape owing to self-gravitation, and is the innermost large moon of Saturn, completing an orbit around the ringed planet in under 24 hours. Mimas is very low in density, only barely denser than water-ice, suggesting that it’s made up largely of volatiles: low-density ices that are easy to deform under the force of gravity.

Were Mimas composed largely of rocks or even metals, it would have to be larger and more massive to self-gravitate into a sphere: as large as 400 or 500 kilometers in radius, in the most extreme cases.

round
Mimas, as imaged here during the closest fly-by of Cassini in 2010, is only 198 kilometers in radius, but is quite clearly round owing to its self-gravitation. However, it lacks sufficient mass to truly be in hydrostatic equilibrium. (Credit: NASA/JPL-Caltech/Space Science Institute)

Round, however, is only part of the story. You can still have large features that lead your object to depart from the shape that self-gravitation would otherwise lead to on a world that becomes rounded. Mimas, in fact, demonstrates this, with its Death Star-like appearance owing to its enormous crater: so large it’s almost a third of Mimas’s diameter. The crater walls are over 5 km high and the crater floor is more than 10 km deep; in fact the surface on the opposite side of Mimas from this crater is highly disrupted. The impact that created this crater must have nearly destroyed Mimas entirely, and its gravitation is insufficient to pull it back into a more spherical shape.

This example illustrates an important distinction: the difference between being “round” and being in “hydrostatic equilibrium.” Self-gravitation can pull you into a round shape easily if you’re over 200 kilometers in radius and icy or over 400 kilometers in radius and rocky. But being in hydrostatic equilibrium is a more difficult bar to clear: you have to have your shape primarily determined by a combination of self-gravitation and rotation: the same shape a self-gravitating drop of spinning liquid water would take on.

The four largest asteroids, all shown here, have been imaged with NASA’s Dawn mission and the ESO’s SPHERE instrument. Ceres, the largest asteroid, is the smallest known body in hydrostatic equilibrium. Vesta and Pallas are not, but Hygeia may yet be. (Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA; ESO)

The smallest body verified to be in hydrostatic equilibrium is the largest asteroid: the dwarf planet Ceres, with a radius of about 470 kilometers. On the other hand, the largest body known not to be in hydrostatic equilibrium is Saturn’s bizarre moon Iapetus, with a radius of around 735 km, whose planet-spanning equatorial ridge would never occur if gravity and rotation alone determined its shape.

For a solid body like a rocky planet or moon, the big question is whether your gravity can make you behave in a plastic fashion. In physics and materials science, plastic doesn’t mean “made out of the by-products of oil,” but rather describes how certain materials deform. When you subject a material to stresses arising from tension, compression, bending, or torsion, those materials will normally elongate, compress, buckle, twist, or otherwise deform.

If your material deforms plastically, those distortions and deformations can become permanent. If you have enough mass together in one place, gravitation will be sufficient to pull you back into hydrostatic equilibrium, so that your overall shape is once again determined by your rotation and gravity alone. If not, you can still be round, but not in hydrostatic equilibrium.

These two global images of Iapetus show its large impact feature and its equatorial ridge, despite its obvious roundness. In concert with its other properties, these features demonstrate that Iapetus is not in hydrostatic equilibrium, making it the largest world in the Solar system to not be. (Credit: NASA/JPL-Caltech/Space Science Institute)

For icy objects, you can be round at about 200 kilometers, but you won’t be in hydrostatic equilibrium until you’re about 400 kilometers in radius. For rocky objects, you won’t be round unless your radius is about 400 kilometers, but you might not reach hydrostatic equilibrium unless your radius is greater: up to 750 kilometers may be needed.

Objects that live in that in-between region could either be in hydrostatic equilibrium or not, and we’re not certain about the status of many of the known ones. Rock-and-ice Hygeia, with a radius of only 215 km, might be in hydrostatic equilibrium. Saturn’s moon Enceladus, at 252 kilometers, is close, but the asteroids Pallas and Vesta, at 256 and 263 km, severely depart from even being round. Pluto’s large moon Charon, with a radius of 606 km, might not have quite achieved hydrostatic equilibrium. The largest two Uranian moons, Titania and Oberon are probably in hydrostatic equilibrium; the next three, Umbriel, Ariel, and Miranda, may or may not be.

However, once you get up to about 800 kilometers in radius, everything known above that size is not only round, it’s in hydrostatic equilibrium as well.

Saturn, as photographed here by Cassini during the 2008 equinox, isn’t just round, but is in hydrostatic equilibrium. With its low density and rapid rotation, Saturn is the most flattened planet in the Solar System, with an equatorial diameter that’s more than 10% larger than its polar diameter. (Credit: NASA/JPL/Space Science Institute)

The dwarf planets Haumea, Eris, and Pluto (along with Makemake, at only 715 km in radius) are all in hydrostatic equilibrium. Neptune’s Triton, Earth’s Moon, Saturn’s Titan, and the four Galilean moons of Jupiter are all in hydrostatic equilibrium as well. So are all eight of the planets, and so is the Sun. In fact, we’re pretty confident that this is a universal rule: if you’re more than about 800 kilometers in radius, regardless of your composition, you’re going to be in hydrostatic equilibrium.

But here’s a fun fact: Many objects — including many planets and stars — rotate so quickly that it’s very clear that they aren’t round, but rather take on a squashed shape known as an oblate spheroid. Earth, due to its 24 hour rotation, isn’t quite a perfect sphere, but has a larger equatorial radius (6378 km) than a polar radius (6356 km). Saturn’s rotation is even faster, completing a rotation in just 10.7 hours, and its equatorial radius (60,268 km) is almost one full “Earth” larger than its polar radius (54,364 km).

The Moon and Mercury, however, are both incredibly slow rotators. They’re only ~2 km larger in radius in the equatorial direction than the polar one, making them very spherical rocky planets. But do you know which body is the most perfect sphere in the Solar System? The Sun. With an average radius of 696,000 kilometers, its equatorial radius is only ~5 km larger than its polar radius, making it a perfect sphere to 99.9993% precision.

round
This image of the Sun, taken on April 20, 2015, shows a number of features common to all stars: magnetic loops, prominences, plasma filaments, and regions of higher and lower temperatures. However, the slowly-rotating Sun is the most perfect sphere in the Solar System, with a polar and equatorial diameter that are identical to 99.9993% precision. (Credit: NASA/Solar Dynamics Observatory)

Although there are many factors at play in determining the shape of an object, there are really only three main categories that bodies fall into.

  1. If you’re too low in mass and/or too small for your composition, you’ll simply take on whatever shape you happened to have by the luck-of-the draw in forming; practically all objects below ~200 kilometers in radius have this property.
  2. If you’re more massive, that initial shape will get reconfigured into a round one, a threshold that you cross between ~200 and 800 km in radius, depending on your composition. However, if a major distortive event happens, like an impact, a deposition, or a change to your orbital properties, you will likely keep an imprinted “memory” of that event.
  3. Finally, above ~800 kilometers in radius, you’ll be in hydrostatic equilibrium: massive enough so that gravity and rotation primarily determine your shape, with only small imperfections superimposed atop that.

In terms of mass, 0.1% of Earth’s mass will do it; bring that much together and you’ll always be in hydrostatic equilibrium. Roundness, on its own, isn’t quite enough to make you a planet, but all planets have more than enough mass to pull themselves into a round shape. The irresistible force of gravity is enough to ensure it couldn’t be any other way.


SPACE WEATHER forecasters are braced for a cloud of hot plasma and magnetic field from the Sun to lash the planet.

Solar storm warming: Breaking News image

SPACE WEATHER forecasters are braced for a cloud of hot plasma and magnetic field from the Sun to lash the planet.

Earth’s ‘magnetic song’ captured during solar storm

A so-called coronal mass ejection (CME) was seen escaping the Sun on Wednesday and may deliver a “glancing blow” to the planet. CMEs are large clouds of charged particles and magnetic field that stream from the Sun’s corona – the outermost layer of the star’s atmosphere. According to the US Space Weather Prediction Center (SWPC), CMEs can reach the planet at speeds between 250 km per second and 3,000 km per second.

Astronomers at SpaceWeather.com have now warned yesterday’s CME could reach the planet by Saturday.

The warning comes after a large filament erupted from the Sun’s southern hemisphere.

The filament split the Sun’s atmosphere wide open and released a cloud of debris into space.

The website’s astronomers wrote: “Imagine a canyon 50,000 miles long with towering walls of red-hot plasma.

Solar storm warming: Breaking News image

A coronal mass ejection may strike the planet on Saturday

Sun fact sheet: Incredible facts about the Sun

“Yesterday, there was one on the Sun.

“It formed when a filament of magnetism lifted off from the southern hemisphere.

“The erupting filament split the Sun’s atmosphere, carving out the canyon as it ascended.

“The glowing walls remained intact for more than six hours after the explosion.”

The debris trailing from the blast was photographed by NASA’s STEREO-A spacecraft and the Solar and Heliospheric Observatory (SOHO).

Space Weather added: “First-look data suggest it might deliver a glancing blow to Earth’s magnetic field on November 28.”

When CMEs interact with the Earth’s magnetosphere – the region of space dominated by Earth’s magnetic field – they may induce a geomagnetic storm (solar storm).

The SWPC explained: “A geomagnetic storm is a major disturbance of Earth’s magnetosphere that occurs when there is a very efficient exchange of energy from the solar wind into the space environment surrounding Earth.

“These storms result from variations in the solar wind that produces major changes in the currents, plasmas, and fields in Earth’s magnetosphere.”

NASA image of the Sun

Solar storm effects on tech - NASA infographic

The strongest solar storms are typically associated with the arrival of a CME.

And depending on the CME’s strength, scientists will rank the resulting storm on a scale of “G1 Minor” to “G5 Extreme”.

At the low end of the scale, Minor storms can cause some disturbance to satellite operations and weak power grid fluctuations may occur.

Weak storms can also create beautiful aurora at northerly latitudes.

Don’t Miss: Near-Total Lunar Eclipse, Snow Moon, Comet Leonard, Geminid Meteor Shower

The next full Moon will be early on Friday morning, November 19, 2021, appearing opposite the Sun (in Earth-based longitude) at 3:58 a.m. EST. While this will be on Friday for much of the Earth, it will be Thursday night from Alaska’s time zone westward to the International Date Line in the Pacific Ocean. The Moon will appear full for about three days around this time, from Wednesday night through Saturday morning.

“This should be a good month for skywatching, with Venus, Jupiter, and Saturn in the evening sky, a potentially visible comet, and one of the best meteor showers of the year!”
— Gordon Johnston

Near-Total Lunar Eclipse

The Moon will be so close to opposite the Sun on November 19 that it will pass through the southern part of the shadow of the Earth for a nearly total lunar eclipse. The partial shadow of the Earth will begin falling on the upper left part of the Moon at 1:02:09 a.m. EST, but the slight dimming of the Moon will not be noticeable until the full shadow of the Earth begins falling on the upper part of the Moon at 2:18:41 a.m. The arc of the shadow of the round Earth will spread across the Moon until the peak of the eclipse at 4:02:53 a.m. when over 97% of the Moon will be in full shadow and only a small sliver of the left side of the Moon will shine in the partial shadow of the Earth.


On November 19, 2021 (late evening of the 18th in some time zones), the Moon passes into the shadow of the Earth, creating a partial lunar eclipse so deep that it can reasonably be called almost total. Credit: NASA’s Scientific Visualization Studio

Sleep Loss Does Not Impact Ability to Assess Emotional Information

Summary: Sleep loss does not numb a person’s response to emotional situations, but it can result in difficulties in regulating emotional response.

Source: Washington State University

It’s no secret that going without sleep can affect people’s mood, but a new study shows it does not interfere with their ability to evaluate emotional situations.

It is often assumed that feeling more negative will color people’s experience of emotional images and events in the environment around them. However, Washington State University researchers found that while going 24 hours without sleep impacted study participants’ mood, it did not change their performance on tests evaluating their ability to process emotional words and images.

“People do become less happy through sleep deprivation, but it’s not affecting how they are processing emotional stimuli in their environment,” said Anthony Stenson, a WSU psychology doctoral student and lead author of the study in Plos One.

The findings have implications for healthcare providers, law enforcement and people in other long-hour professions who need to be able to control their own emotions during stressful and emotionally trying situations. Sleep loss in not likely to make them numb to emotional situations, the researchers found, but it is likely to make them less able to control their own emotional responses.

For the study, about 60 adult participants spent four consecutive days in the Sleep and Performance Research Center at the WSU Elson S. Floyd College of Medicine. All participants were allowed to sleep normally the first night and then given a set of baseline tests to judge their mood as well as their emotional regulation and processing ability. Then, the researchers divided the participants into two groups: one group of 40 people spent the second night awake, while a control group of 20 were allowed a normal sleep period. The tests were then re-administered at different intervals.

The emotional regulation and processing tests both involved viewing a series of images with positive and negative emotional connotations. In the emotional regulation tests, participants were given a prompt to help them recontextualize negative images before seeing them and asked to control their feelings. The sleep-deprived group had greater difficulty reducing the emotion they felt when instructed to do so. 

The processing tests involved responding to words and images with emotional content, for example rating the emotions conveyed by a smiling family, a growling dog or a crying child  All participants performed similarly on these tests whether they were sleep deprived or not.

The distinction between processing the emotional content of the world around you and being able to regulate your own emotional responses is an important one, especially for some professions, said co-author Paul Whitney, a WSU professor of psychology.

“I don’t think we want our first responders being numb to the emotional nature of the situations they encounter, and it looks like they are not,” he said. “On the other hand, reacting normally to emotional situations, but not being able to control your own emotions, could be one reason sleep loss sometimes produces catastrophic errors in stressful situations.”

A lot of previous research has looked at how sleep deprivation impacts so called “cold” cognitive tasks—supposedly emotionally neutral tasks like recalling facts. These studies have also found that regulation, which is considered a “top-down” cognitive process, is a major problem with cold cognitive tasks. For instance, mental flexibility is compromised by sleep deprivation. This is the ability an emergency room doctor might need to quickly change tactics if a patient isn’t responding to a treatment.

The current study shows that top-down regulation is a problem as well with “hot” or emotional cognitive processes. Future research is needed to understand whether the effects of sleep loss on the two top-down processes are linked.

This study is the result of an ongoing collaboration among WSU psychology researchers and sleep experts at WSU College of Medicine. Other authors include psychology post-doctoral fellow Courtney Kurinec as well as psychology Professor John Hinson and College of Medicine Professor Hans Van Dongen. All are also affiliated with the WSU Sleep and Performance Research Center.

Aurora Borealis

A “moderate” geomagnetic storm is forecast for the Earth on Monday, which could cause a few fluctuations to the power grid at higher latitudes and could also affect some satellites, the National Oceanic and Atmospheric Administration (NOAA) said.

The northern lights, aka the aurora borealis, could also be visible in some parts of the nation Monday night in northern-tier states from New England to Washington, according to SpaceWeather.com.

The storm is rated a “G2,” which is the second level of NOAA’s five-level storm scale. (G1 storms are minor, while G5s are considered extreme.) 

The storm is courtesy of a solar flare: On Saturday, a solar flare from a sunspot hurled a coronal mass ejection toward Earth, which is causing the geomagnetic storm Monday, SpaceWeather.com said.  

High-latitude power systems may experience voltage alarms and transformer damage could be possible if the storm lasts long enough, NOAA said.