5 big surprises when comparing the worlds in our Solar System

Within our Solar System, it’s hard to appreciate just how big, massive, and well-separated our planets, moons, asteroids, and more are.

A logarithmic chart of distances, showing Voyager, our Solar System, the Oort Cloud, and our nearest star: Proxima Centauri. If we imagine building larger and larger particle accelerators, every factor of 10 in radius gets us another factor of 10 in energy, but typically at the expense of another factor of 100 in cost. On planetary and stellar scales, this can get prohibitively large very quickly.

These 5 counterintuitive facts can help put things in perspective.

This comparison image shows the four inner planets of our Solar System to scale by their actual size, as acquired by MESSENGER (for Mercury and Venus), Apollo 17 (for Earth), and the OSIRIS instrument aboard Rosetta (for Mars).

1.) Earth is more massive than all the other rocky planets, combined.

This cutaway view of the four terrestrial planets (plus Earth’s moon) shows the relative sizes of the cores, mantles, and crusts of these five worlds. Despite the fact that the Earth is only 5% larger in diameter than Venus, it has more mass than Mercury, Venus, Mars, and the Moon combined. If you could pass through the Earth’s interior as a projectile that didn’t interact electromagnetically with the Earth, you would see your trajectory change slightly as you transitioned across one internal layer to another.

Adding together:

• Mercury,
• Venus,
• Mars,
• the Moon,
• plus the asteroid and Kuiper belts,

would approximately equal Earth’s mass.

In comparing JWST images of Uranus (left) and Neptune (right), features such as rings, moons, and cloudy “bright spots” on the planets are clearly visible. Although Neptune appears much smaller than Uranus, that’s because it’s an extra ~50% farther away from JWST than Uranus is; in reality, they are nearly the same physical size.

2.) Neptune is the smallest gas giant, but Uranus is far less massive.

Although our best-ever views of the planets of Uranus and Neptune still come from the Voyager 2 encounters with these worlds from the late 1980s, the reality is that these two planets are extremely similar in color, composition, and size, with the famous “azure” image of Neptune not representative of its true color. Uranus is just 3% larger than Neptune, but substantially less massive.

Uranus beats Neptune by 3% in diameter.

Although they are approximately the same physical size, Neptune is about 30% denser than Uranus. This is despite the fact that Uranus appears to be more centrally condensed than Neptune. An orbiter mission to Uranus and Neptune would be the ideal way to determine why they have these physical properties.

Neptune’s interior is richer in heavy elements; overall, it’s 17% more massive than Uranus.

This animation shows the relative spin rates and tilts of the planets as they rotate about their axes. Although Mercury and Venus are the slowest rotators, Mercury co-rotates in the same direction that it orbits the Sun, while Venus counter-rotates.

3.) Venus is the slowest rotating planet, but a day on Mercury lasts longer.

While all of the planets in the Solar System orbit the Sun in the same direction, Venus, uniquely, rotates in the opposite direction. For each orbit completed by Venus, although it is the slowest-rotating planet, it experiences roughly two “days” of sunrises and sunsets.

Venus takes over four times as long as Mercury to rotate 360°.

Due to Venus’s rotation in the opposite direction that it revolves around the Sun, it is the only planet where the Sun appears to rise in the west and set in the east. The time from sunrise to sunrise on Venus is approximately 117 Earth-days, or 59 Earth-days shorter than a Mercurian day.

But from sunrise to sunrise, Mercury’s duration beats Venus’s: 176 to 117 Earth-days.

This view of Saturn, taken from above its pole from the Cassini spacecraft, with the Sun located far off to the right, shows the planet from an unusual angle: an angle never achieved from the perspective of Earth. Saturn, although only ~16% smaller than Jupiter, has just 30% of Jupiter’s mass.

4.) Saturn is the least dense planet, by far.

By size, it’s clear that the gas giant worlds vastly outstrip any of the terrestrial planets, and this is true for mass as well. Although Saturn is nearly the same physical size as Jupiter, it contains just 21% of the planetary mass in the Solar System, compared to Jupiter’s whopping 71%.

Saturn is five times Neptune’s mass, but fourteen times its volume.

This cutaway diagram of Saturn showcases how small its rock-and-ice core is, and how much of its volume is dominated by hydrogen. Only the innermost ice-and-rock layers are not element-dominated by hydrogen, helping Saturn achieve, by far, the lowest density of any planet.

Its puffy atmosphere is 96% hydrogen, by volume.

The animation depicts a mapping of the positions of known near-Earth objects (NEOs) at points in time over the past 20 years and finishes with a map of all known asteroids as of January 2018. Despite how crowded a diagram such as this appears, the space between asteroids, on average, is enormous when compared to their actual sizes.

5.) The asteroid belt is more empty than the Earth-Moon system.

While the near-Earth asteroids are already posing potential hazards to Earth, most of the asteroids that are out there are heavily influenced by Jupiter. The wrong gravitational interaction, which can always occur as time goes on, could turn any of these asteroids into potential Earth-orbit-crossing hazards. Despite the dense appearance of this map, the asteroid belt itself is incredibly sparse.

Over 20 million asteroids over 100 meters wide are estimated.

Although we’ve cataloged most of the large (greater than 1 km) asteroids in the Solar System, the population of inner near-Earth asteroids that are greater than 0.1 km in size has not been well-determined at all. The number density of the smaller objects on this graph has only been estimated; a dedicated mission, such as NEO Surveyor, will be vital toward learning what truly poses a predictable hazard to Earth.

The average distance between them is nearly 1 million km: more than double the Earth-Moon distance.

This diagram shows the Earth and Moon, as well as the distance between them, to scale. The gravitational force between the Earth and the Moon is proportional to the distance between them squared, which means if the Moon were only half as far away, the gravitational force and the acceleration of the Moon would be quadrupled, but if it were twice as far away, the gravitational force (and hence, the acceleration of the Moon) would be quartered. The Moon is located approximately 60 times farther away from Earth’s center than Earth’s surface is.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.