Smallest Planet: The Ground Breaking Discovery of Mercury

Mercury is the smallest planet in our solar system. Yet, that’s not the only exciting thing about this terrestrial world. It’s also the closest planet to the Sun, so you’d think it’d be the hottest. But it’s not. Mercury is a tiny planet with a lot going on. From ice on its scorching surface to a super fast orbit, come along as we discover more about the smallest planet.

10 Things to Know About Mercury

You can see Mercury on dark nights with your unaided eye, so the ancients even saw this small world. The Greeks knew Mercury as Hermes, a messenger of the gods who mediated between the lands of the living and those of the dead. He had winged feet to carry messages quickly. And that’s not so different from the smallest planet’s speedy orbit around the Sun.

1. The Smallest Planet Has the Fastest Orbit

Mercury jets around the Sun once every 88 Earth days. The rocky world zips through space faster than all the other planets at almost 29 miles (47 kilometers) per second. That’s right, per second!

The smallest planet has a highly eccentric, oval orbit that pulls it close to the Sun and then pushes it further into space.

  • Closest: 29 million miles away (47 million kilometers)
  • Furthest: 43 million miles away (70 million kilometers)

Mercury’s Slow Rotation on Its Axis

Even though the little planet zooms around the Sun, it turns slowly on its axis. One rotation takes 1,408 hours or about 59 Earth days. However, the rotation looks different when Mercury draws close to the Sun and moves at its fastest in its elliptical orbit.

For example, if you were standing on Mercury’s surface, you might see the Sun rise for a short time, set, and then rise a second time. And the same happens later in the “day” where you might see two sunsets.

A full solar day on Earth is 23.934, or about 24 hours. That’s how long it takes for the Sun (from any given surface position) to move to its highest point in the sky through the night and back to that same high point the next day. However, that same day-night cycle on Mercury takes about 176 Earth days. That’s approximately two full Mercurian years!

While Mercury has a speedy orbit, it has a minimal rotation axis tilt. At only two degrees, the smallest planet spins virtually upright. So, it has excellent posture but no real seasonal weather variations.

Image: NASA, JPL Caltech, and Richard Barkus

2. Is the Smallest Planet Bigger Than Our Moon?

Mercury is so minute that it’d look like a blueberry if Earth were nickel-sized. The smallest planet is only about one-third our home planet’s width at a tiny radius of 1,516 miles (2,440 kilometers.)

But you might still be wondering if Mercury is bigger than our Moon. The answer is YES! But not by much. Mercury is only about 40% larger than the Moon.

Here’s a size comparison between Mars, Mercury, and the Moon. You can also compare the smallest planet to the enormous asteroid Vesta and the tiny dwarf planet Ceres. They lie in the main asteroid belt in between Mars and Jupiter.

Mercury Size comparison with moons
Image: NASA and JPL

3. How Hot Is the Smallest Planet?

Mercury is so close to the Sun; from its surface, the giant star looks more than three times the size we see it from Earth. In addition, sunlight appears about seven times brighter on Mercury than it does to us.

And while the smallest planet has scorching dayside temperatures, it’s not as hot as neighboring Venus. Now that’s one hot planet!

Mercury size comparison
Image: NASA and JPL

Mercury doesn’t have an atmosphere. Instead, its thin exosphere consists of atoms that the solar wind and crashing meteoroids blast off the smallest planet’s surface. Since there’s no atmosphere, the world is a target for passing space debris collisions. But it also has no cloud-blanket insulation to retain heat.

So, temperatures on the tiny world’s surface are extreme. Dayside surface temperatures reach a scorching 800 degrees Fahrenheit (430 degrees Celsius). But that lack of atmosphere means temperatures drop significantly at night. They plunge to minus 290 degrees Fahrenheit (minus 180 degrees Celsius).

Does Mercury Really Have Ice on Its Poles?

Scientists believe Mercury has water ice in the dark craters at its poles despite those wicked hot daytime temperatures. They think it’s cold enough to maintain ice fields in the deep shadows where sunlight never reaches.

The image below shows the smallest planet’s north polar region with yellow highlights in the craters. Puerto Rico’s Arecibo Observatory took radar observations to determine water ice evidence. The MESSENGER spacecraft later collected more proof of the ice-filled craters.

Ice on Mercury
Image: Carnegie Institution of Washington, Johns Hopkins University Applied Physics Laboratory, and NASA

4. Mercury’s Solid Surface & Iron Core

The cratered surface of Mercury looks very similar to our Moon. Both have more impact craters than you can count! But what’s fun about Mercury’s craters is that they’re named after famous musicians, artists, and authors. There’s even a crater named after Dr. Seuss!

In addition, Caloris is one of the “very large impact basins,” and it’s so gigantic that the whole state of Texas can fit inside. It’s about 960 miles (1,550 kilometers) in diameter, and Texas is only about 773 miles (1,244 kilometers) from east to west.

Mercury’s Solid Surface & Iron Core
Image of the Caloris Basin: Image: Carnegie Institution of Washington, Johns Hopkins University Applied Physics Laboratory, and NASA

Mercury does have smooth terrain, too. It’s not all pockmarks! There are also hundreds of miles of enormous cliffs. And some soar up to a mile high. Astronomers believe the cliffs rose while the planet’s core cooled and contracted over time.

Much of Mercury has a greyish-brown coloring. Some bright streaks, or crater rays, are also present. These streaks form after a surface impact from a comet or asteroid. The impact digs a hole and crushes rock. As the pulverized materials fly into the air and fall back down, they form crater rays. The fine particles reflect more sunlight than larger rocks, so they appear as brighter streaks that darken over time.

Mercury has a greyish-brown coloring
Mena Crater Rays: NASA and JPL

The Smallest Planet’s Iron Core

It seems like an impossible task to explore Mercury’s interior when a spacecraft can’t even touch down on its surface. But NASA’s planetary scientists observed how it spins. Then, they measured how a spacecraft orbits the planet. And they formed conclusions about the rocky world’s iron core from its gravity and spin.

They’ve known for a long time that Mercury has an Earth-like metallic core. The outer cores are also similar with a liquid metal makeup. But it wasn’t until a 2019 study at the Goddard Space Flight Center that scientists discovered evidence of the solid core. MESSENGER data provided the proof they needed.

They also found that Mercury’s core is almost the same size as Earth’s. And that is very surprising since the planet is so much smaller than ours. Mercury’s metal core is nearly 85 percent of the planet’s volume. That’s more similar to a cannonball than to our home planet!

Scientists think the similarities and differences of planets’ cores provide clues about the solar system’s formation. And they also point to how terrestrial worlds change over time.

The Smallest Planet’s Iron Core
Image: NASA’s Goddard Space Flight Center

5. Is the Smallest Planet Getting Smaller?

In 2016, scientists discovered that Mercury is shrinking. They examined MESSENGER images to find fault scarps. These are cliff-like landforms that look similar to stair steps. And planetary scientists say they’re small enough to be geologically young.

Here’s the big news: Young fault scarps indicate that the smallest planet is still contracting. So, Earth isn’t the only solar system planet that is tectonically active.

Mariner 10 images showed large fault scarps in the 1970s, but they formed as the planet’s interior cooled. That’s when the vast cliffs formed. But these smaller fault scarps indicate more recent tectonic activity. Scientists think the rocky world may even experience “Mercury-quakes.”

Is the Smallest Planet Getting Smaller?
Image: NASA, JHUAPL, Carnegie Institution of Washington, USGS, and Arizona State University

6. The Smallest Planet’s Magnetic Field

Mercury has an offset magnetic field relative to its equator. So, the magnetic polar cap at the south pole is much larger than that at the north. That’s the area where field lines remain open to the interplanetary medium.

As a result, the South Pole is more exposed to charged particles. They get accelerated and heated by interactions between the magnetosphere and roaring solar winds. Those charged particles then contribute to the planet’s exosphere and to surface materials becoming “space weathered.”

And that’s where magnetic tornadoes come in. They funnel plasma from the solar wind to the planet’s surface. Ions strike the ground where they strike neutrally charged atoms, sending them whirling and looping into space.

The Smallest Planet’s Magnetic Field
Image: Image: Carnegie Institution of Washington, Johns Hopkins University Applied Physics Laboratory, and NASA

7. Can You Breathe on the Smallest Planet?

There’s nothing to breathe on Mercury. The planet has no atmosphere to speak of. Additionally, the thin exosphere is essentially a vacuum. So, no, you cannot breathe on the smallest planet.

The rocky world’s extreme temperature ranges also make it inhospitable to life as we know it. But if, for some reason, life could still exist in super hot or super cold weather, the Sun’s radiation would dash any last hopes for survival. The extremities of Mercury make it very unlikely that any organisms could adapt and thrive.

8. How Many Moons Does Mercury Have?

Mercury has no moons and no rings. No natural satellites orbit the planet due to its proximity to the Sun. If any space objects came close enough, there are two likely outcomes.

  • Any object would crash into the smallest planet.
  • The Sun’s immense gravity would strip them away.

9. Is Mercury Smaller Than Dwarf-Planet Pluto?

Mercury is larger than Pluto. The International Astronomical Union (IAU) is the governing body that sets planetary definitions. And it classifies a dwarf planet as generally smaller than Mercury.

Here are the dwarf planet guidelines.

  • It orbits the Sun.
  • It has enough mass to maintain a nearly rounded shape.
  • It has not cleared the neighborhood (pulled orbiting objects into itself through accretion.)
  • It is not a moon.

Check out the illustration below showing the approximate sizes of solar system planets, dwarfs, and celestial objects.

Is Mercury Smaller Than Dwarf-Planet Pluto?
Image: IAU

10. What Space Missions to Mercury Are There?

There have been multiple space missions to Mercury. But its harsh conditions close to the Sun and quickly speeding orbit create challenges. Some of the Mercury missions include the following.

  • NASA’s Mariner 10: Three close flybys in 1974 and 1975 mapped almost half the smallest planet’s surface.
  • NASA’s MESSENGER: Orbited from 2011 through its intentional crash in 2015. It collected many images, mapped the surface, and discovered ice in the polar regions. It also acquired evidence of volcanic and tectonic activity and the magnetic field’s off-center status.
  • The European Space Agency (ESA) has a joint mission to Mercury with Japan. BepiColombo left Earth in October 2018 with plans to orbit Mercury in 2025. It carries two spacecraft to study the smallest planet.
    • ESA’s Mercury Planetary Orbiter (MPO) will focus on Mercury’s surface and interior.
    • The Japan Aerospace Exploration Agency (JAXA) Mercury Magnetospheric Orbiter (MIO) will focus on Mercury’s magnetic field.

BepiColombo hopes to learn more about the smallest planet’s history, composition, atmosphere, geophysics, and magnetosphere. The spacecraft made its first Mercury flyby on October 1, 2021.

What Space Missions to Mercury Are There?
Image: ESA, BepiColombo MTM, and CC BY-SA 3.0 IGO

Finally, to wrap up our discovery of the smallest planet, Mercury, here are the first and last images taken by NASA’s MESSENGER in 2011 and 2015. Over its mission, the spacecraft took over 277,000 photographs as it orbited the smallest planet. On April 30, 2015, MESSENGER plunged to the planet’s surface in its final act.

discovery of the smallest planet
Image: NASA, Johns Hopkins University Applied Physics Laboratory, and Carnegie Institution of Washington