🔑 Key Takeaways

• The smallest planet by size (radius) is Kepler-37b, tinier than Mercury and just slightly bigger than our Moon.
• The smallest planet by mass is Draugr, orbiting a pulsar, weighing only twice as much as our Moon.
• Recent discoveries like Gliese 367 b, Proxima Centauri d, and Barnard’s Star planets are rewriting our understanding of tiny rocky worlds.
• Studying these smallest planets helps scientists refine how planets form and survive in extreme environments.
• With new tools like JWST and the upcoming Roman Space Telescope, astronomers expect to find even smaller worlds.

What Do We Mean by “Smallest Planet”?

Earth, the smallest planet, and Jupiter—are displayed side by side against a dark background, illustrating their relative sizes.” class=”wp-image-21536″/>

When you hear “smallest planet,” what comes to mind? Maybe Mercury, the runt of our Solar System. Or even our Moon.
But in astronomy, smallest can mean different things:

• By radius (physical size) → Kepler-37b holds the crown, at only 31% the size of Earth.
• By mass (weight) → Draugr wins, with just 2% the mass of Earth.
• By complete measurement (both mass and size) → Kepler-138b is the first confirmed “sub-Earth” with both data points.

This distinction matters, because how we discover exoplanets (transit, pulsar timing, radial velocity) determines what we measure.

A Short History of the Hunt for the Smallest Planet

The First Clues: Draugr Around a Pulsar

In 1992, astronomers Aleksander Wolszczan and Dale Frail shocked the world. They found planets orbiting a pulsar—a dead star that spins like a cosmic lighthouse. One of them, later named Draugr, turned out to be the least massive exoplanet ever found, just twice the mass of our Moon.

Imagine that: a tiny world clinging to life around a dead star, bathed in radiation. Not a place for humans, but proof that planets can form—or reform—even in the wreckage of a supernova.

COROT-7b: The First Rocky Exoplanet

Fast forward to 2009. The European CoRoT satellite spotted a new kind of planet: COROT-7b, a rocky world about 1.7 times Earth’s size.
This was no cozy Earth twin. Orbiting so close to its star that a “year” lasts just 20 hours, its surface is probably covered in oceans of molten lava.

Still, it was a milestone: the first time we confirmed that rocky planets exist outside our Solar System.

Kepler-42d: A Mars-Sized Surprise

In 2012, NASA’s Kepler telescope unveiled a system of three tiny planets orbiting a red dwarf star.
The smallest, Kepler-42d, is only about 0.57 Earth radii—comparable to Mars. All three planets zipped around their star in less than two days.

Astronomers joked that the system looked less like a Solar System and more like “Jupiter and its moons.”

Kepler-37b: The Reigning Champion of Small

Then came the big headline in 2013: Kepler-37b, a planet smaller than Mercury.

• Size: 0.31 Earth radii (about 1,930 km across).
• Orbit: 13 days, 0.1 AU from its star (much closer than Mercury is to the Sun).
• Temperature: ~445 °C—hot enough to melt zinc.
• Atmosphere: None. Too small to hold onto one.

What made this discovery possible was a technique called asteroseismology. By studying “starquakes” (tiny vibrations in the star’s brightness), scientists could measure the host star’s size precisely, and therefore calculate the planet’s radius.

For the first time, we had proof that planets even smaller than Mercury exist and survive for billions of years.

New Records and Recent Discoveries

Gliese 367 b: The Iron Planet

Gliese 367 b, found just 30 light-years away, is only 0.72 Earth radii.
But what makes it stand out is its composition: it’s ultra-dense and iron-rich, almost like a scaled-up version of Mercury. Scientists think it lost its outer layers, leaving just a metallic core.

Proxima Centauri d: Our Tiny Neighbor

Proxima Centauri, the closest star to our Sun, hosts several planets. One of them, Proxima Centauri d, has a mass of only 0.25 Earth masses.
That makes it one of the lightest planets known—and it’s practically next door at 4.2 light-years away.

Barnard’s Star: A System of Mini-Worlds

In 2025, researchers confirmed four rocky planets orbiting Barnard’s Star, all smaller than Earth.
With masses between 0.19 and 0.34 Earth masses, this is the first known system composed entirely of sub-Earth planets.

This discovery suggests that tiny planets are not rare—they might be incredibly common in our galaxy.

Why Smallest Planets Matter

So why do astronomers care about these tiny, scorching, airless rocks?
Because they are test cases.

• They prove that planet formation can produce bodies far smaller than Earth.
• They help us understand atmospheric loss—why some planets keep their air while others can’t.
• They serve as signposts for detection technology: if we can spot a Moon-sized planet 200 light-years away, what’s next?

Ultimately, the smallest planets expand our picture of planetary diversity. For every Jupiter-like giant, there may be countless moon-sized rocks.

Smallest Planet vs Largest Planet: A Cosmic Scale

Let’s put things in perspective:

The difference is staggering. From worlds smaller than our Moon to planets bigger than Jupiter, the galaxy’s diversity is beyond imagination.

Firsthand Reflection: Seeing Small Worlds as Milestones

When I read NASA’s press release about Kepler-37b back in 2013, I remember thinking: This changes everything.
It wasn’t a habitable world. It wasn’t Earth 2.0. But it was proof that planets even smaller than Mercury could exist and be detected.

Since then, every new “smallest planet” discovery has reminded me of one thing: the galaxy is full of surprises.
If these tiny, hostile rocks are common, then Earth-like planets might also be plentiful—waiting to be found.

What the Future Holds for Smallest Planets

• James Webb Space Telescope (JWST) is already scanning atmospheres of small rocky planets, like those in the TRAPPIST-1 system.
• Roman Space Telescope (launch late 2020s) will use microlensing to detect cold, tiny worlds, maybe even Mars- or Moon-sized.
• Extremely Large Telescopes (ELTs) on Earth will measure the mass and density of sub-Earths with more precision.

The search is moving from “Where are they?” to “What are they like?”

❓ Frequently Asked Questions (FAQ)