Gliese 523b: the 'Mega-Earth' With 23 Times Earth's Mass
On August 21, 2026, a team of astronomers announced the discovery of Gliese 523b, a rocky planet so extraordinary it helps define an entire category: the "mega-Earth." With 23.5 times our planet's mass packed into a body just 2.55 times its size, it's a world where walking would be, quite literally, a much greater effort.
Gliese 523b (or GJ 523b) is a mega-Earth: a rocky planet far more massive than Earth. It has about 23.5 Earth masses but only 2.55 times the radius, which makes it extraordinarily dense. That combination gives an estimated surface gravity of about 3.6 times Earth's: there, a 70 kg person would feel as if they carried more than 250. It orbits a dwarf star about 87 light-years away.
What Makes It So Unusual
What's strange about Gliese 523b isn't just its mass, but the combination of mass and size. A planet with more than 20 times Earth's mass should, by expectation, have accumulated a thick envelope of gas and become something like Neptune. Instead, GJ 523b is rocky and compact, with only a thin atmosphere. Fitting that much mass into so little volume makes it one of the densest planets known.
That's why astronomers highlight it: the term "mega-Earth" had been used for over a decade without a clear example to define it. According to the discovery team, from the Wisconsin Center for Origins Research, Gliese 523b is the one that finally lets us say concretely what a mega-Earth is. It adds other oddities too: it belongs to a young system (about 170 million years old) and orbits nearly perpendicular to its star's spin, which is uncommon.
How Much Gravity It Would Have at Its Surface
Here's the most striking figure. A planet's surface gravity depends on its mass and radius through a simple formula: g = mass ÷ radius² (compared with Earth). With GJ 523b's numbers:
What does that feel like? See it in the widget.
How much would you weigh on Gliese 523b?
Adjust your weight on Earth and compare it with what you'd feel under GJ 523b's estimated gravity (~3.6 g), alongside other worlds for reference.
Gravity relative to Earth (=1): Moon 0.17 · Mars 0.38 · Earth 1 · GJ 523b ~3.6 (estimated from mass 23.5 and radius 2.55 Earths). The "weight" you feel is your mass × the local gravity.
The Takeaway
Gliese 523b is a reminder of how diverse the worlds beyond the Sun are: a rocky planet with more than twenty times Earth's mass, so compact that its gravity would more than triple what we feel here. It's no place for a comfortable walk, but it's a perfect case for understanding how a planet's mass and size set the force that keeps you on the ground.
And that force can be calculated with the same law of gravitation that rules here. If you want to explore how gravitational pull changes with mass and distance — the same principle behind GJ 523b's gravity — a gravitational force simulator lets you play with the numbers and see it for yourself.
Data based on the discovery announcement of Gliese 523b (August 2026) by researchers at the Wisconsin Center for Origins Research, reported by astronomy outlets. The mass (~23.5 Earths) and radius (~2.55 Earths) are the reported values; the surface gravity is an estimate calculated from them. Informational and educational.
Educational note on the Gliese 523b discovery announced in August 2026; figures follow the published announcement and may be refined.
Did this help? You can buy me a coffee.