Scientists Puzzled by Dense Mega-Earth with Minimal Atmosphere

A planet named GJ 523b is challenging scientists due to its unusual characteristics. Although more than twice the size of Earth, it has a surprisingly thin atmosphere. Researchers expected its massive size to result in a thick gas layer.

GJ 523b is around 170 million years old and is 2.5 times larger than Earth. Weighing about 23 times more than our planet, it measures around 60 percent the size of Neptune. The University of Wisconsin-Madison researchers describe it as mostly dense rock with a massive core.

This isn’t what we expected at all. Dense planets like this aren’t uncommon, but they’re usually small rocky planets similar to Earth or Mercury. This planet is two and a half times bigger than the Earth.

What Is a Mega-Earth?

A Mega-Earth is a large and dense terrestrial exoplanet. It boasts more mass than a typical “super-Earth” and consists mainly of rock and metal. Professor Thomas Beatty explains that GJ 523b helps clarify what defines a Mega-Earth.

People have been using the phrase ‘Mega-Earth’ for more than a decade, but we’ve never had a planet that let us say concretely what one is. GJ 523b finally does.

Understanding the planet requires multidisciplinary expertise. Geologists must study how iron and rock behave under high pressures not replicable on Earth. Atmospheric scientists are also crucial to determining the planet’s composition.

NASA’s Transiting Exoplanet Survey Satellite (TESS) first identified GJ 523b. It detects potential planets by monitoring stars for periodic light dips, suggesting an orbiting planet blocking the star’s light.

There’s this periodic dipping of the star’s light. We think that’s a planet passing in front of the star and transiting. It’s blocking some of the light from the star, and the star gets dimmer.

The team confirmed this using the WIYN telescope in Arizona and data from the James Webb Space Telescope, investigating GJ 523b’s density and atmospheric properties.

Why Is Its Atmosphere So Small?

Traditionally, rock and metal cores gather hydrogen-rich atmospheres. In our solar system, large planets like Jupiter and Saturn developed extensive atmospheres after surpassing 20 times Earth’s mass. GJ 523b contradicts this pattern.

The question is, why didn’t this planet do that, if it’s 20 times the size of the Earth?

Experts suggest GJ 523b could have lost its atmosphere due to extreme heat from close orbiting its star in the past. Another theory proposes a collision between two planets, with the resulting heat stripping the atmosphere.

A planet can’t hold on to its atmosphere if it’s really hot, and so you could be left with this big glob of rock made by these two planets with very little atmosphere.