Mystery of Planet GJ 523b: Dense Yet with Little Atmosphere

An unusually dense planet more than twice the size of Earth is leaving scientists perplexed. Despite its massive size, the planet has very little atmosphere. Known as GJ 523b, the planet is around 170 million years old and measures over 2.5 times the size of Earth. It has an estimated mass 23 times greater than Earth and is roughly 60 percent the size of Neptune, according to researchers from the University of Wisconsin-Madison. The university’s statement describes it as mostly dense rock surrounding a massive core.

Lead author Max Kroft, a graduate student in Professor Thomas Beatty’s laboratory at UW-Madison, commented on the surprising nature of the findings. “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 Earth,” Kroft said.

What Is a Mega-Earth?

A Mega-Earth is a significantly large and dense terrestrial planet positioned outside our solar system. Unlike typical “super-Earths,” Mega-Earths are thought to consist largely of solid materials such as rock and metal. Professor Beatty noted that GJ 523b could help refine the definition of this category. “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,” Beatty explained. “GJ 523b finally does.”

Understanding this planet requires expertise beyond astronomy. Geologists are needed to study how iron and rock behave under high pressures not reproducible on Earth. Atmospheric scientists can help determine how much of GJ 523b is actually rock, as Beatty mentioned.

GJ 523b was initially identified by NASA’s Transiting Exoplanet Survey Satellite (TESS). The satellite detects planets by observing stars for regular dips in brightness, indicating a planet crossing in front of its star. “There’s this periodic dipping of the star’s light,” Kroft said. “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.”

Kroft continued the investigation using the WIYN telescope in Arizona and data from the James Webb Space Telescope to analyze the planet’s density and atmosphere.

Why Is Its Atmosphere So Small?

Scientists expect a rock-metal core to gather a hydrogen-rich atmosphere. For example, Jupiter and Saturn in our solar system have massive atmospheres as they grew beyond 20 times Earth’s mass. “The question is, why didn’t this planet do that, if it’s 20 times the size of Earth?” Kroft asked.

Several theories have been suggested by researchers. One possibility is that GJ 523b once orbited very close to its star, and extreme heat may have stripped its atmosphere away. Another idea is that the planet formed from a collision between two planets, with the resulting heat removing the atmosphere. “A planet can’t hold on to its atmosphere if it’s really hot, leaving you with a big glob of rock and minimal atmosphere,” Kroft said.