Mysteriously Dense Planet GJ 523b Challenges Scientific Expectations

Researchers are closely studying an unusual planet named GJ 523b due to its surprising composition. Despite being over twice the size of Earth, it has little atmosphere, contrary to expectations for such a massive body. According to the University of Wisconsin-Madison, this planet is around 170 million years old and more than 2.5 times the size of Earth, with a mass approximately 23 times greater. It measures about 60 percent of the size of Neptune, consisting mainly of dense rock surrounding a substantial core.

This isn’t what we expected at all, said Max Kroft, a graduate student in the laboratory of Astronomy professor Thomas Beatty at UW-Madison. 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.

Understanding Mega-Earths

A Mega-Earth is a term for a large and dense terrestrial planet found outside our solar system. These are more massive than typical “super-Earths” and largely composed of solid materials like rock and metal. Professor Beatty explained that GJ 523b provides a clearer definition of what constitutes 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, Beatty said. GJ 523b finally does.

Beatty noted the need for interdisciplinary expertise to understand GJ 523b thoroughly. This includes geologists to analyze the behavior of iron and rock under high pressures, along with atmospheric scientists to determine the rock composition.

Discovery and Analysis

The planet GJ 523b was initially spotted by NASA’s Transiting Exoplanet Survey Satellite (TESS). This satellite detects exoplanets by observing the regular dimming of stars as planets transit by. Kroft described this process: 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.

After its initial discovery, Kroft used the WIYN telescope in Arizona and data from the James Webb Space Telescope to analyze GJ 523b’s density and atmosphere.

Atmospheric Enigma

Traditionally, large rocky planets like GJ 523b attract a thick, hydrogen-rich atmosphere. Our solar system’s giants, like Jupiter and Saturn, developed voluminous atmospheres after exceeding 20 times Earth’s mass.

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

The researchers suggest several theories. The planet might have formed closely to its star, with intense heat stripping its atmosphere. Another possibility is the planet’s formation following a collision between two planets, where the resulting heat removed most of the gaseous envelope.

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, Kroft said.