Astronomers baffled: A planet with a mass of 23 Earths defies theories

 

Since the discovery of the first exoplanet in the 1990s, scientists have expanded their search for distant worlds to reveal how planets form and evolve, and the number of discovered planets has now exceeded 6,000 exoplanets.

Today, NASA’s TESS telescope mission helped identify a new candidate around the star GJ 523 before ground-based observations revealed it to be an extraordinary world by all accounts.

The planet, named GJ 523b, has a radius about 2.55 times that of Earth, but a mass 23.5 times that of our planet. Its density is about 7.8 grams per cubic centimeter, a high density indicating that it is mostly composed of rock and heavy materials, with a relatively thin atmosphere.

Researchers call this type of world a "mega-earth," an informal description of large, dense planets that appear primarily rocky.

A great scientist who was supposed to become a conqueror

The main surprise lies in the mass of "GJ 523b". Traditional models of planet formation suggest that a planet begins with a core of rock and minerals, and then this core can attract hydrogen and helium from the gaseous disk and dust surrounding the star.

In our solar system, scientists believe that giant planets like Jupiter and Saturn began forming cores that reached about 20 times the mass of Earth before they started accumulating vast quantities of gas. Therefore, it was expected that if GJ 523b reached a mass exceeding this limit, it would form a dense atmosphere around itself.

But the planet did not follow this path.

"This is not what we expected at all," says Max Croft, lead researcher and graduate student at the University of Wisconsin-Madison, noting that dense planets are not necessarily rare, but are usually small and similar to Earth or Mercury, while this planet is about twice the size of Earth and one and a half times the size of Earth.

Young age and fast orbit add to the mystery

GJ 523b orbits its star once every 17.75 days, and the planetary system is only about 170 million years old, which is young compared to the age of the solar system, which is about 4.6 billion years.

A team of researchers at the Center for the Origins of the Universe at the University of Wisconsin-Madison was able to confirm the nature of the planet and measure its mass by combining TESS data with observations from the 3.5-meter Wien telescope at the Kitt Peak National Observatory in Arizona, using a high-resolution spectrometer to measure the small changes in the star's speed caused by the planet's gravity.

From these measurements, it was possible to estimate the mass of the planet, which revealed that its mass is about 23.5 times the mass of Earth.

TESS had detected the periodic dimming of the star's light, a signal that occurs when a planet passes in front of its star and blocks a small portion of its light—a transit. From the magnitude and frequency of the dimming, the planet's size and orbit could be estimated, and spectral measurements were then used to determine its mass.