Super-Earths face significant challenges in forming near metal-poor stars, according to a new study. The discovery challenges previous assumptions about planet formation and sheds new light on how the universe’s earliest stars may have influenced the distribution of planets in our galaxy.

Using metallicity (the abundance of elements heavier than helium), astronomers can trace the history of stars and planetary systems, providing new insights into the conditions needed to form a super-Earth.

Metallicity and planet formation

In astronomical terms, metallicity measures the presence of elements heavier than helium in a star. Stars rich in these elements, like our Sun, are younger, while stars poor in metals come from the early universe. Previous studies have suggested a gradual decline in planet formation as metallicity decreases, but this new study reveals a sharp boundary beyond which super-Earth formation becomes extremely unlikely.

The study’s lead author, Kirsten Boley, recently received her doctorate in astronomy from The Ohio State University.

“When stars develop life, they enrich the surrounding space until there is enough metal or iron to form planets. But even for stars with lower metallicities, it was thought that the number of planets they could form would never reach zero,” Boley explained.

Super world formation rate

To test existing predictions, Bowley’s team compiled a catalog of 10,000 metal-poor stars using data from NASA’s Transiting Exoplanet Survey Satellite (TESS). Based on known trends, they expected to find about 68 super-Earths. But contrary to expectations, the researchers found none. “Essentially, we found a rock where we would expect to see a slow or gradual tilt that continues to move,” Boley said. “The expected event rates just don’t match up.”

This “rock” dates back almost halfway across the universe, meaning super-Earth formation was possible only seven billion years ago.

“Seven billion years ago is probably the best point where we start to see a reasonable amount of super-Earth formation,” Boley said.

Metal-poor stars and super-Earths

The implications of these findings are profound. Stars that formed before this metallicity threshold would not have the materials needed to form a super-Earth, sharply limiting the number of such planets in our galaxy.

“We now know that in a star like ours, when you go through the minus 0.5 metallicity region, you don’t expect a lot of planet formation,” Boley said.

The results of the search for life

The discovery clarifies where those searching for extraterrestrial life should focus their search. “You don’t want to be looking in places where you don’t think life would be possible or where you don’t even think you’re going to find a planet,” Boley said.

Studying planet formation provides important clues, from whether a planet can hold water to its magnetic field—the basic conditions for life. While the study raises important questions about planet formation, more research is needed. Fortunately, future projects like NASA’s Nancy Grace Rome Space Telescope and the European Space Agency’s PLATO mission will help expand the search for planets in habitable zones.

“These instruments are really going to be vital in quantifying the number of planets and getting as many follow-up observations as possible,” Boley said.

Understanding the Super World

Super-Earths are exoplanets that are larger than Earth but much smaller than ice giants such as Uranus or Neptune. Despite their name, the planets are not necessarily similar to Earth in terms of conditions or habitability. The term “super-Earths” primarily refers to their size, which ranges from about 1.5 to 10 Earth masses.

These planets are of great interest to scientists because they are among the most common types of exoplanets found in our galaxy, even though they are not in our solar system. Some super-Earths may have a rocky, thin atmosphere like Earth, while others may have a thick, gaseous atmosphere like Neptune. These planets could potentially host liquid water, making them prime targets in the search for life outside our solar system.

The study was conducted in collaboration with researchers from Ohio State, Caltech, the University of Arizona, McGill University, Penn State and the University of Southern Queensland, with support from the National Science Foundation and NASA.