A recent study by astronomers reveals new findings about the challenges in planet formation, indicating that substantial planets, those larger than Earth, struggle to form in the vicinity of stars with low metallicity levels.
Astronomers use the sun as a reference point to understand when a star was created by measuring its metallicity, which reflects the presence of heavier elements. Stars or nebulas rich in metals are relatively young, whereas those low in metals are often ancient, reflecting the universe’s early stages.
Previous research indicated a weak correlation between metallicity and the ability of stars to form planets, suggesting that a decrease in metallicity could hinder the formation of specific types of planets, such as sub-Saturns or sub-Neptunes.
This study uniquely identifies that, according to existing theories, forming super-Earths near metal-poor stars is considerably more challenging, indicating a definite threshold for the environment needed for their existence, explained lead author Kiersten Boley, who recently earned a PhD in astronomy from The Ohio State University.
“As stars evolve, they enrich space with metals and iron, which are essential for planet formation,” noted Boley. “However, it was previously assumed that even stars with lower metallicities could still produce some planets.”
Other research claimed that planet formation in our Milky Way typically occurs when stars have metallicity levels ranging from negative 2.5 to negative 0.5. However, this hypothesis had not been validated until now.
To explore this theory, the research team analyzed a catalog of 10,000 of the most metal-poor stars identified by NASA’s Transiting Exoplanet Survey Satellite (TESS). If their predictions were accurate, the analysis would suggest finding about 68 super-Earths among a larger sample spanning 85,000 metal-poor stars.
Surprisingly, the researchers found no super-Earths, according to Boley. “We discovered a sudden drop-off in occurrences where we anticipated a slow, gradual decline,” she remarked. “The expected occurrence rates were vastly different from what we observed.”
This research was published in The Astronomical Journal.
This sudden drop-off provides a timeline indicating that the metallicity was insufficient for planet formation for a significant period, suggesting super-Earths did not arise during the early universe. “Around seven billion years ago seems to be the right time frame for a notable emergence of super-Earths,” Boley explained.
Since most stars that formed in the universe’s earlier phases also have low metallicities, they had to wait for the Milky Way to become enriched by multiple generations of dying stars to create the right conditions for planet formation. This finding establishes an upper limit on the quantity and distribution of small planets in our galaxy.
“In similar types of stars to the ones we’ve studied, we now know not to expect abundant planet formation once you exceed the negative 0.5 metallicity level,” Boley noted. “This is significant as we can now back it up with data.”
These findings also have important implications for the search for extraterrestrial life, as a clearer understanding of the parameters of planet formation can guide scientists on where life may thrive in the universe.
“You wouldn’t want to explore regions where life cannot exist or where planets are unlikely to be found,” Boley emphasized. “These insights open up a wide array of inquiries.”
Such inquiries might explore whether these exoplanets contain water, the composition of their cores, and the presence of a magnetic field, all of which are essential for fostering life.
To extend their research to other planet formation processes, the team will likely need to investigate other kinds of super-Earths for more extended periods than currently possible. Luckily, future missions such as NASA’s Nancy Grace Roman Space Telescope and the European Space Agency’s PLATO mission will enhance the search for terrestrial planets in habitable zones similar to Earth.
“These instruments will be crucial in determining how many planets exist and enabling as many follow-up observations as possible,” Boley stated.
Other contributors to this research include Ji Wang from Ohio State; Jessie Christiansen, Philip Hopkins, and Jon Zink from The California Institute of Technology; Kevin Hardegree-Ullman and Galen Bergsten from The University of Arizona; Eve Lee from McGill University; Rachel Fernandes from The Pennsylvania State University; and Sakhee Bhure from the University of Southern Queensland. This study received support from the National Science Foundation and NASA.
