Formation of super-Earths limited near metal-poor stars

In a new study, astronomers report fresh evidence about the limits of planet formation, finding that after a certain point, planets larger than Earth have difficulty forming near low-metallicity stars.

Using the Sun as a baseline, astronomers can measure a star’s formation by determining its metallicity, or the level of heavy elements present within it. Metal-rich stars or nebulae formed relatively recently, while metal-poor objects were likely present early in the universe.

Previous studies have found a weak link between metallicity rates and planet formation, noting that as a star’s metallicity decreases, planet formation also decreases for certain populations of planets, such as sub-Saturns or sub-Neptunes.

Yet this work is the first to observe that, under current theories, the formation of super-Earths near metal-poor stars becomes significantly more difficult, suggesting a strict cutoff for the conditions needed for a super-Earth to form, said lead author Kiersten Boley, who recently earned a doctorate in astronomy at The Ohio State University.

“As stars go through a life cycle, they enrich the surrounding space until there is enough metal or iron to form planets,” Boley said. “But even for stars with lower metallicity, it was generally thought that the number of planets they could form would never reach zero.”

Other studies have suggested that planet formation in the Milky Way should begin when stars are between a metallicity of -2.5 and -0.5, but so far this theory has not been proven.

To test this prediction, the team constructed and studied a catalog of 10,000 of the most metal-poor stars observed by NASA’s Transiting Exoplanet Survey Satellite (TESS) mission. If these results were correct, extrapolating known trends to search for small, short-period planets around a region of 85,000 metal-poor stars would have allowed them to discover about 68 super-Earths.

Surprisingly, the researchers found no such cases, Boley said. “We basically found a cliff where we expected to see a slow or gradual slope that continues,” she said. “The expected rates of occurrence don’t match up at all.”

The study was published in The Astronomical Journal.

This cliff, which provides scientists with a period when metallicity was too low for planets to form, spans about half the age of the universe, meaning super-Earths did not form early in its history.

“Seven billion years ago is probably the sweet spot where we start to see a good bit of super-Earth formation,” Boley said.

Moreover, since the majority of stars formed before this epoch have low metallicity and would have had to wait until the Milky Way was enriched by generations of dying stars to create the conditions for planet formation, the results successfully propose an upper limit on the number and distribution of small planets in our galaxy.

“In a type of star similar to our sample, we now know that we should not expect abundant planet formation once we pass a region of negative metallicity of 0.5,” Boley said. “That’s quite striking, because we now have data to prove it.”

What’s also striking are the study’s implications for those searching for life beyond Earth, as a more precise understanding of the intricacies of planet formation can provide scientists with detailed knowledge of where in the universe life might have thrived.

“You don’t want to look in areas where life wouldn’t be possible or in areas where you don’t even think you’re going to find a planet,” Boley said. “There’s just a plethora of questions you can ask if you know these things.”

Such research could include determining whether these exoplanets contain water, the size of their cores and whether they have developed a strong magnetic field, all conditions conducive to the creation of life.

To apply their work to other types of planet-formation processes, the team will likely need to study different types of super-Earths for longer periods of time than they currently have to deal with. Fortunately, future observations could be made possible by upcoming projects like NASA’s Nancy Grace Roman Space Telescope and the European Space Agency’s PLATO mission, both of which will expand the search for terrestrial planets in habitable zones like our own.

“These instruments will be really critical to determining how many planets exist and getting as many follow-up observations as possible,” Boley said.

Other co-authors include Ji Wang of Ohio State; Jessie Christiansen, Philip Hopkins and Jon Zink of the California Institute of Technology; Kevin Hardegree-Ullman and Galen Bergsten of the University of Arizona; Eve Lee of McGill University; Rachel Fernandes of Pennsylvania State University; and Sakhee Bhure of the University of Southern Queensland.