In a brand new study, astronomers report novel evidence regarding the boundaries 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 when a star formed by determining its metallicity, or the extent of heavy elements present inside it. Metal-rich stars or nebulas formed relatively recently, while metal-poor objects were likely present in the course of the early universe.
Previous studies found a weak connection between metallicity rates and planet formation, noting that as a star’s metallicity goes down, so, too, does planet formation for certain planet populations, like sub-Saturns or sub-Neptunes.
Yet this work is the primary to look at that under current theories, the formation of super-Earths near metal-poor stars becomes significantly harder, suggesting a strict cut-off for the conditions needed for one to form, said lead creator Kiersten Boley, who recently received a PhD in astronomy at The Ohio State University.
“When stars cycle through life, they enrich the encompassing space until you’ve got enough metals or iron to form planets,” said Boley. “But even for stars with lower metallicities, it was widely thought that the variety of planets it could form would never reach zero.”
Other studies posited that planet formation within the Milky Way should begin when stars fall between negative 2.5 to negative 0.5 metallicity, but until now, that theory was left unproven.
To check this prediction, the team developed after which searched a catalog of 10,000 of probably the most metal-poor stars observed by NASA’s Transiting Exoplanet Survey Satellite (TESS) mission. If correct, extrapolating known trends to go looking for small, short-period planets around one region of 85,000 metal-poor stars would have led them to find about 68 super-Earths.
Surprisingly, researchers on this work detected none, said Boley. “We essentially found a cliff where we expected to see a slow or a gradual slope that keeps going,” she said. “The expected occurrence rates don’t match up in any respect.”
The study was published in The Astronomical Journal.
This cliff, which provides scientists with a timeframe during which metallicity was too low for planets to form, extends to about half the age of the universe, meaning that super-Earths didn’t form early in its history. “Seven billion years ago might be the sweet spot where we start to see an honest little bit of super-Earth formation,” Boley said.
Furthermore, as nearly all of stars formed before that era have low metallicities and would have needed to attend until the Milky Way had been enriched by generations of dying stars to create the best conditions for planet formation, the outcomes successfully propose an upper limit on the number and distribution of small planets in our galaxy.
“In an identical stellar type as our sample, we now know to not expect planet formation to be abundant when you pass a negative 0.5 metallicity region,” said Boley. “That is type of striking because we even have data to indicate that now.”
What’s also striking is the study’s implications for those looking for life beyond Earth, as having a more precise grasp on the intricacies of planet formation can supply scientists with detailed knowledge about where within the universe life may need flourished.
“You do not need to go looking areas where life would not be conducive or in areas where you do not even think you are going to search out a planet,” Boley said. “There’s only a plethora of questions that you would be able to ask when you know these items.”
Such inquiries could include determining if these exoplanets hold water, the dimensions of their core, and in the event that they’ve developed a robust magnetic field, all conditions conducive for generating life.
To use their work to other kinds of planet formation processes, the team will likely need to review various kinds of super-Earths for longer periods than they’ll today. Fortunately, future observations might be attained with the assistance of upcoming projects like NASA’s Nancy Grace Roman Space Telescope and the European Space Agency’s PLATO mission, each of which can widen the seek for terrestrial planets in habitable zones like our own.
“Those instruments will probably be really vital when it comes to determining what number of planets are on the market and getting as many follow-up observations as we are able to,” said Boley.
Other co-authors 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 was supported by the National Science Foundation and NASA.
