Stars that don’t have enough elements heavier than hydrogen and helium can’t form super-Earths—rocky planets slightly larger than Earth. The occurrence rate of super-Earths drops off sharply when stars have less than a third of the Sun’s heavy-element content.
The discovery of this “metallicity cliff,” which will be published in the Astronomical Journal, can help astronomers narrow down the search for rocky super-Earth exoplanets and understand the dawn of planet formation in the Milky Way.
Falling Off the Metallicity Cliff
All stars are primarily made of hydrogen and helium, but the small percentage of a star’s mass made up of heavier elements, which astronomers call metals, can make a large difference in the star’s evolution. The Sun, for example, is 1.3% metals by mass.
The mass abundance of iron is one of the most common proxies for a star’s total metal abundance, and “metallicity” is typically calculated as the mass ratio of iron to hydrogen in the star compared to the same ratio in the Sun. Metal-poor stars have fewer metals than the Sun, and metal-rich ones have more.
“Iron is a great tracer for the available metals that are in a protoplanetary disk where planets form,” explained Kiersten Boley, an astronomer at Ohio State University in Columbus and lead researcher on the new discovery.
Because planets form from the leftovers of star formation, a star’s metallicity has been thought to play a determining role in the types of planets that a star system can make, Boley explained. Past exoplanet surveys by the Kepler and K2 missions found that a star’s metallicity correlates with the likelihood that it hosts a gas giant planet—after all, gas giants have rocky cores, too.
However, because the Kepler and K2 missions did not find a bevy of planets smaller than 4 Earth radii (super-Earths), astronomers couldn’t determine whether the connection between planet occurrence and metallicity holds for smaller, rocky planets.
NASA’s Transiting Exoplanet Survey Satellite (TESS) mission, an all-sky planet hunter, overcame this challenge. The mission has detected hundreds of super-Earths already. Boley and her colleagues analyzed around 110,000 TESS stars, including nearly 85,000 stars with 1%–30% of the Sun’s metal content. They also included stars closer to the Sun’s metallicity to compare with the Kepler and K2 surveys. They then calculated the occurrence rate of super-Earths around stars with different metallicities.
“We searched all of these stars, and we found zero planets.”
On the basis of the Kepler-derived planet occurrence trends, the researchers predicted that TESS would have discovered 68 super-Earths around metal-poor stars.
But there were none.
“When we’re looking in the same metallicity regime as Kepler, our [planet occurrence] data points match up pretty nicely,” Boley said. But in the metal-poor regime, “we searched all of these stars, and we found zero planets,” Boley said.
As they looked to more and more metal-poor stars, the super-Earth occurrence rate dropped off sharply. “We found this cliff,” Boley said. Using these data, the team calculated that stars need at least 32% of the Sun’s metal content to form a super-Earth.
When Did Planets Happen?
“Qualitatively, [the cliff] fits very nicely within the current paradigm of planet formation,” said Erik Petigura, an exoplanet astronomer at the University of California, Los Angeles, who was not involved with the research.
“We think super-Earths [form] from the bottom up: Dust grows to pebbles, which grow to asteroid size objects, which grow to moon sized objects, and finally to super-Earths,” he explained.
A key question in planet formation theories is how efficient disks are at converting metal-rich material—dust—into planets. Do planets use up all the available dust in a disk, or is some of it lost in the process?
“The cliff encodes this efficiency,” Petigura said. A third of the Sun’s metal content is more than enough, by mass, to form a super-Earth, so it’s telling that without that much, no super-Earths form.
Furthermore, the new analysis shows that planet formation is limited by the availability of dust only up to a point. “More metals do not mean more planets beyond a certain value,” he said, and that value is roughly the Sun’s metallicity.
Further exploration of the metallicity-planet connection, for example, for Earth-sized planets, will require even larger surveys of stars. Smaller planets are harder to find, and metal-poor stars are rare.
“Roughly when the Milky Way was 7 billion years old…that’s probably when super-Earth formation began.”
“Such stars exist,” Petigura said. “In fact, the oldest stars in the Milky Way galaxy have metal contents that are a factor of 1,000 times lower than the Sun, but…they are very rare and faint, and not amenable to planet searches.”
Stars continuously fuse hydrogen and helium into heaver elements and then release that material into the galaxy when they die, so the overall metal content in the galaxy increases with time. Today’s stars have accrued metals from previous generations of stars. Looking backward through galactic history, there was likely a time when there simply wasn’t enough metal content to form super-Earths at all, Boley explained.
“Roughly when the Milky Way was 7 billion years old…that’s probably when super-Earth formation began, at least within this region of the Milky Way,” Boley said. Future exoplanet surveys, with the Roman Space Telescope, for example, could further constrain the advent of planet formation in the galaxy, not just for super-Earths but for Earth-like planets, too.
—Kimberly M. S. Cartier (@AstroKimCartier), Staff Writer
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