The Milky Way keeps its planets close to its chest. Stars in a thin, flat disk bisecting the galaxy have more planets on average than stars in a thicker, enveloping disk — and astronomers now think they know why.
Stars that currently live in the galaxy’s thick disk were born during a time of galactic chaos, says MIT astrophysicist Tim Hallatt. The stars’ violent upbringing hindered their ability to grow and retain planets, he and astrophysicist Eve Lee, formerly of McGill University in Montreal, report January 22 in the Astrophysical Journal.
The Milky Way’s stars live mostly in two neighborhoods. Young, hip stars stick together in a thin disk, orbiting as if they’re all sitting on the same flat spinning record. Older stars, about 10 billion years and up, appear to have moved to the suburbs, residing in a thick disk of stars whose orbits take them above and below that main plane.
Most of the stars in the thin disk host at least one planet, astronomers think. Observations show that nearly half have a planet whose size is between Earth’s and Neptune’s. “As far as we can tell, the dominant outcome of planet formation in the Milky Way is to produce these super-Earths and sub-Neptunes,” Hallatt says.
But as a population, stars in the thick disk seem to have about half as many of these relatively small planets as the thin disk stellar population does.
“The puzzle is, these planets are very common,” Hallatt says. “And yet when we look at this other dominant population of stars in the Milky Way, they’re less common. So what’s going on?”
Hallatt thinks it’s a question of when these stars were born, not where they live now. Thick disk stars were born in an epoch when the Milky Way was furiously producing stars, a time astronomers call cosmic noon. “It was the most intense period of star formation ever.”
All those newborn stars sent powerful winds of radiation into their cosmic neighborhoods. That radiation could have wreaked havoc on any protoplanets trying to form around the stars, Hallatt says.
He and Lee, who is now at the University of California, San Diego, calculated how much radiation an average star at cosmic noon would have experienced from its neighbors. They found that this background radiation was 1 million to 10 million times what stars experience in a modern star-forming region.
That much radiation could erode a planet-forming disk within a few hundred thousand years, the pair calculated.
“These stars, having been born at cosmic noon, had less opportunity to form planets because their disks were destroyed,” Hallatt says. Astronomers think that such disks around modern stars last for millions of years before they finish forming planets.
Hallatt focused this study on super-Earths and mini-Neptunes, but he thinks the conclusion holds for larger planets, too. “If our theory is correct, and these disks really didn’t live very long at cosmic noon, we would expect it to be even harder to form giant planets.”
The idea is clever and makes sense, says astrophysicist Thomas Haworth of Queen Mary University of London, who studies planet-forming disks in the local universe. Most studies of planetary systems’ early lives have focused on either the newborn star or the planet-forming disk in isolation. Connecting those environments to the eventual planets is hard.
“There has been this question all throughout of, do the planets care?” Haworth says. “It’s fantastic to be able to make that link … to draw a conclusion that says, here’s a real discernable impact of the radiation environment on the planets.”
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