Intense radiation destroyed protoplanetary disks, resulting in little planet formation in the thick disk.
The Milky Way keeps its planets close to its heart. Stars in a thin, flat disk that extends out across the galaxy contain, on average, more planets than stars in a thick disk that circles the galaxy—and astronomers now think they know why.
Tim Hallett, an astrophysicist at MIT, notes that stars currently residing in the galaxy's thick disk were born during a period of galactic chaos. Their violent growth compromised their ability to grow and sustain planets, he and astrophysicist Abe Lee, formerly of McGill University in Montreal, report Jan. 22 in the journal Astrophysical Journal.
Different stellar populations
The Milky Way's stars reside primarily in two regions. Young, regenerating stars are concentrated in a thin disk, moving in orbits as if they were all sitting on a spinning flat record. In contrast, older stars—around 10 billion years old or older—appear to have moved to the outskirts, residing in a thick disk, with their orbits taking them above and below the central plane.
Astronomers believe that most stars in the thin disk host at least one planet. Observations suggest that nearly half of them harbor a planet between the size of Earth and Neptune. “By all accounts, the result of the planet-forming process in the Milky Way is the creation of supergiants and sub-Neptunian stars,” says Hallett. But as a population, stars in the thick disk appear to have about half as many of these small planets as those in the thin disk. “The mystery is, these planets are very common, and yet when you look at the other dominant population in the Milky Way, they are less common. So what’s going on?” Hallett asks.
Hellet believes it has nothing to do with the current location of the stars, but rather with the time of their birth. Stars in the thick disk were born during a time when the Milky Way was at its peak of star formation—a period astronomers call “cosmic noon.” “It was the most intense period of star formation ever,” Hellet notes.
Effect of radiation on protoplanetary disksם
All those newborn stars sent powerful winds of radiation into their cosmic surroundings. This radiation could have destroyed protoplanetary disks that had formed around the stars, thus impairing their ability to form planets. Hellett and Abe Lee, now at the University of California, San Diego, calculated how much radiation an average star was exposed to during cosmic noon from its neighbors. They found that the background radiation level was between a million and ten million times higher than that of stars in modern star-forming regions.
That much radiation could erode a planetary disk in a few hundred thousand years. "These stars, born during the cosmic noon, had a limited opportunity to form planets because their disks were destroyed," explains Hellet. Astronomers believe that such disks around modern stars persist for millions of years before the planet formation process is complete.
"If our theory is correct, and if these disks did not last long during the cosmic noon, we would expect it to be even more difficult to form giant planets," concluded Hallett.
One response
This sentence, for example: "The formation of supergiants and sub-Neptunian stars" concerns planets, not stars, so I think it is better to use the term "planets" so as not to confuse readers.