Astronomers have discovered that the history of planet formation in the Milky Way is intricately linked to the galaxy's cosmic past. Most stars in the Milky Way's thin disk, where a majority of its stars reside, are believed to host at least one planet. Meanwhile, stars currently inhabiting the galaxy's thick disk were born during an era known as cosmic noon, characterized by intense galactic activity. This period of galactic chaos played a significant role in shaping the types of planets formed around these stars.
The Milky Way's stars predominantly live in two neighborhoods: the thin disk and the thick disk. The thin disk hosts most of the Milky Way's stars, while older stars, approximately 10 billion years old and above, have settled into the thick disk. During cosmic noon, a time marked by furious star production, stars in the thick disk were born. Astronomers suggest that nearly half of all stars in the Milky Way possess a planet sized between Earth and Neptune, known as super-Earths and sub-Neptunes.
“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
The radiation environment during cosmic noon was significantly more intense than what stars experience today. Background radiation levels were estimated to be 1 million to 10 million times higher than those in modern star-forming regions. This intense radiation could have eroded protoplanetary disks within a few hundred thousand years, severely limiting planet formation. In contrast, similar disks around modern stars endure for millions of years before completing planet formation.
"These stars, having been born at cosmic noon, had less opportunity to form planets because their disks were destroyed.” – Hallatt
The thick disk's stellar population appears to have about half as many small planets compared to those in the thin disk. This discrepancy suggests that the tumultuous environment of cosmic noon may have stunted planet formation in these regions. As a result, fewer small planets came to fruition around stars born during this epoch of high radiation.
“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.” – Thomas Haworth
Today, astronomers recognize that most stars in the thin disk host at least one planet, underscoring the prolific nature of planet formation in quieter cosmic environments. The process has predominantly resulted in super-Earths and sub-Neptunes, highlighting a discernible pattern within our galaxy.
“It was the most intense period of star formation ever.” – Hallatt
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