The Milky Way galaxy, our cosmic home, exhibits a fascinating dichotomy in its stellar population, a division that has profound implications for the prevalence of planets. The galaxy’s stars predominantly reside in two distinct regions: a thin, flat disk, reminiscent of a spinning record, and a thicker, more diffuse disk enveloping the former. These two regions are not merely spatial designations; they represent distinct epochs in the galaxy’s history and harbor stellar populations with markedly different characteristics, particularly regarding the planets they host.
The thin disk is primarily populated by younger stars, a vibrant community where planet formation appears to be a common occurrence. Observations suggest that almost half of the stars in this region are accompanied by planets ranging in size between Earth and Neptune, often referred to as super-Earths or mini-Neptunes. This observation points towards a potential galactic norm: the dominant outcome of planet formation in the Milky Way seemingly favors the creation of these intermediate-sized worlds. This prolific planet formation within the thin disk stands in stark contrast to the thick disk.
The thick disk, in contrast, is home to a more ancient population of stars, their ages estimated to be around 10 billion years or older. These stellar veterans, unlike their younger counterparts in the thin disk, appear to be significantly less endowed with planets, particularly the super-Earth and mini-Neptune types. Studies indicate that thick disk stars possess only about half the number of these smaller planets compared to the thin disk stars. This disparity raises a crucial question: why are these prevalent planets, so abundant in the thin disk, comparatively scarce in the thick disk? The answer, according to a recent study, lies not in their current location, but in the turbulent environment in which these older stars were born.
The thick disk stars originated during a period of intense star formation in the Milky Way’s history, a period astronomers refer to as “cosmic noon.” This epoch, occurring billions of years ago, witnessed an unprecedented burst of stellar birth, a cosmic baby boom unlike any other. The sheer number of stars forming in close proximity created a chaotic and intensely radiative environment. Newborn stars emitted powerful winds of radiation, flooding their surroundings with energy that had devastating consequences for nascent planetary systems.
This intense radiation, according to the study, played a crucial role in hindering planet formation around thick disk stars. The researchers calculated the radiation levels experienced by an average star during cosmic noon, finding them to be a staggering 1 million to 10 million times higher than what stars experience in current star-forming regions. This extreme radiation bombardment effectively eroded the protoplanetary disks, the swirling clouds of gas and dust where planets coalesce, around young stars. The researchers estimate that these disks were eradicated within a few hundred thousand years, a timescale too short for planets, especially smaller ones, to fully form. Modern protoplanetary disks, in contrast, are thought to persist for millions of years, providing ample time for planet formation to reach completion.
The study primarily focused on super-Earths and mini-Neptunes, but the researchers believe the findings extend to larger planets as well. The harsh radiative environment of cosmic noon would have made it even more challenging for gas giants to accumulate sufficient mass before their protoplanetary nurseries were dissipated. This theory provides a compelling explanation for the observed planet deficit around thick disk stars. Their turbulent upbringing, in a galaxy teeming with newborn stars and bathed in intense radiation, severely curtailed their planet-forming potential.
This research offers a significant advance in our understanding of planet formation within the Milky Way. It emphasizes the crucial role of the galactic environment in shaping the destiny of planetary systems. While previous studies have largely focused on individual stars and their protoplanetary disks in isolation, this study highlights the profound influence of the wider galactic context. The intense radiation field during cosmic noon, a consequence of the heightened star formation rate, dramatically altered the lifespan of protoplanetary disks, leaving a lasting impact on the planet populations around thick disk stars. This insight provides a crucial link between the early galactic environment and the ultimate planetary outcome, demonstrating how the galaxy’s history can be etched into the planetary systems we observe today.
