Stars and planets are inextricably linked. They form together and stars shape the fate of planets. Stars create the dusty protoplanetary disks that give birth to planets of all kinds.
And when a star dies, planets are either blown apart, swallowed, or doomed to spend an eternity in cold and darkness.
There’s a fundamental question at play in this issue: Exactly how does the birth, life, and death of a star affect planets?
If we can understand that, we can understand how Earth formed, and how it will meet its end. By extension, we’ll understand the birth, life, and death of exoplanets.
A new white paper examines this issue and outlines observation strategies needed to acquire a deeper understanding.
It’s titled “Bridging stellar evolution and planet formation: from birth, to survivors of the fittest, to the second generation of planets.” It was submitted to the ESO’s Expanding Horizons initiative, and the lead author is Akke Corporaal from the European Southern Observatory.
“Stars and planets form, live, and evolve in unison,” the authors write. “Throughout the life of a star, dusty circumstellar discs and stellar outflows influence the further evolution of both the star(s) and their orbiting planet(s).”
The mechanics of dust plays a huge role in the lives of planets. This includes how dust moves around in the protoplanetary disk as planets form. It also includes how dust moves around as stars age away from the main sequence.
In these environments, stars swell up and release powerful winds that shape the dust in their surroundings. This includes winds coming from red giant branch (RGB) stars and asymptotic giant branch (AGB) stars. It also includes post-RGB disks and post-AGB disks.
“The physical processes that occur during each of these stages establishes how the Solar System as well as exoplanetary systems were formed, are evolving, and will eventually die,” the authors explain.
Decades ago, astronomers could only wonder about the types of telescopes that are operating today. We know more about distant solar systems—and our own—than earlier astronomers could envision.
Astronomy has taken big strides towards understanding stars, planets, and the dusty environments that hide the details of their relationships. ALMA has given us views of dozens of protoplanetary disks.
The JWST has used its dust-penetrating infrared vision to reveal new structures in these disks, including rings and spiral arms around some stars.
But even with all this progress, and with more facilities coming online to drive further progress, important details are hidden and will remain hidden.
“Despite these expected advancements, current and planned facilities will keep key windows into dust processing, including planet formation and evolution in dusty environments across the Hertzsprung-Russell diagram, inaccessible,” the authors write.
Things like dust grain growth, dust clumping, and interactions between planets and their dusty environments are beyond the spatial resolution and reach of existing and planned facilities.
Dust grain growth is important because it’s the link between gaseous stars and rocky planets. Dusty disks can be dense environments where dust grains collide and stick together. Eventually they reach pebble size, which creates a problem.
They experience radial drift, where gas drag in the disk draws the pebbles back into the star. But the pebbles can clump together in high pressure zones in the disk which helps them overcome gas drag and radial drift. Understanding exactly how this happens is necessary to understand the relationships between stars, dust, and planets.
“Mapping the kinematics and dust formation at currently unresolvable scales close to the host star is crucial for our understanding of dust physics, and thus for stellar and planetary formation and evolution,” the authors write.
Dust also acts as a thermostat in a dusty disk. Dust grains absorb UV and visible light from stars then re-emit it as infrared, changing the temperature of the disk. As dust grains become larger, their characteristics change.
They shade and heat the disk differently, which shifts the location of the frost line. This has a huge effect on which types of planets can form, and where they can form. Tangentially, dust grains are also where water and organic chemicals form, which are eventually taken up in planet formation.
Dust isn’t only important when a solar system is forming. Later in their evolution, when stars reach their RGB/AGB stages, dust plays another role. Red giants produce fierce stellar winds that can produce another dusty disk. It’s possible that more planets can form in these disks.
To penetrate deeper into the relationship between stars, dust, and planets, the authors propose a near-infrared (NIR) to mid-infrared (MIR) interferometer.
It would have an angular resolution of about 0.1 mas (milliarcseconds). Compare that to the JWST’s 0.07 arcseconds. In fact, the proposed interferometer would have a five-factor increase in angular resolution better than both the VLTI and CHARA arrays, our “sharpest eyes on the sky” as the authors put it.
“Imaging the very inner regions (0.01 au-10 au) of planet-forming discs and post-RGB/post-AGB discs at 0.1 mas scales would allow us to test and refine our understanding of macrostructure formation and planet-disc interaction,” the authors explain.
Unfortunately, key structures in the inner disks are currently unresolvable, even for the nearest planet-forming disks only 500 light-years away. The critical aspect of this future observing effort is to resolve dusty structures at sub-au scales that are out of reach currently.
The authors have mapped out what future progress should look like. During the 2030s, instruments like the Extremely Large Telescope and the VLT can be used to find close-in exoplanets, that is, exoplanets not widely separated from their stars. This dusty environment is critical to understanding the relationship between stars, dust, and planets. During the 2040s, the proposed infrared interferometer would probe even more deeply into this environment.
There are a plethora of open questions in need of answers.
“In particular, it is unclear how we can link the RGB, AGB, and post-RGB/AGB phases in terms of dust, disc, and outflow physics, and how planetary systems are shaped and evolved in such dusty environments,” the authors write.
Written by Evan Gough/Universe Today.
