PLANETS MAY FORM AROUND DYING STARS
When a Sun-like star exhausts the fuel in its core, it enters its death throes and throws off its outer layers into space. This violent phase of stellar life is called the asymptotic giant branch (AGB). Despite that volatility, astronomers have suspected that under some circumstances, a new disk of material could form around the dying star, giving rise to a second generation of planets. An analysis published Feb. 1 in
Astronomy and Astrophysics gives some of the first observational hints that this is happening.
The study focuses on 85 post-AGB stars in the Milky Way, all of which have a binary companion — and are also surrounded by vast disks of material. It appears that the second star’s gravity draws back in some of the material that the post-AGB star ejected, forming a warm, glowing, rotating disk of gas and dust around the stars.
Tantalizingly, about 10 percent of these secondgeneration disks don’t glow as brightly as expected in the infrared. Researchers call these transition disks, and they’re a sign that a planet is orbiting deep within, clearing the innermost and hottest material from the disk.
The team found lower levels of dusty elements like iron on the surface of the stars themselves. This also points to an orbiting planet: Simulations show that a planet’s gravity causes dust to build up just outside its orbit, preventing the dust from spiraling down onto the star.
These two factors make it “very likely” that the disks have a planet within, says the study’s first author, astronomer Jacques Kluska of KU Leuven in Belgium. It’s not clear whether these are first-generation or second-generation planets. Post-AGB disks last just tens of thousands of years — barely enough time to form a planet, according to current theory. This makes the team think they may be seeing surviving first-generation planets.
But the presence of a first-generation planet also creates better conditions for second-generation planets to form later: By causing dust to build up in one region of the disk, it can expedite the snowballing process of accumulating matter by which a planet takes shape.