When young stars merge from a cloud of molecular hydrogen, a disk of residual material called a protoplanetary disk surrounds them. This disc is where planets form, and astronomers are getting better at peering into these veiled environments and watching embryonic worlds take shape. But young stars aren’t the only stars with disks of raw material spinning around them.
Some old dying stars also have discs. Can a second generation of planets form under these conditions?
Planets form after stars form, but soon after. In our solar system, the Sun formed about 4.6 billion years ago and the Earth about 4.5 billion years ago. The fate of the Earth is linked to the fate of the Sun.
When the Sun expands into a red giant, it blasts layers of matter out into space and eventually expands enough to destroy Earth and the other inner planets. Jupiter and the outer planets will survive, but they will likely spend the rest of their lives orbiting a white dwarf, the remnant of the Sun.
No new planet can form around the white dwarf in this scenario.
But our Sun is a relative rarity. Many stars exist in binary pairs. Binary stars are the same age, but they have different masses. Since a star’s initial mass determines its future, stars in a binary pair have different lifetimes.
If one of these stars has a mass similar to that of our Sun, it becomes a red giant and expels material into space as it dies. What happens to all that material if the star has a binary partner?
This is where a new study comes into play. Its title is “A Population of Transition Discs Around Evolved Stars: Fingerprints of Planets”. The first author is KU Leuven astronomer Jacques Kluska. The journal Astronomy & Astrophysics published the article.
The gravitational pull of the second star can cause material ejected from the dying star to form a new rotating disk very similar to the protoplanetary disk around the star when it was young.
Astronomers already knew this could happen. What’s new is evidence that a second generation of planets can form in the disk. According to this new study, new worlds form about 10% of binary stars in this situation.
“In ten percent of the evolved binary stars with disks that we have studied, we see a large cavity in the disk,” first author Kluska said in a press release. “It’s an indication that something is floating there that has collected all the material in the cavity area.”
There is probably only one thing that can form in these disks: planets. Observations of the dying star strengthen the likelihood that the object is a planet.
“In evolved binary stars with a large cavity in the disk, we saw that heavy elements such as iron were very sparse on the surface of the dying star,” Kluska said. “This observation leads to the suspicion that dust particles rich in these elements have been trapped by a planet.”
Astronomers don’t yet know if these are planets, but the evidence is intriguing. If it turns out that a second generation of worlds is formed in this way, it is a significant discovery. This means that our theory of planetary formation, called the nebular hypothesis, is correct but does not go far enough.
“Confirmation or refutation of this extraordinary way of planet formation will be an unprecedented test for current theories.