SCIENCES: Dying stars can spawn new planets, study finds

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.