Most of the stars in the Milky Way are single stars. But between one-third and one-half of them are binary stars. Can habitable planets form in these environments?
New research shows that habitable planets could exist around binary stars, but they would form differently than worlds around single stars.
A young binary star system about 1,000 light-years away is at the heart of this research. It’s called NGC 1333-IRAS2A, and it’s a low-mass binary protostar. The binary pair is so young it’s still gathering mass. It’s the focus of several studies into protostars and protostellar disks because it’s young and still forming.
The new study is titled “Binarity of a protostar affects the evolution of the disk and planets,” published in the journal Nature. The lead author is Professor Jes K. Jørgensen from the Niels Bohr Institute at the University of Copenhagen. Professor Jørgensen is co-author of several papers on NGC 1333-IRAS2A.
The study is based on ALMA (Atacama Large Millimetre/submillimetre Array) observations of NGC 1333-IRAS2A. These observations are only snapshots of a process that takes millions of years. But with these observations, and the knowledge gleaned from the study of young protostars in general, the research team created computer simulations of the binary protostar that reach backward and forward in time.
The study shows that planet formation is different around binary stars than solitary stars like our Sun. It’s because of the way the young stars behave as they form.
“The observations allow us to zoom in on the stars and study how dust and gas move towards the disc. The simulations will tell us which physics are at play, how the stars have evolved up till the snapshot we observe, and their future evolution,” explained Postdoc Rajika L. Kuruwita from the Niels Bohr Institute, who is the study’s second author.
Young protostars are surrounded by protoplanetary disks made of gas and dust. Inside the disks, planets form mostly via accretion. After millions of years of chaos and collision, planets coalesce and take up orbits. It’s a highly complex process that scientists are studying intently. Solar systems like ours are simple in one way: there is only one star. The star’s mass and gravity influence the morphology and behaviour of the protoplanetary disk and the planets that form in the disk.
But in a system with two protostars, there’s even more complexity.
In a single star system, the star accretes material more uniformly. There are still variations in accretion, but things progress more predictably with only one massive object. But as this study shows, binary protostars behave much differently as they form. Rather than a steady accretion process, the star formation process is marked by cyclical bursts of brightness as the stars orbit their common center of mass and periodically absorb large amounts of material. Those punctuated episodes of absorption trigger outbursts of energy which distort the disk. And that has implications for any planets forming in the disk of material around the stars.
This image is a screenshot of one of the study’s MHD (magneto-hydrodynamic) simulations of the binary protostar. A gas bridge (yellow) connects the pair, and the white lines indicate an outflowing burst of material resulting from an episode of rapid accretion. These powerful bursts shape and disrupt the protoplanetary disks where planets form. Image Credit: Jørgensen, Kuruwita et al. 2022.
“The falling material will trigger significant heating. The heat will make the star much brighter than usual,” says Kuruwita. “These bursts will tear the gas and dust disc apart. While the disc will build up again, the bursts may still influence the structure of the later planetary
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