Most stars in the Milky Way are single stars. But between a third and a 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 from worlds around single stars.
A young binary star system located about 1,000 light-years away is at the heart of this search. It’s called NGC 1333-IRAS2A, and it’s a low-mass binary protostar. The binary pair is so young that it continues to gain mass. It is the subject of many studies of protostars and protostellar disks because it is young and still forming.
The new study is titled “The binarity of a protostar affects the evolution of the disk and the 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 just snapshots of a process that takes millions of years. But with these observations and knowledge gained from studying young protostars in general, the research team created computer simulations of the binary protostar that go back and forth in time.
The study shows that planet formation is different around binary stars compared to solitary stars like our Sun. It’s because of the way young stars behave when they’re training.
“Observations allow us to zoom in on stars and study how dust and gas move towards the disk. The simulations will tell us what physics is at play, how the stars have evolved to the snapshot we observe, and how they will evolve in the future,” explained Niels Bohr Institute Postdoc Rajika L. Kuruwita, who is the second author. of the study.
Young protostars are surrounded by protoplanetary disks made up of gas and dust. Inside disks, planets form mainly by accretion. After millions of years of chaos and collisions, the planets merge and take orbits. It is a very complex process that scientists are studying carefully. Solar systems like ours are simple in a way: there is only one star. The star’s mass and gravity influence the morphology and behavior of the protoplanetary disk and the planets that form within the disk.
But in a system with two protostars, there is even more complexity.
In a single star system, the star accretes matter more uniformly. There are still variations in accretion, but things progress more predictably with a single massive object. But as this study shows, binary protostars behave very differently when they form. Rather than a regular process of accretion, the process of star formation is marked by cyclical bursts of brightness as stars orbit around their common center of mass and periodically absorb large amounts of matter. These punctuated episodes of absorption trigger bursts of energy that warp the disk. And that has implications for all the planets that form in the disk of matter around stars.
“The fall of material will trigger significant heating. The heat will make the star much brighter than usual,” says Kuruwita. “These gusts will tear apart the disk of gas and dust. As the disk recovers, the gusts may still influence the structure of the subsequent planetary system.
Episodes of increased influx of material are cyclical. For tens or hundreds of years, every thousand years or so, the material movement towards the stars becomes very strong. Binary stars brighten tens or hundreds of times their normal luminosity during these episodes before fading.
“The fall of material will trigger significant heating. The heat will make the star much brighter than usual,” says Kuruwita. “These gusts will tear apart the disk of gas and dust. As the disk recovers, the gusts may still influence the structure of the subsequent planetary system.
NGC 1333-IRAS2A is a kind of laboratory to observe the formation of young systems. There are no planets yet, so it is too early to conclude what effect this activity has on planetary formation or whether habitable planets may form there. But other objects could also be part of the habitability equation, and the research team intends to use ALMA to further study the system, especially comets.
Comets in our solar system are known to carry some of the building blocks of life. Scientists have detected the amino acid glycine on comet 67P/Churyumov-Gerasimenko. They also found ammonium salts and aliphatic compounds. These findings lend weight to the long-held idea that comets can spread the materials necessary for life in solar systems.
“Comets are likely to play a key role in creating possibilities for the evolution of life. Comets often have a high ice content with the presence of organic molecules. One can well imagine that organic molecules are preserved in the comets at times when a planet is barren and subsequent comet impacts will introduce the molecules to the surface of the planet,” Prof Jørgensen said.
Recent research shows that building blocks can forms on icy grains in the space. But in a system like NGC 1333-IRAS2A, episodes of pronounced heating could disrupt or alter the chemistry of this process.
“The heating caused by the bursts will trigger the evaporation of the dust grains and the ice that surrounds them. This can alter the chemical composition of the material from which planets are formed,” Jørgensen said.
ALMA can detect some of these chemicals, especially in gaseous form. And he can see complex chemistry. In this study, the authors detected several complex chemicals around protostars.
“The wavelengths covered by ALMA allow us to see fairly complex organic molecules, therefore molecules of 9 to 12 atoms and containing carbon. These molecules can be building blocks for more complex molecules that are essential for life as we know it,” Jørgensen said. “For example, the amino acids that have been found in comets.”
Humanity will have to observe NGC 1333-IRAS2A for millions of years to see what kind of planets are forming. But we won’t have to wait that long to figure out some of the chemistry of the system and what kind of building blocks are there. The James Webb Space Telescope, ALMA, the upcoming Square Kilometer Array (SKA) and the European Extremely Large Telescope (E-ELT) will all work together to detect the elusive chemistry. Not only in this young binary protostellar system, but also in others.
“The SKA will make it possible to directly observe large organic molecules. The James Webb Space Telescope operates in the infrared, which is particularly well suited to observing molecules in ice. Finally, we continue to have ALMA, particularly suited to the observation of molecules in gaseous form. Combining the different sources will provide a host of exciting results,” Jørgensen said.
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