Planets in binary systems can be habitable, but they form differently

Most of the 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 can exist around binary stars, but that they will form differently than worlds around individual stars.

At the heart of this research is a young binary star system about 1,000 light-years away. It is called NGC 1333-IRAS2A, and it is a low-mass binary star. The binary pair is so small that it is still clustering. It is the focus of many studies of protostars and stellar disks because they are young and still forming.

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The new study, titled “Proto-star duality influences disc and planet evolution,” is published in the journal Nature. The lead author is Professor Jess K. Jorgensen of the Niels Bohr Institute at the University of Copenhagen. Professor Jorgensen has co-authored several papers on NGC 1333-IRAS2A.

The study is based on ALMA (Atacama Large Millimeter/submillimeter Array) observations of NGC 1333-IRAS2A. These observations are only snapshots of a process that takes millions of years. But with these observations, and knowledge gleaned from studying young protostars in general, the research team created a computer simulation of the binary protostar arriving back and forth in time.

The study showed that the composition of planets is different around binary stars than around single stars such as our sun. It’s because of the way young stars behave as they form.

“Observations allow us to zoom in on stars and study how dust and gas move toward the disk. Simulations will tell us which physics is at play, how stars have evolved up to the snapshot we observe, and their future evolution,” explained Postdoc Rajika L. Kuruwita of the Niels Bohr Institute, second author of the study.

Young protostars are surrounded by protoplanetary disks made of gas and dust. Within the disks, planets are formed mostly by accretion. After millions of years of chaos and collision, the planets unite and spin in orbits. It is a very complex process that scientists study with interest. Solar systems like ours are simple in one way: There is only one star. A star’s mass and gravity influence the shape and behavior of the protoplanetary disk and the planets that form in the disk.

But in a system with two protostars, there is more complexity.

In a single star system, the star acquires material more uniformly. There are still differences in accumulation, but things progress more predictably with one huge body. But as this study shows, binary protostars behave much differently as they form. Rather than a steady accretion process, star formation is characterized by periodic bursts of brightness as stars orbit their common center of mass and periodically absorb large amounts of material. Intermittent bouts of sucking lead to bursts of energy that distort the disc. This has implications for which planets form in the disk of material around stars.

This image is a screenshot of an MHD simulation of a binary protostar.  The pair is connected to a bridge of gas (yellow), the white lines indicate an intermittent outflow of material.  These powerful eruptions form and disrupt protoplanetary disks.  Image credit: Jørgensen, Kuruwita et al.  2022.
This image is a screenshot of an MHD simulation of a binary protostar. A gas bridge (yellow) connects the pair, and the white lines indicate a flowing stream of material resulting from a ring of rapid buildup. These powerful eruptions form and disrupt the protoplanetary disks that make up the planets. Image credit: Jørgensen, Kuruwita et al. 2022.

“The falling matter will cause significant heating. The heat will make the star much brighter than usual. These explosions will tear the disk of gas and dust apart. As the disk builds up again, bursts may continue to affect the structure of the subsequent planetary system.” .

This figure from the study shows some activity in the binary protostar.  The stars revolve around their common center of gravity, which is indicated by the black dot.  When a star absorbs a large amount of material, it glows in brightness and produces an outflow.  Because of the binary motion of the protostars, the outflows are not dipolar.  Image credit: Jørgensen, Kuruwita et al.  2022.
This figure from the study shows some activity in the binary protostar. The stars revolve around their common center of gravity, as indicated by the black dot. When a star absorbs a large amount of material, it glows in brightness and produces an outflow. Because of the binary motion of the protostars, the outflows are not dipolar. Image credit: Jørgensen, Kuruwita et al. 2022.

Episodes of falling material increase are periodic. For tens or hundreds of years, every thousand years or so, the movement of material in the stars becomes very strong. Binary stars shine tens or hundreds of times their normal brightness during these rings before waning.

“The falling matter will lead to significant heating. The heat will make the star much brighter than usual,” Kuroeta says. “These explosions will tear the disk of gas and dust apart. As the disk builds up again, bursts may continue to affect the structure of the subsequent planetary system.” .

MHD simulations from the study show accretion and outburst flows from young binary protostars. Credit: Jørgensen, Kuruwita et al. 2022.

NGC 1333-IRAS2A is a bit like a laboratory for observing the formation of modern systems. There are no planets yet, so it’s too early to conclude what effect this activity has on planet formation or whether habitable planets can form there. But other objects may also be part of the habitability equation, and the research team intends to use ALMA to study the system further, particularly comets.

Comets in our solar system are known to carry some of the building blocks for life. Scientists have discovered the amino acid glycine on comet 67P / Churyumov-Gerasimenko. They also found salts of ammonia and aliphatic compounds. These discoveries lend weight to the age-old idea that comets can spread the material needed for life throughout solar systems.

Comet 67P as seen by Rosetta on July 7, 2015. By ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0, CC BY-SA 3.0-igo, https://commons.wikimedia.org/w/index.php?curid= 41733207
Comet 67P as seen by Rosetta on July 7, 2015. By ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0, CC BY-SA 3.0-igo, https://commons.wikimedia.org/w/index.php?curid= 41733207

Comets likely play a major role in creating possibilities for life to evolve. Comets often have a high ice content with organic molecules present. Professor Jorgensen said: “It can be imagined that organic molecules are preserved in comets during the eras when the planet is barren and that the effects of subsequent comets will introduce the molecules to the planet’s surface.”

Recent research shows that building blocks can form on icy grains in space. But in a system like NGC 1333-IRAS2A, pronounced heating spells can disrupt or alter the chemistry in that process.

“Heating from the eruptions will cause the surrounding dust and ice grains to evaporate. This may change the chemical composition of the material that the planets are made of,” Jorgensen said.

ALMA can detect some of these chemicals, particularly in their gaseous form. And he can see some complicated chemistry. In this study, the authors discovered several complex chemicals around protostars.

This figure from the study shows some of the molecules that have been discovered around VLA1, one of the stars in the binary pair.  The team discovered them in warm gas in the protostar's atmosphere.  Image credit: Jørgensen, Kuruwita et al.  2022.
This figure from the study shows some of the molecules that have been discovered around VLA1, one of the stars in the binary pair. The team discovered them in warm gas in the protostar’s atmosphere. Image credit: Jørgensen, Kuruwita et al. 2022.

“The wavelengths covered by ALMA allow us to see very complex organic molecules, and thus molecules that are 9-12 atoms and contain carbon. These molecules could be building blocks for more complex molecules that are key to life as we know it,” Jorgensen said. “For example, the amino acids that are found in comets.”

Humanity will have to watch NGC 1333-IRAS2A for millions of years to see what kind of planets it forms. But we won’t have to wait that long to understand some of the chemistry in the system and what kind of building blocks there are. The James Webb Space Telescope, ALMA, the upcoming Square Kilometer Array (SKA) and the European Very Large Telescope (E-ELT) will work together to reveal the elusive chemistry. Not only in this small initial binary system but in others as well.

SKA will allow large organic molecules to be directly monitored. The James Webb Space Telescope operates in the infrared, and is particularly suitable for observing particles in ice. Finally, we still have ALMA, which is particularly suitable for observing molecules in gas form. Combining the different sources will provide a wealth of exciting results, Jorgensen said.

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