University of Michigan: Planets of binary stars: Targets in the search for alien life

Planetary systems around stars the size of our sun are obvious targets for astronomers trying to locate extraterrestrial life forms, and nearly every second star in that category is a binary star.

New research from Denmark and the University of Michigan indicates that planetary systems are formed in a very different way around binary stars—two stars gravitationally bound to each other—than around single stars such as the sun.

The researchers began observing the binary star system NGC 1333-IRAS2A in an attempt to understand the fundamental aspects of the births of stars and planetary systems, says study co-author Edwin Bergin, professor and chair of the U-M Department of Astronomy.

“We were trying to peer as close as we could to the most luminous star in the binary—the primary star—and observe the beginnings of planetary system formation, and we got it, but in a different way,” Bergin said. “The result that we have has something to do with how planetary systems form, and it’s that the binary star system can influence planetary formation tremendously.”

Jes Kristian Jørgensen, professor of astrophysics and planetary science at the Niels Bohr Institute at the University of Copenhagen and principal investigator of the study, says the result can also help aid in the search for extraterrestrial life.

“The result is exciting since the search for extraterrestrial life will be equipped with several new, extremely powerful instruments within the coming years,” he said. “This enhances the significance of understanding how planets are formed around different types of stars. Such results may pinpoint places which would be especially interesting to probe for the existence of life.”

The results from the project, which also has participation of astronomers from Taiwan, are published in Nature.

The team used observation time on the ALMA telescopes in Chile to observe the binary star system NGC 1333-IRAS2A in the Perseus molecular cloud. The distance from Earth to the binary star is about 1,000 light-years, which is quite a short distance in an astronomical context. Formed some 10,000 years ago, it is a very young star. The two stars of the binary system are 200 astronomical units apart. An AU equals the distance from Earth to the sun. In comparison, the furthest planet of the solar system, Neptune, is 30 AU from the sun.

The binary star is surrounded by a disc consisting of gas and dust. Some of this material is moving away from us, while other material is approaching. The observations can only provide researchers with a snapshot from a point in the evolution of NGC 1333-IRAS2A. However, the team has complemented the observations with computer simulations reaching both backward and forward in time.

“The observations allow us to zoom in on the stars and study how dust and gas move toward the disk. The simulations will tell us which physics is at play and how the stars have evolved up till the snapshot we observe and their future evolution,” said Rajika Kuruwita, a postdoctoral researcher at the Niels Bohr Institute.

Notably, the movement of gas and dust does not follow a continuous pattern. At some points in time—typically for relatively short periods of 10 to 100 years every thousand years—the movement becomes very strong. The binary star becomes ten to one hundred times brighter, until it returns to its regular state.

Presumably, the cyclic pattern can be explained by the duality of the binary star. The two stars encircle each other, and at given intervals their joint gravity will affect the surrounding gas and dust disc in a way that causes huge amounts of material to fall toward the star.

“The falling material will trigger significant heating. The heat will make the star much brighter than usual,” Kuruwita said. “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 system.”

The stellar system is still too young for planets to have formed. The team hopes to obtain more observational time, allowing them to investigate the formation of planetary systems. The researchers only had five hours of ALMA observation time, and hope to get 100 hours for a new project, Jørgensen said. The astronomers would also like to observe comets, which often have a high content of ice with organic molecules present.

ALMA is not a single instrument but 66 telescopes operating in coordination. This allows for a much better resolution than could have been obtained by a single telescope. ALMA, which allows scientists to see complex organic molecules, will also allow scientists to study how comet impacts could introduce organic molecules to barren planets.

“The binary is clearly influencing the evolution of the disk of material around the main system. And that suggests, since most stars form as binaries, that the binary will influence the ultimate composition of planets that are born,” Bergin said. “Is it any different if you were born as a single star? We don’t know that. We have observed that most stars are born in binaries but many of these binaries drift apart over time.

“So how important is this for the formation of planetary systems and the formation of habitable worlds? Another way of saying this in the ‘Star Wars’ parlance is how many Tatooines are out there? We don’t know, but what we now know is that, hey, this matters.”

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