Astronomers have gained new insight into the formation of planets around twins that orbit each other.
Despite the fact that we are most familiar with planets orbiting a single central star – like the arrangement of our solar system – more than 50% of the stars in the universe exist in a binary system, meaning they have a companion star. These binary systems can also have planets around them that either orbit one of the stars in a “circumstellar orbit”, or both stars loop in a much wider “circumbinary orbit”.
Using the Atacama Large Millimeter/submillimeter Array (ALMA) — made up of a combination of 66 radio telescopes located in northern Chile — and the 10-meter Keck II telescope in Hawaii, astronomers collected data on the two twin star systems. What they found could change our understanding of the conditions that can either foster or hinder such planet formation in binary systems.
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The formation of binary stars is not very different from the formation of a single star. These bodies are formed when dense clouds of cold interstellar gas form overdense patches that collect more mass and eventually collapse under their own gravity, giving birth to a star child called a “protostar.”
This protostar continues to gather material from its prenatal cocoon of gas and dust until it has enough mass to initiate nuclear fusion of hydrogen into helium in its core, the process that defines a main-sequence star. Importantly, some of these interstellar clouds are large enough to host two or even three main sequence stars.
Any material left over from this cloud of gas and dust after these stars formed then surrounds them as what astronomers call a “protoplanetary disk.” As the name suggests, planets form from these disks. Like the planets themselves, disks can be circumstellar, surrounding only one star, or circumbinary, surrounding the entire system.
Scientists are currently not aware of the factors that allow these discs to stick around long enough to give birth to planets, nor are they sure what ultimately causes the discs to disperse. As it turns out, circumstellar disks in pre-main sequence protostellar binaries could be an ideal laboratory for investigating these questions.
Properties of these early disks, such as their sizes, substructures, and even their inclinations (compared to protostar characteristics such as rotation rate and magnetic field strength) can reveal details of the complex interactions that shape such planet-birth environments.
Moreover, the sheer ubiquity of multi-star systems in the universe means that the study of planet formation around binaries is vital to understanding this process at a deeper level.
One of the binary systems the team honed in on with ALMA and Keck II was DF Tau, composed of two protostars with a mass of about 0.6 times the mass of the Sun located about 150 light-years from Earth in the Taurus star-forming region.
The two DF Tau stars are separated by a distance equivalent to about 14 times the distance between Earth and the Sun; they take about 44 Earth years to complete their highly elongated orbits.
Fascinating, ALMA found that the interstellar cloud responsible for the birth of these stars has split into two circumstellar disks. One is magnetically locked to the central star, DF Tau A, and is actively supplying it with material to facilitate its growth. The second star appears to have separated from the second star, DF Tau B. The central region of the disk appears to be eroded as the young star spins rapidly.
This suggested to the team that there might be a connection between the rotation of young stars and also the magnetic locking of the disks to them, and hence the early dissipation of the disks. In addition, it appears that misalignments between the orbit of DF Tau, its circumstellar disks, and the inclinations of its stars may affect the general evolution of the disk.
The second binary system the team focused on was the very young, 2.8-million-year-old FO Tau system (for context, recall that the Solar System is 4.6 billion years old).
This system is also located roughly 450 light years away. Its stars, FO Tau A and B, are in a more circular orbit than the DF Tau stars. They are also more separated, with FO Tau B orbiting FO Tau A at a distance equivalent to about 22 times the distance between Earth and the Sun.
Using ALMA, astronomers found that FO Tau’s disks are aligned with the binary star’s orbit. Both stars show rotation rates on the slower side, and the two circumstellar disks remain magnetically locked to their protostars. This suggests that systems like FO Tau with slower stars and more circular orbits may be better suited to forming planetary bodies around both of its stellar components than fast systems with elongated orbits.
ALMA observations of other simple and binary stellar disks have revealed complex substructures within the disks, including features such as spiral patterns, gaps, and ring formations.
Although these structures are not visible for DF Tau and FO Tau at this time, the determination of larger-scale properties in these two close binary systems has significantly advanced our understanding of planet-forming environments.
The team’s results were revealed at the 244th meeting of the American Astronomical Society (AAS).