Binary stars are fairly common throughout the Universe, but astronomers have puzzled over how these stellar pairs can form planetary systems. Some of these questions are receiving answers due to observations made by the ESO’s Atacama Large Millimeter/submillimeter Array (ALMA). Most notably, it was revealed that the planet-forming discs surrounding each star in the HK Tauri binary star system are massively misaligned. The research was performed by Eric Jensen of Swarthmore College in Pennsylvania and Rachel Akeson of NASA’s Exoplanet Science Institute at CalTech, and the paper will be published in Nature.
Located 450 light-years away in the constellation Taurus, HK Tauri consists of a pair of 5 million year old stars that are separated by a distance about 13 times greater than Neptune and our Sun. These baby stars are surrounded by clouds of dust and gas that are being swirled around due to gravity, forming flat discs. The swirling dust will begin to stick together, and over time form members of a planetary system.
HK Tauri A is the brighter of the two, and the glare washes out much of the protoplanetary disc surrounding it which required the disc to be imaged in millimeter-wavelength light. HK Tauri B’s disc is oriented so that we see it on its edge. This dust cuts blocks a lot of the brightness from the star, allowing the disc to be imaged at visible and near-infrared wavelengths. Although, the fact that the pair of stars have planet-forming discs that are on different planes by a staggering 60 degrees was a surprising insight into planetary formation.
“This clear misalignment has given us a remarkable look at a young binary star system,” Akeson said in a press release. “Although there have been earlier observations indicating that this type of misaligned system existed, the new ALMA observations of HK Tauri show much more clearly what is really going on in one of these systems.”
As HK Tauri is a binary system, typical planetary formation gets confounded. The vast misalignment of the disc planes could lead to highly unusual orbits because of gravitational pull from the companion star. This could lead to planets that precess, which is when the planet’s axis also has an axis which it spins around (sort of like when a spinning top gets wobbly before it falls). Preliminary analysis indicates that the conditions in this system could very well be enough to interfere with those future orbits.
Astronomers have never before been able to witness a binary system at this early stage in planetary formation, and more research is needed to determine if the misalignment seen in HK Tauri is standard or not.
“Although understanding this mechanism is a big step forward, it can’t explain all of the weird orbits of extrasolar planets — there just aren’t enough binary companions for this to be the whole answer. So that’s an interesting puzzle still to solve, too!” Jensen added.