The patterns and observations show that both stars and planets form a cloud of material that collapses into a disk. If the process proceeds in an orderly manner, then all planets will form from the same disk and thus they will orbit in the same plane. And – because the material on the same disc will fall in the star, bringing its impulses – the stars will rotate with its equator along the same plane. That should lead to a system ordered with the star equator in line with the plane of any planet orbiting it.
Except if not. Anything that affects the uniform entry of the material – from agglomeration to a starring star – may upset this process. We have seen the results: planets formed by planets and planetary orbits that do not line up with a star's equator.
Now, researchers report a complex four-star system in which a planet-forming disk is aligned perpendicular to the stars so that they are orbiting above their poles.
Practice vs. theory
To some extent, this type of system has been predicted. The calculations suggested that binary star systems should have complex interactions with the surrounding discs. In some cases, this will strictly impose order because gravitational force will force a disc to line up with star equations. But if the disk starts far enough from this plane, the gravitational interactions will overturn the plane of the perpendicular disc, so it goes above and below the two stars' poles.
Although we had calculations that suggest this is a probability, I have not actually noticed it in any binary star system. This has prompted an international team of astronomers to look closely at the star system HD 98800.
Even without planets, the HD 98800 is quite complex, consisting of two pairs of binary stars. This resulted in a truly atrocious nomenclature with a binary named "A" and the two stars called "a" and "b", while the second binary is called "B" and grouped with stars "a" and "b". Consequently, the paper is full of discussions about the BaBb binary.
In any case, the two binary systems have stars that orbit the centers of mutual weight at a distance approximately equal to the distance of the Sun Earth. The two binary systems are separated about 54 times that distance, or about 1.8 times the distance between Sun and Neptune. All four stars in the system appear to be young and it has been discovered that a material disc has blinded the BaBb pair in the 1980s. The inner edge of this disc is set by the two stars it orbits, while the outer edge is limited by the second set of stars further.
But the properties of the disc – its composition and its orientation towards the two stars – was not clear on the basis of these initial observations. Thus, the researchers turned to the ALMA telescope matrix. The hardware also allowed them to perfect orbits of the stars in the system and track the material in the surrounding disk.
ALMA for those with data problems
ALMA is sensitive to carbon monoxide emission wavelengths, which is a common material in planets formed by planets. This allowed researchers to create a high-resolution disk map, including orbital speed, which causes different regions of the disc to emit red and blue light, depending on whether they are moving toward or away from us. Then they built different disc models based on ALMA data.
The patterns indicate a number of things. First of all, they suggest that the disc can not only be solid material, as it would be quickly removed from orbit. This implies that there is gas around, allowing friction and gravitational attraction to keep the disk intact. That means we found a planet-forming disk and not a scrap disk that failed to co-exist on planets.
The second hint that comes from the models is that there are two possible orientations of the disk towards the binary stars, and one is perpendicular to the plane of the star orbit. In other words, the disc beats over and under the pillars of the stars. Given that this was already anticipated by orbital calculations, scientists suspect that this is what we are forgetting.
Finally, the authors simulated a disc that is slightly aligned from a true vertical and found that gravitational interactions will return it in a vertical alignment in less than 1000 years. This indicates that the current configuration is a stable one that will naturally form on any disc that started anywhere near this alignment.
How would a disk form in an off-kilter orientation? Conveniently, the additional binary in this four-star system provides a possible explanation. Had it been a relatively late arrival – that is, the two binary systems formed separately and only after gravitationally bound – then its arrival could have discarded the disc. Alternatively, the disk could have been thrown into the center by the same processes that cause it to happen with solo stars: the uneven distribution of the material into the cloud of material that gives rise to the systems.
One of the last observations came out of Alma's observations: the disk material seems to have condensed into dust particles, a primordial step that allows the formation of the planet. So at some point in the future, there may be planets orbiting the system with two suns and two very bright, very close stars.
Nature Astronomy, 2019. DOI: 10.1038 / s41550-018-0667-x (About DOI).