Bizarre 4-Star System Forms Planets in a Vertical Disk Orbit

One of the common features of star systems is that both stars and their planets tend to orbit in the same plane. That means the equator of the star tends to align with the plane of the planets in orbit around it. In our own solar system, for example, all of the major planets follow this trend (although the dwarf planet Pluto has an eccentric orbit caused by gravitational interactions with Neptune).

Now, scientists have discovered an unusual pair of binary stars (four stars total) that appear to have created planets that orbit in a vertical plane over the poles of the stars rather than the horizontal dimension one would typically expect. The system in question, HD 98800, consists of two binary pairs (HD 98800 A and HD 98800 B). The stars in each pair are also labeled with “a” and “b”, which (as Ars Technica details) means the paper is filled with a discussion of the sigh BaBb binary. Given the relatively young age of the stars in question (7-10 million years), we can at least say the abbreviation is amusing, if difficult to follow.

It’s been known for years that various events could disrupt the formation of a standard protoplanetary disk around a star, but this is the first time we’ve observed a misaligned vertical orientation as predicted by theoretical models. We’ve been aware of HD 98800 and its protoplanetary disk for years, but this is the first time researchers have attempted to analyze the orientation and composition of the ring using the ALMA (Atacama Large Millimeter Array) telescope system.

Finding a protoplanetary disk in this orientation confirms theoretical models, but it doesn’t tell us how common these types of planets actually are. Binary stars have proven to be more common in the universe than some models initially predicted; astronomers currently estimate that up to 85 percent of stars may be binary stars. If planet formation around binary stars can happen in both these disrupted perpendicular disks and the conventional protoplanetary disk configuration, this has implications for our search for exoplanets. Polar planetary disks might even be common in this kind of stellar pairing, which would inform our searches for planets by giving us a better idea which orbits to check in the first place.

It isn’t currently clear if the HD 98800 system has planets, though the work from these researchers does indicate the presence of an actual planet-forming disk rather than the hypothetical ‘debris disk’ of failed remnants that hadn’t coalesced into a planet and would never do so. The vertical orientation of the disk is also believed to be a stable feature, at least over several hundred years — attempts to simulate a disk inclined off the vertical axis resulted in a vertical realignment within a short period of time (relative to geologic timescales).

Feature image by the University of Warwick / Mark Garlick

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