Astronomers Have Discovered The First-Ever Star System With A Perpendicular Planet-Forming Disk

The planets in our solar system orbit the sun on a plane that extends from the star’s equator. This means that the sun’s orbital path — also known as the ecliptic — is aligned with that of its planets, making them coplanar.

But astronomers have discovered a nearby star system in which things revolve in a completely different manner. In this particular case, the first one to ever be observed, the system’s protoplanetary disk — from which planets are born — has been flipped and is now oriented toward the stellar poles, reports

This unusual situation was uncovered in a binary star system located 150 light-years away in the direction of the Crater constellation (“The Cup”). Known as HD 98800, this system is actually made up of four stars, grouped into two binaries — HD 98800 A and HD 98800 B, sitting at about 1.8 times the distance between the sun and Neptune.

The recent discovery pertains to the latter binary, whose stars have been dubbed HD 98800 Ba and HD 98800 Bb. To simplify matters, the binary is now being referred to as BaBb, notes Ars Technica.

While astronomers have known for about three decades that BaBb is orbited by a large disk of gas and dust, it turns out that the binary’s protoplanetary disk is quite unique. Instead of circling around the equator of the double-star system, the disk is perpendicular to the orbit of the two stars.

In other words, it orbits at a right angle to the plane of the two stars, looping over and under their poles.

A 2008 illustration of the HD 98800 four-star system, with the BaBb binary shown surrounded by a protoplanetary disk.
A 2008 illustration of the HD 98800 four-star system, with the BaBb binary shown surrounded by a protoplanetary disk.Featured image credit: NASA/JPL-Caltech/T. Pyle (SSC)

One possible explanation for this bizarre phenomenon has to do with the uneven distribution of material within the protoplanetary disk. This is what likely caused it to develop in an almost vertical position with respect to the orbit of the stars.

Another possibility is that the system’s other binary, AaAb, may have pushed the disk into a perpendicular orbit — particularly if it joined the other two stars at a later date, as opposed to all four stars being formed together and at the same time.

First Observed Polar Circumbinary

Because the disk orbits two stellar bodies instead of just one, it is described as “circumbinary.” And, since it stretches out into space from the poles of the double star system, it has been dubbed a “polar circumbinary.”

While scientists have studied many coplanar circumbinary disks before, this is the first time that a circumbinary disk in polar configuration has ever been encountered. Until now, the existence of such constructions was merely theoretical.

The incredible discovery was made by an international team of astronomers led by Dr. Grant Kennedy, a research fellow at the University of Warwick in the U.K. The scientists observed the disk around the BaBb binary with the help of the European Southern Observatory’s (ESO) Atacama Large Millimeter/sub-millimeter Array (ALMA) in Chile and published their findings in the journal Nature Astronomy.

“We used to think other solar systems would form just like ours, with the planets all orbiting in the same direction around a single sun. But with the new images we see a swirling disk of gas and dust orbiting around two stars,” said study co-author Dr. Daniel Price, a researcher at Monash University in Australia.

Possible Planets In The Future

Perhaps the most exciting news about this discovery is that the unusual binary may be on the verge of forming planets.

As the scientists explain in their paper, nearly all young stars exhibit a protoplanetary disk, which holds the possibility that the system could eventually be populated with planets — churned out from the gas and dust material contained within the disk.

“We know that at least a third of the [disks] orbiting single stars form planets. Some of these planets end up being misaligned with the spin of the star, so we’ve been wondering whether a similar thing might be possible for circumbinary planets,” said Kennedy, who is a researcher at the University of Warwick’s Department of Physics and Centre for Exoplanets and Habitability.

After scouring the HD 98800 system with ALMA, the team observed that the material in BaBb’s disk appears to have condensed into dust particles — an early and crucial step to planet formation.

“We take this to mean planet formation can at least get started in these polar circumbinary disks,” explained Kennedy.

“If the rest of the planet formation process can happen, there might be a whole population of misaligned circumbinary planets that we have yet to discover.”