Odd system with 4 stars, vertical planet-forming disk found 146 light-years away

Seasons, days & nights on planets in such systems would be extraordinarily different than Earth's.


Typically, the disks of gas and dust that make up planet-forming disks are found in a single, equatorial plane of star systems. Astronomers have now discovered rings like these that exist in dramatically-different angles and rotate around the polar ends of stars, nothing like our own system.

In a new study, researchers explain what an unusual arrangement like this implies for stellar systems and their planets. Potentially, seasons on planets in such systems would be extraordinarily different than Earth's. Day and night, too, would take a whole different meaning.

As stars are formed, they are surrounded by a cloud of gas and dust, which is wielded by the gravitational pull of the growing star into an orbiting spiral around it.

The gas cloud clumps over time, moving in random directions, with particles colliding and merging to form denser structures. Over time, a flattened disk called the protoplanetary disk is formed, which usually spins along the equator and in the same direction as the star at its centre. Naturally, the planets that are formed in these disks also end up orbiting their star in the same direction.

 Odd system with 4 stars, vertical planet-forming disk found 146 light-years away

An artist's impression of a view of the double star system and surrounding disc. Image credit: University of Warwick

In some cases, simulations have shown that any extra material that these disks collect over time can throw their system out of whack to varying degrees. Astronomers believe that this is why some exoplanets have been seen having 'misaligned' or crooked orbital paths around their parent star.

For the first time, researchers at the Atacama Large Millimeter/submillimeter Array (ALMA) observatory in Chile have found evidence of one such misaligned protoplanetary disk in a four-star system called HD 98800, roughly 146 light-years from Earth.

"If planets were born here, there would be four suns in the sky," Daniel Price, study co-author of the study from Monash University, Australia said in a statement.

"It's one of those examples that nature manages to be more creative than we expect," Dr Grant Kennedy, lead author of the study from University of Warwick, told Space.com.

The researchers used the ALMA telescope to capture high-resolution images of the unusual system, which has a protoplanetary disk around two of the system's four stars. This ring is roughly the same size (diameter) as our solar system's asteroid belt.

A planet embedded in the protoplanetary disk of the star system HD 98800 would get this striking view of the disk and pair of stars. Image credit: University Warwick

A planet embedded in the protoplanetary disk of the star system HD 98800 would get this striking view of the disk and pair of stars. Image credit: University Warwick

The two other stars in the system appear to be outside the disk, with the two pairs of stars orbiting each other at right angles.

If a planet ever did develop in a system like HD 98800, anyone seeing the sky from its surface would see a thick band of the disk running straight up from the planet's horizon.

Interestingly, the pair of stars that would be visible in the sky would move in and out of the disk, giving any objects on the ground two shadows at most times.

"Perhaps the most exciting thing about this discovery is that the disc shows some of the same signatures that we attribute to dust growth in discs around single stars," Kennedy said in the statement. "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, and things like weird seasonal variations to consider."

The new study was published in the journal Nature Astronomy on 14 January. 


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