Cosmic butterfly or interstellar burger? This planet-forming disk is the largest ever seen

The largest planetary construction site ever seen, spanning hundreds of billions of miles in size, may very well be cast in an enormous shadow that accentuates its bizarre appearance. In short, it looks like a cosmic butterfly — and, for years, it was ignored.

The object, known as IRAS 23077+6707, was originally cataloged as a source of infrared emission by the Infrared Astronomy Satellite (IRAS) in the 1980s. Then in 2016, while conducting a survey of active galaxies in the region of the constellation of Cepheus, astronomer Ciprian Berghea of the U.S. Naval Observatory serendipitously rediscovered it with the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS).

Berghea didn't know for sure what it was, but it appeared to have two parallel lobes with a dark lane between them — typical of an edge-on planet-forming disk. Bright parts in such an object represent dust-scattered light in the upper echelons of the disk, while the dark lane is the equivalent of our solar system's ecliptic plane, where most of the material is concentrated. It is this dense section of material that blocks and absorbs the light of a system's central star. Upper and lower planes of the disk disperse gradually rather than exhibit a sharp edge, while two filaments trace those flared portions, which are also flared. Because of all this, the arrangement looks uncannily like a butterfly — but, in a way, those bright regions split by a dark lane also gives the impression of a hamburger. So, as per his Romanian heritage growing up near Transylvania, Berghea nicknamed IRAS 23077+6707 Dracula’s Chivito," a chivito being a hamburger-like sandwich from his native country.

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Now, thanks to observations with the Submillimeter Array (SMA) in Hawaii, astronomers including Berghea have confirmed that this particular chivito is indeed a planet-forming disk seen from the edge, but it's no ordinary disk. It's the most immense planet-forming disk ever seen.

"What we found was incredible — evidence that this was the largest planet-forming disk ever discovered. It is extremely rich in dust and gas, which we know are the building blocks of planets," said Kristina Monsch, an astronomer with the Harvard–Smithsonian Center for Astrophysics, in a statement. Monch is the lead author of one of two new papers describing the disk. 

To give a sense of the scale of this particular world-building yard, astronomers assume that it sits somewhere between 800 to 1,000 light-years away based on the fact that it is positioned in the sky close to the Cepheus star-forming region. If correct, then the angular size of the disk in our sky corresponds to the disk's radius being thousands of astronomical units (AU). To provide further context, one AU is defined as the average distance between Earth and the sun, or 149.6 million kilometers (93 million miles), while the outermost known planet, Neptune, is 30 AU from our sun.  

"From the SMA data we can also weigh the dust and gas in this planetary nursery, which we found has enough material to form many giant planets — and out to distances over 300 times farther out than the distance between the sun and Jupiter!" said Monsch.

What's more, the disk is rotating. The SMA measured radio waves emitted from carbon monoxide gas within the disk, and some of these radio waves were redshifted, indicating that they are emitted by clouds of gas moving away from us; meanwhile, the submillimeter radio waves from carbon monoxide in other parts of the disk were blueshifted, meaning that they are moving toward us. This behavior is the hallmark of a rotating system.

"The data from the SMA offer us the smoking-gun evidence that … it is rotating around a star likely two to four times more massive than our own sun,” said Monsch. It's possible that this star is still growing as material from the disk falls inwards and accretes onto it.

Besides the disk's gargantuan size, another peculiar aspect is that the western lobe of the disk is noticeably dimmer than the other lobe by a factor of six. Monsch, Berghea and their colleagues are not sure why that is, but there are some possibilities being considered. One strong candidate is that it is just an illusion that the two halves are uneven in brightness, a geometric effect caused by the disk not being perfectly edge-on to us such that we can see a little more of the eastern half than the western half.

But there's another explanation too, which is that half of the disk lies in shadow.

This assumes that the disk, rather than being pregnant with planetary potential, has already given birth and that a giant planet is now plowing through the disk. This planet may be sweeping up raw material as it grows, carving a ringed path, or gap, in the disk in the process.

Such a gap would effectively bisect the disk, resulting in instabilities that'd cause the inner disk to become warped like a vinyl record that has been bent too much. This misalignment would block some of the light from the central young star, resulting in the inner disk casting a shadow onto the outer disk. Therefore, the asymmetry in the brightness of the studied disk could be indirect evidence for the presence of a giant planet. It's fitting that IRAS 23077+6707 has some resemblance to the shape of a butterfly; like a caterpillar enters a chrysalis and emerges as a butterfly, the chrysalis of a protoplanetary disk can enable gas and dust — the remains of ancient generations of stars — to reform and blossom into the cosmic butterflies of new planets.

And, beyond all this, the existence of IRAS 23077+6707 raises a tantalizing question. Computer simulations predict that we should see more edge-on planet-forming disks than we actually do — so, are there more supersized disks out there that we haven't recognized yet?

The observations of IRAS 23077+6707 are reported in two papers, one that was published on May 14 in The Astrophysical Journal Letters, and another that has been accepted for publication in a future issue of the same journal.