On the outskirts of the Rho Ophiuchi cloud complex, which is about 400 light-years from Earth, a Flying Saucer glows.

Okay, the Flying Saucer isn’t a spaceship. It’s a young star officially known as 2MASS J16281370-2431391; its colloquially name is inspired by its appearance in visible pictures. But just because it isn’t an alien spacecraft doesn’t mean something interesting isn’t happening. In fact, an international team of scientists with the European Southern Observatory (ESO) recently glimpsed something they thought was physically impossible. 

Viewing the star’s protoplanetary disk—a collection of gas and dust associated with early planet formation—with the Atacama Large Millimeter/submillimeter Array (ALMA), the researchers observed and imaged the glow emanating from the disk’s carbon monoxide molecules. Surprisingly, the team observed a negative signal.  

“This disk is not observed against a black and empty night sky,” said Stephane Guilloteau, who is the lead author of a Letter to the Editor appearing in Astronomy & Astrophysics. “Instead it’s seen in silhouette in front of the glow of the Rho Ophiuchi Nebula. This diffuse glow is too extended to be detected by ALMA, but the disk absorbs it. The resulting negative signal means that parts of the disk are colder than the background. The Earth is quite literally in the shadow of the Flying Saucer!”

The ALMA data was combined with other observations of the background glow from Spain’s IRAM 30-m telescope. With the two datasets, the researchers discovered the disk dust grain temperature was -266 C, colder than previous temperature models predicted.

“Although dust is the main agent to control the protoplanetary disk temperature, our knowledge of dust temperatures essentially relies on modeling of disk images and (Spectral Energy Distribution),” the researchers write in Astronomy & Astrophysics. “Despite (or even because of) their sophistication, these models suffer from many uncertainties because of the large number of assumed properties: radial distribution, dust grain growth, dust settling, composition and porosity, disk flaring geometry, etc.”

According to the ESO, most current models predict temperatures between -258 and -253 C.

While the researchers haven’t pinpointed the exact reason behind the low temperature, they have a few ideas. One idea postulated is the temperature may depend on the disk’s grain sizes. The larger grains being cooler, and the smaller ones warmer.

“It is too early to be sure,” said coauthor Emmanuel di Folco.

Further observations are needed to understand the role temperature plays in planet formation.