‘Flying Saucer’, a planet-forming disk surrounding a young star, contains extremely cold planetary building blocks, so cold that these do not jive with current planetary formation models.

Astronomers have found that the star, named 2MASS J16281370-2431391, is some 400 light-years away from Earth in the Rho Ophiuchi star formation region. Sporting a protoplanetary disk edge-on to us, the star comprises a glowing halo with a dark band at the centre. The dark band is the one that has perplexed astronomers.

“This disc is not observed against a black and empty night sky. 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 disc absorbs it”, said study lead Stephane Guilloteau, of the Laboratoire d’Astrophysique de Bordeaux, France.

The resulting negative signal means that parts of the disc are colder than the background. The Earth is quite literally in the shadow of the Flying Saucer, said Guilloteau.

While observing the star, researchers noticed something strange about the dust in the edge-on protoplanetary disk. The astronomers studied the Flying Saucer with the Atacama Large Millimeter/submillimeter Array in Chile and the IRAM 30-meter telescope in Spain.

In order to highlight their study, the researchers have prepared a video to zoom in on the Flying Saucer. The team discovered that temperatures in the disk were at minus 447 degrees Fahrenheit (minus 266 degrees Celsius), which is quite low and only a few degrees above absolute zero.

Scientists so far used to think that for planet formation at the disk, the optimum temperature should be between minus 432 and minus 423 F (minus 258 to minus 253 C). The surprisingly low temperatures suggest that scientists might need to take another look at how planets form within the disks surrounding newborn stars, researchers said.