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Scientists from the European Southern Observatory (ESO) are employing the Atacama Large Millimeter/submillimeter Array (ALMA) and IRAM telescopes. They are attempting to create the first ever direct measurement of big dust grains in the outer regions of the planet-forming disc around 2MASS J16281370-2431391, a star located in the Rho Ophiuchi star formation area. This is situated about 400 light-years away.

The "Flying Saucer" or the disc of this star has helped the team to find that the grains are minus 266 degrees Celsius, or colder than expected. This is a name for the glowing disc of gas and dust around 2MASS J16281370-2431391, a representation of the early stages of planetary systems. Carbon monoxide molecules have led to the brightness of the disc.

The team created sharp images that beamed a negative signal, which they felt is "typically impossible", but was explained by lead author Stephane Guilloteau.

"This disc is not observed against a black and empty night sky," he said in a press release. "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. 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!"

There is a difference in the temperature of the disc dust grains. The temperature in this particular example was calculated at minus 266 degrees Celsius, contradicting the predictions of current models, pointing to a temperature of minus 258 to minus 253 degrees Celsius.

The discrepancy is most probably due to the properties of the large dust grains, different from those assumed by modern models. They can give us an insight into their evolution and change over time.

"To work out the impact of this discovery on disc structure, we have to find what plausible dust properties can result in such low temperatures. We have a few ideas - for example, the temperature may depend on grain size, with the bigger grains cooler than the smaller ones. But it is too early to be sure," said study co-author Emmanuel di Folco.

The findings were published in the Jan. 19 issue of Astronomy & Astrophysics Letters.