Scientists find unexpectedly cold grains in 'Flying Saucer'

An international team has found unexpectedly cold dust grains in a planet-forming disc nicknamed the "Flying Saucer", about 400 light-years from Earth.

The team led by Stephane Guilloteau at the Laboratoire d'Astrophysique de Bordeaux, France, measured the temperature of large dust grains around the young star named "2MASS J16281370-2431391" in the spectacular Rho Ophiuchi star formation region.

This star is surrounded by a disc of gas and dust. Such discs are called protoplanetary discs as they are the early stages in the creation of planetary systems.

This particular disc is seen nearly edge-on, and its appearance in visible light pictures has led to its being nicknamed the "Flying Saucer".

The astronomers used the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the glow coming from carbon monoxide molecules in the disc.

They were able to create very sharp images and found something strange -- in some cases they saw a negative signal! Normally a negative signal is physically impossible, but in this case there is an explanation, which leads to a surprising conclusion.

"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 lead author Stephane Guilloteau in a statement.

This is the first direct measurement of the temperature of large grains (with sizes of about one millimetre) in such objects.

If these low dust temperatures are found to be a normal feature of protoplanetary discs, this may have many consequences for understanding how they form and evolve.

For example, different dust properties will affect what happens when these particles collide, and thus their role in providing the seeds for planet formation.

Whether the required change in dust properties is significant or not in this respect cannot yet be assessed.

"Further observations are needed but it seems that the cooler dust found by ALMA may have significant consequences for the understanding of protoplanetary discs," the authors noted in a paper published in the journal Astronomy & Astrophysics Letters.