BORDEAUX, France, Feb. 4 (UPI) -- For the first time, researchers have directly measured the temperature of dust grains in a protoplanetary disk. The readings from the ALMA and IRAM telescopes suggest the grains surrounding a young, faraway star are surprisingly frigid -- negative 266 degrees Celsius, just 7 degrees above absolute zero.
Astronomers call the planet-forming disk of gas and dust surrounding 2MASS J16281370-2431391 the Flying Saucer. Because the disk appears edge-on from the vantage of Earth, it looks like a stereotypical alien spaceship.
The saucer spins around 2MASS J16281370-2431391, a young star found 400 light-years from Earth in the action-packed Rho Ophiuchi star formation region.
Astronomers used the Atacama Large Millimeter/submillimeter Array to image the glow of carbon monoxide molecules found in the Flying Saucer. In doing so, they created extremely sharp images of the disk -- but also negative signals, an impossibility.
This puzzling development led to a surprising discovery.
"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," astronomer Stephane Guilloteau, a researcher with the Laboratoire d'Astrophysique de Bordeaux in France, explained in a press release. "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!"
One oddity led to another, as researchers discovered the remarkably frigid temperatures of the disk's dust were responsible for the negative signals. The surprises continued when astronomers conducted direct temperature measurements, with dust particles registering as low as negative 266 degrees Celsius -- several degrees lower than current protoplanetary disk models suggest is possible.
"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," said Emmanuel di Folco, also an astronomer at the Bordeaux laboratories. "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."
Because cooler, larger dust particles can significantly affect the way particles coalesce and begin to form planets, researchers must go back to the drawing board -- or back to the telescopes. Astronomers say they will need to study the temperature and dust particle sizes in more protoplanetary disks to work out this new problem.
The new head-scratching discovery is detailed in the journal Astronomy & Astrophysics.