Astronomers have found a protoplanetary disk surrounding a young star with a rather chilly secret: the planetary building blocks it contains are cold, so cold in fact that it doesn’t jibe with current planetary formation models.

The star, called 2MASS J16281370-2431391, is located some 400 light-years from Earth in the Rho Ophiuchi star formation region. It is a stellar baby, sporting a stunning protoplanetary disk edge-on to us. The result is a glowing halo with a dark band in the middle and it’s this dark band that has surprised astronomers.

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Its appearance looks like something out of a retro sci-fi comic book, earning 2MASS J16281370-2431391′s protoplanetary disk the far more palatable moniker “Flying Saucer.” When studying the Flying Saucer with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile and the IRAM 30-meter telescope in Spain, researchers detected something weird about the dust in the edge-on protoplanetary disk.

“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 Stephane Guilloteau, of the Laboratoire d’Astrophysique de Bordeaux, France, and study lead. “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!”

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Guilloteau’s team carried out the first ever direct measurements of the relatively large grains of dust in the Flying Saucer (measuring approximately one millimeter across), located around 15 billion kilometers (9 billion miles) from the star and found they had settled to a low temperature of -266 degrees Celsius — that’s only 7 degrees above absolute zero. These grains will eventually go on to form planets as the system matures, but current theoretical models predict this dust should be at least 10 to 15 degrees (−258 to −253 degrees Celsius) above absolute zero. Although still cold, in the field of planetary formation models, this discrepancy is huge.

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“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 co-author Emmanuel di Folco, also of Laboratoire d’Astrophysique de Bordeaux. “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.”

This may sound like a minor complication in the field of planetary science, but the temperature of the dust in protoplanetary disks can greatly impact the size and developmental characteristics of the planets that eventually form. Cooler dust, for example, could allow larger planets to coalesce closer to their parent star in compact protoplanetary disks.

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Now we need more studies of more protoplanetary disks to see if this temperature versus particle size trend is typical for planet-forming systems, hopefully adding more detail to planet formation models.

Source: ESO