Astronomers using the Atacama Large Millimeter/submillimeter Array have detected a pair of trailing symmetric spiral arms in the protoplanetary disk surrounding a young star called Elias 2-27. This spiral feature was produced by density waves — gravitational perturbations in the disk.

Elias 2-27 is a member of the Ophiuchus star-forming complex at a distance of 453 light-years.

Also known as 2MASS J16264502-2423077 and ISO-Oph 67, the star is classified as a young stellar object from analysis of its spectral energy distribution.

Although Elias 2-27 is only 50-60% of the Sun’s mass, it is known to harbor an unusually massive protoplanetary disk.

The star is estimated to be at least one million years old and still encased in its parent molecular cloud, obscuring it from optical telescopes.

The Atacama Large Millimeter/submillimeter Array (ALMA) was able to peer deep into the mid-plane of Elias 2-27’s disk and discover the clear signature of spiral density waves.

Previously, astronomers noted compelling spiral features on the surfaces of protoplanetary disks, but it was unknown if these same spiral patterns also emerged deep within the disk where planet formation takes place.

“These observations are the first direct evidence for density waves in a protoplanetary disk,” said Dr. Laura Pérez, of the Max Planck Institute for Radio Astronomy in Bonn, Germany.

Nearest to Elias 2-27, ALMA found a flattened disk of dust, which extends past what would be the orbit of Neptune in the Solar System.

Beyond that point, the telescope detected a narrow band with significantly less dust, which may be indicative of an exoplanet in formation.

Springing from the outer edge of this gap are two sweeping spiral arms that extend more than 6 billion miles (10 billion km) away from the star.

“Finding density waves at these extreme distances may have implications for planet-formation theory,” Dr. Pérez said.

The standard picture of planet formation begins with small planetesimals coming together under gravity.

In the outer reaches of a protoplanetary disk, where there is a dearth of planetesimals, disk instabilities may also lead directly to the formation of a planet.

ALMA’s detection of spiral density waves may be evidence that such a process is taking place.

The team’s results were published this week in the journal Science.

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Laura M. Pérez et al. 2016. Spiral density waves in a young protoplanetary disk. Science 353 (6307): 1519-1521; doi: 10.1126/science.aaf8296