Spiral Density Waves: Clue to Planet Formation?

Have a look at the spiral of pinwheeling dust that can be seen around the young star Elias 2-27. We’re looking at gravitational perturbations in a protoplanetary disk that, as this National Radio Astronomy Observatory news release says, mimic the vast arms we expect in a spiral galaxy. But here we’re looking at a process with implications for planet formation, one that draws on data from the Atacama Large Millimeter/submillimeter Array (ALMA). This is the first time a spiral density wave has been detected in a protoplanetary disk’s planet formation areas.

Image: ALMA peered into the Ophiuchus star-forming region to study the protoplanetary disk around the young star Elias 2-27. Astronomers discovered a striking spiral pattern in the disk. This feature is the product of density waves – gravitational perturbations in the disk. Credit: L. Pérez (MPIfR), B. Saxton (NRAO/AUI/NSF), ALMA (ESO/NAOJ/NRAO), NASA/JPL Caltech/WISE Team.

Some 450 light years from Earth in the Ophiuchus star-forming region, Elias 2-27 is about half the mass of the Sun, though its protoplanetary disk is massive. Although the young star (about a million years old, according to current estimates) is shrouded by the molecular cloud from which it grew, ALMA was able to peer into the mid-plane of the disk to identify the spiral density waves. The spiral arms extend as much as 10 billion kilometers away from the host star.

All this catches the eye because while we can account for star formation from the collapse of gas and dust under the influence of gravity, we need a mechanism to keep enough material from falling into the protostar to ensure that it doesn’t spin up enough to shred itself. The protostellar disk projects angular momentum outward, and is where we can expect planets to form. But standard core accretion models have problems explaining the formation of planets 20 to 30 AU out, where the disk may not be dense enough to allow the process to be efficient.

Gravitational instabilities in the outer disk, however, can produce the kind of dense spiral arms we see here, with new material being pushed out into regions far from the star and collapsing under its own gravity to begin planet formation. Andrea Isella (Rice University) explains:

“We don’t completely understand how planets form, but we suspect there are two ways: Either small particles stick together until they form something like the Earth or Mars, or accreting gas forms a planet like Saturn or Jupiter. But this process works only very close to the star, within a few astronomical units (roughly the distance from the Sun to the Earth), because that’s where all the material is, and it has to have enough density… If a disk is massive enough to be gravitationally unstable, a spiral will form naturally.”

Thus the spiral arms of Elias 2-27 may be the manifestation of an instability that gives birth to a particular kind of exoplanet. Near to the star, the ALMA observations found a flattened dust disk that extends out beyond 30 AU, followed by a narrow band of sharply diminished dust that may indicate a planet in formation. The spiral arms extend outward from the edge of this gap in the disk. Lead author Laura Pérez (Max Planck Institute for Radio Astronomy) notes that an upcoming program will use ALMA data to home in on similar protoplanetary disks as we try to find out whether Elias 2-27’s spiral density waves actually do reveal planet(s) in formation.

The paper is Pérez et al., “Spiral density waves in a young protoplanetary disk,” Science Vol. 353, Issue 6307 (30 September 2016), pp. 1519-1521 (abstract). A Rice University news release is also available.