Spirals Assist in the Birth of Planets

In regions of higher density as occur along the density waves which have just been observed, the planet formation could progress much more rapidly, due to the increased gravitational force in the region concerned as well as the higher probability of collisions.

“The structure we have observed with Elias 2-27 is the first direct indication of spiral density waves in a protoplanetary disc,” says Laura Pérez. “It shows that instabilities can form within the disc, leading to sub-regions with much higher density and thus the formation of further planets.” These instabilities occur not only on the dimensions of planet formation: probably the best-known example is density waves in spiral galaxies that give rise to their distinctive spiral arms.

Conversely, planets which have already formed in the disc can trigger spiral density waves while they orbit the central star. To differentiate between these two roles – spiral arms as the trigger for planet formation on the one hand, or being formed themselves by young planets on the other – a deeper understanding is required, and observations such as the ALMA image just published will make their contribution here.

“After years of being able to measure only the integrated thermal radiation around young stars, we now see them in all their beauty and diversity, in the meantime with a spiral structure as well. This helps us to gain a better understanding of how planets form,” says Thomas Henning, Director at the Max Planck Institute for Astronomy in Heidelberg, who was also involved in the most recent observations.

“Over the past decades, astronomers have found a considerable variety of exoplanets. We can only explain this variety when we understand the early phases of planet formation – and the impressively detailed ALMA images make an important contribution here,” says Hendrik Linz, also a scientist at the Max Planck Institute in Heidelberg.

Almost touching the sky: A cluster of radio telescopes forming part of the Atacama Large Millimeter Array (ALMA) at an altitude of more than 5100 metres above sea level in the Chilean Andes.

Credit: © ESO/C. Malin

The two sweeping spiral arms around Elias 2-27 extend to more than ten billion kilometres from the young star – further into space than the Kuiper belt in our Solar System. “The presence of spiral density waves at these extreme distances from the star could explain the existence of exoplanets, which orbit their central stars at a similarly large separation,” says Pérez. The conventional models say that these planets should not be able to form locally in the first place.

The young star Elias 2-27 is part of a much larger star formation region bearing the designation Rho Ophiuchi in the Snake-Holder constellation. Elias 2-27 formed only around one million years ago – very recently compared to the age of our Sun, which goes back around 4.6 billion years. Researchers already knew that this star is surrounded by a disc; according to previous observations with a resolution of between 0.6 and 1.1 arc seconds, it could just as well have been an unstructured disc with cylindrical symmetry, however.

The new ALMA observations with a very high resolution of 0.24 arc seconds show radiation with a wavelength of 1.3 millimetres. This is caused by the presence of dust particles that make up between one and ten percent of the total mass of the disc. Using this radiation, the astronomers were able to track the spiral pattern mentioned over a range spanning from approximately 100 astronomical units (i.e. 100 times the average distance between the Earth and the Sun) to 300 astronomical units from the central star.

As mentioned above, the spiral structure could be caused by a planet which already exists within the disc. ALMA has indeed discovered a narrow band with much less dust, but it is not large enough to accommodate a planet which in turn would be large enough to generate the spiral pattern observed.

On the other hand, the gravity of the disc itself can also cause instabilities which can produce these spiral patterns. Given the total mass of the disc and the form and symmetry of the spiral pattern, the authors also consider this possibility to be highly likely.

“ALMA observations of this type are becoming increasingly frequent and should provide us with more and more images of inhomogeneous sub-structures in protoplanetary discs,” says Karl Menten, Director at the Max Planck Institute for Radio Astronomy and co-author of the article in the Science journal. “We should thus increasingly be able to describe the properties of these structures in more detail and clarify the role they play in planet formation.”