Planets form in disks around their host protostar, but when and how disks form is still an unanswered question. ALMA observations by a team of astronomers led by Nadia Murillo and Shih-Ping Lai have found the youngest disk around a protostar to date, at an earlier stage than predicted by most models.

Material in our own Solar System follows Kepler’s Laws, with the inner planets orbiting the Sun faster than planets further out. This is what is usually referred to as Keplerian rotation. Keplerian disks, that is disks with Keplerian rotation, are very likely to eventually form planets in stable orbits. Discovering these types of disks in the very early stage when the protostar is still deeply embedded in its natal cloud provides insight into the ability of these systems to form planets.

ALMA was used to observe VLA1623, a triple protostellar system located in the rho Ophiuchus star-forming cloud. A disk structure was observed towards VLA1623A, a very young source still wrapped in its cocoon of dust and gas. Thanks to ALMA’s capabilities, analysis of the gas in the disk revealed its motion to be Keplerian and to have a radius of roughly 5 times the radius of Neptune’s orbit, but with a central protostar mass of only 0.2 times our Sun’s mass. This shows that the protostar is still very young and growing.

Models and simulations have previously predicted that Keplerian disks cannot form in the earliest phases of star formation. However, the discovery of VLA1623A’s big Keplerian disk proves otherwise, suggesting that other factors may play a role in disk formation. This is in line with more recent studies that suggest that misalignment of magnetic field and rotation axes or turbulence may enhance early disk formation, producing disks of 100 times the Earth-Sun radius or larger. The recent evidence that nearly all stars, including multiple stars, have at least one planet is in line with our observed result that nature finds a way to make protoplanetary disks.

The research findings are presented in the article “A Keplerian disk around a Class 0 source: ALMA observations of VLA1623A” published in the Astronomy and Astrophysics Journal, Vol. 560, of December 13, 2013.

The image below is an artist’s impression of VLA1623A. A Keplerian disk was found around the central protostar, and powerful gas flow is ejected from the protostar.
Credit: N. Murillo et al.