In November 2014 we were amazed by the very first long baseline image of a baby solar system from the Atacama Large Millimetre/Submillimetre Array (ALMA).
Located in a nearby stellar nursery, the proto-planetary disc around the young star HL Tauri features a striking series of concentric, axisymmetric gaps, or rings, that can be explained by the presence of newborn planets.
Now a team of astronomers from Italy, Australia, and the UK have sought to explain the gaps seen in the ALMA long-baseline observations of the HL Tauri proto-planetary system.
Giovanni Dipierro, Daniel Price, Guillaume Laibe, Kieran Hirsh, Alice Cerioli and Giuseppe Lodato have found through extensive modelling and computer simulations that the observed gaps are explained by the different response of gas and dust to embedded planets in proto-planetary discs.
Astronomers have known for some years that planets can carve gaps in proto-planetary discs, but it has until now proved difficult to explain why HL Tauri does not exhibit the spiral pattern seen in other similar discs.
Dipierro et al. performed global, three dimensional dusty smoothed particle hydrodynamics calculations of multiple planets embedded in dust/gas discs which successfully reproduce most of the structures seen in the ALMA image. Crucially, the scientists have found that their computer simulations explain the shape of the ALMA image because it shows a disc made of dust and not gas. Gaps carved in dust do not exhibit the spiral patterns that show up in observations of gas in young newly formed stellar systems.
They find a best match to the observations using three embedded planets with masses of 0.2, 0.27 and 0.55 MJ (Jupiter masses) in the three main gaps observed by ALMA.
There are however some remaining uncertainties in the exact planet masses from this disc model, so we can expect more observations and further modelling from astronomers keen to understand this nearby system of massive proto-planetary bodies caught in the act of forming planets.
On planet formation in HL Tau (2015) has been accepted for publication in MNRAS Letters and is available as a pre-print at :
The simulation movie can be viewed at :
Image Credit : Dipierro et al. (2015).