The ALMA telescope in Chile has, for the first time, observed a star system that might be in the early stages of planet formation. The picture has astronomers drooling over it because the study of the origins of planets has until now been limited to simulated computer models and observations of planets made after they formed.

ALMA image of the protoplanetary disc around HL Tauri.
ALMA image of the protoplanetary disc around HL Tauri. Image: ALMA (ESO/NAOJ/NRAO)

According to a statement put out by the European Southern Observatory (ESO), the observation was one of the first made with the ALMA, which opened in September 2014 for a ‘Long Baseline Campaign’ (ESO is the institution through which European countries fund the telescope). ALMA uses a technique called very-long baseline interferometry to achieve high resolutions that lets it observe objects hundreds of light-years away in fine detail. It makes these observations in the millimeter/sub-millimeter range of wavelengths; hence its name: Atacama Large Millimeter/sub-millimeter Array.

The image shows a disc of gas, dust and other debris orbiting the star HL Tauri, located about 450 light-years from Earth. A system like this is originally a large cloud of gas and dust. At some point, the cloud collapses under its own gravitation and starts to form a star, further accruing matter from the cloud and growing in size. The remaining matter in the cloud then settles into a disc formation over millions of years around the young star.

In the disc, the gas and dust continue to clump, this time into rocky lumps like planets and asteroids. This is why the disc is called a proto-planetary disc. As a planet forms and its gravitational pull gets stronger, it starts to clear a space in the disc of matter by either sucking it for itself or knocking it out. The gaps that are formed as a result are good indicators of planet formation.

According to the ESO statement, “HL Tauri’s disc appears much more developed than would be expected from the age of the system [less than 100,000 years]. Thus, the ALMA image also suggests that the planet-formation process may be faster than previously thought.”

An annotated image showing the protoplanetary disc surrounding the young star HL Tauri.
An annotated image showing the protoplanetary disc surrounding the young star HL Tauri. Image: ALMA (ESO/NAOJ/NRAO)

In the Solar System, similar gaps exist called Kirkwood gaps. They represent matter cleared by Jupiter, whose prodigious gravitational pull has been pushing and pulling the orbits of asteroids around the Sun into certain locations. In fact, Jupiter’s movement within the Solar System – first moving away, then toward, and then away once more from the Sun – has been used to explain why the material composition of some asteroids between Mars and Jupiter is similar to those of Kuiper Belt objects situated beyond the present orbit of Neptune. Jupiter’s migration mixed them up.

Similarly, the gaps forming around HL Tauri, though they may represent planetesimals, may not result in planets in the exact same orbits as they could move around under the influence of subsequent gravitational disruptions. They could acquire unexpectedly eccentric orbits if their star system comes too close to another, as was found in the nearby binary star system HK Tauri in July 2014. Or, the gaps are probably being emptied by the gravitational effects of an object in another gap.

However, astronomers think the presence of multiple gaps is likely evidence of planet formation more than anything else.

At the same time, the resolution in the image is 7 AU (little more than Jupiter’s distance from the Sun), which means the gaps are very large and represent stronger gravitational effects.

Astronomers will use this and other details as they continue their investigation into the HL Tauri system and how planets – at least planets in this system – form. The Long Baseline Campaign, which corresponds to the long-baseline configuration of the ALMA telescope that enabled this observation, will continue into December.