Spiraling in on a protoplanet with SPHERE

By Iain Hammond
[email protected]

Exoplanet surveys have revealed that planets around other stars are common. Yet, we have little understanding of how planets form and what sets their size and composition. One of the most direct ways to better understand planet formation is to detect young planets still embedded in disks of gas and dust around their host star. These ‘protoplanets’ are still gathering material from the protoplanetary disc and are hence hot, making their detection possible in the near-infrared possible with large telescopes.

There have been many false positives among the detections of planets in formation over the last ten years. Only two protoplanets have been confirmed using direct imaging – PDS 70 b & c, making detections of more protoplanets crucial. Many scientists hypothesised that another yet undiscovered protoplanet could exist in the disc around the star HD 169142. Observations of this system with ALMA show a beautiful set of dust rings in this system, which could be interpreted as signs of a planet carving its way through the disc.

Movie of HD169142 priotoplanet
Figure 1: Animation of images from SPHERE of the protoplanet around HD 169142, showing orbital motion from 2015-2019.

With our discovery and confirmation of a protoplanet in the disc surrounding HD 169142, the number of detected protoplanets has now increased to three. We used near-infrared observations of HD 169142 captured with the SPHERE instrument at the Very Large Telescope in Chile, taken over a period of 4 years (2015-2019). Careful analysis of the data revealed a compact source inside the annular gap at 37 au – slightly further than the orbit of Neptune – and slightly larger than Jupiter. Through the years we could see the plant moving in its orbit (Fig. 1 above), with a velocity that follows Keplerian motion. In the infrared, we can also see a spiral arm in the disc (Fig. 2 below), caused by the planet like a boat moving through water, suggesting that other protoplanetary discs containing spirals may also harbour yet undiscovered planets.

The polarised light images, as well as the planet’s low resolution spectrum, hint that it is enshrouded in an envelope that contains a significant amount of dust. This dust could be in the form of a circumplanetary disc (a small disc that forms around the planet itself) which in turn could form moons. This means that HD 169142 b has very different properties to the protoplanets around PDS 70, meaning we could have captured it at an earlier stage of formation. Given the very small number of protoplanets discovered, this detection and follow-up studies are crucial to understanding how giant planets such as Jupiter form.

Spiral wake
Figure 2: SPHERE near-IR image showing the protoplanet, its spiral wake, and the size of our solar system for comparison.


Michael Murphy is the Australian representative on the ESO Science Technical Committee. Contact: [email protected]

Sarah Sweet is the Australian representative on the ESO Users Committee. Contact: [email protected]

Stuart Ryder is a Program Manager with AAL. Contact: [email protected]

Guest posts are also welcome – please submit these to [email protected]