An international team of researchers from the University of Liège (Belgium) and Monash University (Australia) has just published the results of the analysis of data from the SPHERE instrument of the European Southern Observatory (ESO), which confirms a new protoplanet. This result was made possible thanks to the advanced image processing tools developed by the PSILab of the University of Liège.

The study is published in the Royal Astronomical Society Monthly Notices: Letters.

Planets form from clumps of matter in disks surrounding infant stars. When the planet is still forming, that is, when it is still gathering matter, it is called a protoplanet. To date, only two protoplanets had been unambiguously identified as such, PDS 70 b and c, both orbiting the star PDS 70. This number has now been increased to three, with the discovery and confirmation of a protoplanet in the disc of gas and dust surrounding HD 169142, a star 374 light-years from our solar system.

“We used observations from the European Southern Observatory’s (ESO) Very Large Telescope (VLT) SPHERE instrument obtained on the star HD 169142, which was observed multiple times between 2015 and 2019,” says Iain. Hammond, researcher at Monash University (Australia) who studied at ULiège as part of his doctoral thesis.

“Because we expect planets to be hot when they form, the telescope took infrared images of HD 169142 to look for the heat signature of their formation. With this data, we were able to confirm the presence of a planet, HD 169142 b, about 37 AU (37 astronomical units, or 37 times the distance from Earth to the Sun) from its star, slightly further than Neptune’s orbit.”

In 2019, a team of researchers led by R. Gratton had previously hypothesized that a compact source seen in their images could trace a protoplanet. The new study confirms this hypothesis both through a reanalysis of the data used in their study as well as the inclusion of new, higher quality observations.

The various images, obtained with the SPHERE instrument of the VLT between 2015 and 2019, reveal a compact source which moves in time as expected for a planet orbiting 37 astronomical units from its star. All datasets obtained with the SPHERE instrument were analyzed with state-of-the-art image processing tools developed by the PSILab team at the University of Liège.

“The last dataset considered in our study, obtained in 2019, is crucial for confirming the movement of the planet”, explains Valentin Christiaens, researcher at the PSILab of the University of Liège. “This dataset had not been released until now.”

The new images also confirm that the planet must have carved out an annular space in the disk, as predicted by the models. This discrepancy is clearly visible in polarized light observations of the disk. “In the infrared, we can also see a spiral arm in the disc, caused by the planet and visible in its wake, suggesting that other protoplanetary discs containing spirals could also harbor planets that are still unknown,” explains Hammond.

The polarized light images, as well as the infrared spectrum measured by the research team, further indicate that the planet is buried in a significant amount of dust accumulated from the protoplanetary disk. This dust could be in the form of a circumplanetary disk, a small disk that forms around the planet itself, which in turn could form moons. This important discovery demonstrates that the detection of planets by direct imaging is possible even at a very early stage of their formation.

“There have been many false positives among the detections of planets in formation over the past ten years”, specifies Valentin Christiaens. “Apart from the protoplanets of the PDS 70 system, the status of the other candidates is still highly debated in the scientific community. The protoplanet HD 169142 b seems to have different properties from the protoplanets of the PDS 70 system, which is very interesting. It looks like we captured it at a younger stage in its formation and evolution, as it is still completely buried or surrounded by a lot of dust.”

Given the very small number of planets in formation confirmed to date, the discovery of this source and its follow-up should allow us to better understand how planets form, and in particular giant planets like Jupiter.

Further characterization of the protoplanet and independent confirmation could be obtained through future observations with the James Webb Space Telescope (JWST). JWST’s high sensitivity to infrared light should indeed allow researchers to detect thermal emission from hot dust around the planet.