Planets form from clumps of material in discs surrounding newborn stars. When the planet is still forming, i.e. when it is still gathering material, 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 disk of gas and dust surrounding HD 169142, a star 374 light years from our solar system.
"We used observations from the SPHERE instrument of the European Southern Observatory's (ESO) Very Large Telescope (VLT) obtained on the star HD 169142, which was observed several times between 2015 and 2019," explains Iain Hammond, a researcher at Monash University (Australia) who stayed at ULiege as part of his doctoral thesis. As we expect planets to be hot when they form, the telescope took infrared images of HD 169142 to look for the thermal signature of their formation.
With these 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 the Earth to the Sun) from its star - slightly further than the orbit of Neptune. Back in 2019, a team of researchers led by R. Gratton had previously hypothesised that a compact source seen in their images could trace a protoplanet. Our new study confirms this hypothesis through both a re-analysis of the data used in their study as well as the inclusion of new observations of better quality.
The different images, obtained with VLT's SPHERE instrument between 2015 and 2019, reveal a compact source that is moving over time as expected for a planet orbiting at 37 astronomical units from its star. All data sets obtained with the SPHERE instrument were analysed with state-of-the-art image processing tools developed by the PSILab team at the University of Liege. The last data set considered in our study, obtained in 2019, is crucial for the confirmation of the planet's motion," explains Valentin Christiaens, researcher at the PSILab of the University of Liege. This data set had not been published until now.
The new images also confirm that the planet must have carved an annular gap in the disc - as predicted by the models. This gap is clearly visible in polarised light observations of the disc. "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 may also harbour yet undiscovered planets," says Hammond.
The polarised 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 that it has accreted from the protoplanetary disc. 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 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 last ten years," says Valentin Christiaens. "Apart from the protoplanets of the PDS 70 system, the status of the other candidates is still hotly debated in the scientific community. The protoplanet HD 169142 b seems to have different properties to the protoplanets of the PDS 70 system, which is very interesting. It seems that we have captured it at a younger stage of its formation and evolution, as it is still completely buried in or surrounded by a lot of dust."
Given the very small number of confirmed forming planets to date, the discovery of this source and its follow-up should give us a better understanding of how planets, and in particular giant planets such as Jupiter, are formed.
Further characterisation of the protoplanet and independent confirmation could be obtained through future observations with the James Webb Space Telescope (JWST). The high sensitivity of JWST to infrared light should indeed allow researchers to detect thermal emission from the hot dust around the planet.
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