Can one witness a new world being born, sculpting its stellar cradle as it emerges out of cosmic dust? In the spinning protoplanetary disc around HD 135344B, 440 light-years away from Earth, scientists have now achieved what may be the most convincing evidence to date: the direct observation of a forming planet, growing giant, sculpting its environment.
The detection was achieved through the synergistic combination of three powerful instruments: the Atacama Large Millimeter/submillimeter Array, the SPHERE instrument on the Very Large Telescope of the European Southern Observatory, and the ERIS instrument on the VLT. spiral arms and dust asymmetries in the disc around HD 135344B had already been revealed by ALMA and SPHERE, but not any direct sign of a planetary companion until today.
The observations, published in a paper by Francesco Maio and in Astronomy & Astrophysics, used ERIS’s advanced infrared imaging and coronography to block the star’s blinding light and uncover faint signals in the surrounding disc. The outcome: a point source located at the end of one of the spiral arms, precisely where theoretical models suggest a planet would be if it had indeed etched out such a pattern. “This gives us a much higher level of confidence in the planet’s existence, as we’re observing the planet’s own lightdisc,” said Maio. This gives us a much higher level of confidence in the planet’s existence, as we’re observing the planet’s own light.
The candidate planet, estimated at twice the mass of Jupiter and orbiting at roughly the same distance from its star as Neptune does from the Sun, appears to be driving the observed spiral structures. The ERIS observations, in the L′ band (3.96 μm) with a vortex coronagraph, revealed the planet at a projected distance of ~28 astronomical units well within the disc’s high-viscosity cavity. Multiwavelength analysis shows extreme dust extinction (AV ≳ 10 mag), and infrared brightness of the planet implies a circumplanetary disk, a feature expected to be characteristic of gas giants in their early stages.
The technical capability involved here is impressive. ERIS pairs a new imaging camera with a refurbished integral field spectrometer and an advanced adaptive optics module, providing diffraction-limited imaging and spectroscopy from 1–5 μm. Its capacity for high-contrast imaging, especially when married to advanced data reduction methods like angular differential imaging and principal component analysis, has pushed the boundary of what can be done in the face of the glare from young stars. The vAPP coronagraph in specific has proven especially effective at distilling faint planetary signals from scattered starlight noise.
Theory planet-disc models have long expected giant planets to drive spiral density waves, creating the spectacular structures seen in scattered light and millimeter continuum. But as recent hydrodynamic simulations show, the nature of these structures pitch angle and spirals’ amplitude is controlled by the planet’s mass and disc physical conditions. Alignment with the bottom of the S2 spiral arm, as seen, is extremely strong evidence for the planet-induced spiral scenario, and that similar features do not occur elsewhere in the disc is against other interpretations such as gravitational instability, which is unlikely due to the relatively low mass of the HD 135344B disc.
Multi-instrument synergy was key. While SPHERE’s polarimetric imaging had previously mapped the spiral arms of the disc in stunning detail, and ALMA had mapped the millimeter-scale dust distribution and revealed a gigantic vortex, it was the infrared sensitivity and high-contrast imaging ability of ERIS alone that could pierce the inner regions and actually observe the emission from the planet. The combined data allowed researchers to decouple the confounding interaction of dust, gas, and hidden objects, ensuring spirals and asymmetries and spirals are not mere shadows or instabilities but rather have a tight link with the existence of a massive accreting protoplanet.
The bearings extend far beyond a single discovery. “We will never witness the birth of Earth, but here, near a young star 440 light-years from us, we may be witnessing the formation of a planet in real time,” Maio said. With the progress in high-contrast imaging and multi-wavelength investigations, astronomers have the possibility to learn about the first pages of planetary assembly one disc, one spiral, one newborn world at a time.
