“We will never witness the formation of Earth, but here, around a young star 440 light-years away, we may be watching a planet come into existence in real time,” says Francesco Maio, whose research has flipped the story of the search for planetary cradles on its head. The first direct detection of a proto-planet nestled in the spiral arms of a protoplanetary disc around HD 135344B is a landmark moment in planet formation science. For centuries, astronomers have theorized that young planets chiseled discs’ multi-component rings, gaps, and spirals into being. Short of this, however, it has been tough to get a planetary sculptor red-handed.
The discovery was made using the aid of the Very Large Telescope at the European Southern Observatory and the Enhanced Resolution Imager and Spectrograph. Researchers used the high-order adaptive optics on the instrument and a coronagraph to mask the blinding light of the star so they could detect the hidden, feeble light of a forming planet. The proto-planet was roughly twice Jupiter in diameter and in a Neptune orbit around the Sun some 30 astronomical units away. It was discovered lying at the bottom of a spiral arm where theory predicted planetary builder of this type to be. “What makes this detection potentially a turning point is that, unlike many previous observations, we are able to directly detect the signal of the protoplanet, which is still highly embedded in the disc,” Maio continued. “This gives us a much higher level of confidence in the planet’s existence, as we’re observing the planet’s own light.”
This find not only verifies long-standing hypothesis regarding the origin of spiral structure in protoplanetary discs but also presents an ephemeral opportunity to probe the physics of planet-disc interaction. Spiral density waves, originally addressed by Goldreich and Tremaine, arise when a massive object a forming planet, e.g., perturbs the ambient gas and dust and creates waves that travel outward in the disc. New high-resolution computations and multi-wavelength observations reaffirm that these spiral arms are not surface phenomena; they penetrate deeply into the mid-plane of the disc, where planet formation is strongest. The vertical nature of such waves, for example, counter-rotating poloidal rolls, have far-reaching implications for the dust dynamics, settling, and ultimately the efficiency of planet formation.
The technological achievement which makes it possible cannot be overemphasized. Telescopes such as ERIS and SPHERE at the VLT, and even ALMA submillimeter imaging, have opened an era where exoplanet direct imaging, even of those still enveloped in their natal cocoons, is feasible. Adaptive optics laboratories now spit out Strehl ratios greater than 80% at near-infrared wavelengths, and coronagraphs mask starlight to tens of millions or better, enabling astronomers to investigate the faintest signals at separations of less than a few dozen milliarcseconds. In addition to showing that planetary companions do in fact exist, these advances are now starting to set limits on their masses, temperatures, and atmospheric composition with unprecedented precision.
In a related discovery, the unpredictable variability of Betelgeuse, one of the most mythic red supergiants in the sky, has been explained by a plausible explanation. With the speckle imager ‘Alopeke on the Gemini North telescope, scientists have, for the first time, directly imaged an unseen companion star in the outflowing atmosphere of Betelgeuse. This companion, approximately 1.5 solar masses but six magnitudes fainter than Betelgeuse in the optical, is four astronomical units distant. “Papers that predicted Betelgeuse’s companion believed that no one would likely ever be able to image it,” said Steve Howell of NASA Ames Research Center. But “Gemini North’s ability to obtain high angular resolutions and sharp contrasts allowed the companion of Betelgeuse to be directly detected.”
The observation not only uncovers a two-century-long mystery regarding Betelgeuse’s brightness fluctuations its so-called “cosmic Morse code” but also demonstrates the ability of new imaging technology. Speckle imaging, which relies on rapid, short exposures to freeze atmospheric turbulence, combined with the light-gathering prowess of Gemini’s 8.1-meter mirror, has pushed the boundaries of what can be resolved in the crowded glare of a supergiant star. The tight companion’s orbit deep within Betelgeuse’s outer atmosphere implies dynamical interaction: in 10,000 years, the two stars will collide in a ball of fire, maybe affecting the timing and nature of Betelgeuse’s subsequent supernova.
This outcome does not occur by itself. It is the convergence of theory, simulation, and instrumentation that inform one another. Direct imaging of a proto-planet constructing spiral arms in a protoplanetary disc offers a living laboratory for testing models of planetary system evolution, and imaging the Betelgeuse companion opens up new windows of access to late stellar evolution and mass loss. With the progress in coronagraphy, adaptive optics, and high-resolution imaging technologies, astronomers will be capable of looking, more precisely, into the mechanisms within the universe that formed our solar system and the galaxy as a whole.
