Miraculous view of a nascent planet

Almost 25 years ago to the day, on 23 November 1995, the very first discovery of a planet outside our solar system was reported [1]. That breakthrough triggered a development that is often dubbed the 'exoplanet revolution' - and one that earned the Swiss astronomers Michel Mayor and Didier Queloz the 2019 Nobel Prize in Physics (shared with the Canadian-American cosmologist James Peebles). Today, hundreds of new exoplanets are discovered each year, and the rate of discovery is still accelerating. At the same time, the level of detail with which the atmospheres and orbits of exoplanets can be characterized is ever increasing.

A case in point is a newly published study [2] by a team of international astronomers led by Dr. Tomas Stolker, a former ETH fellow in the group of Professor Sascha Quanz at the Institute for Particle Physics and Astrophysics and now a fellow at Leiden University in the Netherlands. The team has carried out a unique survey at mid-infrared wavelengths and obtained fresh insight into the previously discovered exoplanet PDS 70 b. Intriguingly, this giant planet is still in the process of formation as it sweeps up gas and dust in its orbit around a young star. The new observations help to gain a deeper understanding on the atmospheres of such nascent planets and the physical processes that drive their formation.

The survey that Stolker leads is known as MIRACLES (Mid-InfraRed Atmospheric Characterization of Long-period Exoplanets and Substellar companions) and was designed to systematically characterize the atmospheres of directly imaged exoplanets and brown dwarfs at wavelengths of 4-5 µm [3]. The team - which brings together scientists from Switzerland, Germany, the Netherlands and the US - uses the Very Large Telescope (VLT) at the Paranal Observatory in Chile combined with high-contrast imaging techniques to directly detect the light coming from relatively young planets which are still glowing at infrared wavelengths. By observing at 4-5 µm, the survey provides an illuminating window into the physical and chemical properties of these giant exoplanets.

For the study now reported, the researchers pointed the VLT to PDS 70, which is a young star in the constellation Centaurus at some 370 lightyears away. What makes this star stand out is that in its orbit two planets had been previously discovered, both of which are still growing within the natal environment of the circumstellar disk (see the figure). One of the planets, PDS 70 b, was now characterized more closely as part of the MIRACLES survey. The new data, together with the re-examination of archival imaging data, enabled Stolker et al. to place important constraints on the mass, radius and luminosity of PDS 70 b. The analysis also showed that the planet is embedded in an extended dusty environment, which is replenished by gas and dust from the circumstellar disk.

Results like these underline that the exoplanet revolution is alive and well. In fact, a new imager and spectrograph - ERIS, and to which ETH scientists have made major contributions - will enter into operation at the VLT in the near future. This instrument will make it possible to detect and characterize even fainter and closer-in exoplanets.