Free Astronomy Magazine May-June 2021

45 MAY-JUNE 2021 T his animation shows a representation of the orbits and movements of the planets in the TOI-178 system. New research by Adrien Leleu and his col- leagues with several telescopes, including ESO’s Very Large Telescope, has re- vealed that the system boasts six exoplanets and that all but the one closest to the star are locked in a rare rhythm as they move in their orbits (represented in orange). In other words, they are in resonance. This means that there are pat- terns that repeat themselves rhythmically as the planets go around the star, with some planets aligning every few orbits. In this artist’s animation, the rhythmic movement of the planets around the central star is represented through a musical harmony, created by attributing a note (in the pentatonic scale) to each of the planets in the resonance chain. This note plays when a planet completes either one full orbit or one half orbit; when planets align at these points in their orbits, they ring in resonance. [ESO/L. Calçada] SPECULOOS, both sited at ESO’s Paranal Observatory in Chile. Since exoplanets are extremely tricky to spot directly with telescopes, as- tronomers must instead rely on other techniques to detect them. The main methods used are imaging transits — observing the light emit- ted by the central star, which dims as an exoplanet passes in front of it when observed from the Earth — and radial velocities — observing the star’s light spectrum for small signs of wobbles which happen as the exoplanets move in their orbits. The team used both methods to observe the system: CHEOPS, NGTS and SPECULOOS for transits and ESPRESSO for radial velocities. By combining the two techniques, astronomers were able to gather key information about the system and its planets, which orbit their central star much closer and much faster than the Earth orbits the Sun. The fastest (the innermost planet) completes an orbit in just a couple of days, while the slowest takes about ten times longer. The six plan- ets have sizes ranging from about one to about three times the size of Earth, while their masses are 1.5 to 8 times the mass of Earth. Some of the planets are rocky, but larger than Earth — these planets are known as Super-Earths. Others are gas planets, like the outer plan- ets in our Solar System, but they are much smaller — these are nick- named Mini-Neptunes. Although none of the six exoplanets found lies in the star’s habitable zone, the researchers suggest that, by continuing the resonance chain, they might find additional planets that could exist in or very close to this zone. ESO’s Extremely Large Tel- escope (ELT), which is set to begin operating this decade, will be able to directly image rocky exoplanets in a star’s habitable zone and even characterise their atmospheres, pre- senting an opportunity to get to know systems like TOI-178 in even greater detail. !