An international research team with the participation of Dr Markus Mugrauer from the Astrophysical Institute of Friedrich Schiller University Jena has succeeded in the direct imaging of a young exoplanet. The planet orbits the Sun-like star YSES 2, which is about 360 light years away and located in the constellation Musca (the Fly) in the southern night sky. The team discovered the exoplanet using the SPHERE instrument at the Very Large Telescope of the European Southern Observatory (ESO) in Chile. The researchers have now published their observations in the current issue of the scientific journal “Astronomy & Astrophysics”.
Since the discovery of the first exoplanets a good 30 years ago, the number of newly discovered planets amongst distant stars has been growing steadily. More than 4000 such planets are currently known to exist. However, the vast majority of them have only been detected using indirect observation methods. In contrast, direct observation of exoplanets orbiting their central stars has so far been much less successful.
“Only the use of modern adaptive optics on a large telescope of the eight-metre class, together with special high-contrast observation techniques, allows the direct imaging of such substellar objects in close proximity to their much brighter host stars,” Markus Mugrauer explains. “And that is exactly what is possible with SPHERE at ESO’s Very Large Telescope in Chile. This instrument was designed specifically for the direct imaging and characterisation of exoplanets,” he adds. The newly discovered exoplanet YSES 2b is about 10,000 times fainter than its central star. The discovery was possible because the planet is still young. “During the first few million years of their existence, planets essentially radiate the gravitational energy released during their formation,” says Mugrauer. In accordance with the temperature of planets in this early phase of evolution, this radiation is in the near-infrared spectral range. In this case, the observations were made at about 1.6 and 2.2 µm.
Process behind the formation of YSES 2b is still unclear
There are still questions about YSES 2b in relation to the process of its formation. In astrophysics, two different theories of planet formation in large disks of gas and dust around young stars are currently being discussed. The first is what is known as the core accretion scenario, in which kilometre-sized planetesimals first form from dust and ice particles through clumping and accretion processes, and then grow through subsequent collisions into increasingly more massive bodies, which can eventually attract the gas surrounding them, creating gas planets.
According to the second theory, there is the possibility that a gravitational instability of the disk of gas and dust around a young star can itself lead to planet formation. Compressions of gas and dust masses are first formed in the disks and these then further contract to planets through their self-gravity.
However, neither of the two theories can fully explain the existence of YSES 2b. On the one hand, the planet is too low in mass to have been formed by gravitational disk instability, given its distance of more than 100 astronomical units from its host star. And on the other hand, it is too far away from its host star for its formation to be explained by the core accretion scenario.
Follow-up observations are therefore necessary to investigate how the formation process of the newly discovered exoplanet took place. To this end, the composition of its atmosphere is to be analysed by means of spectroscopic measurements and in addition, its orbit around the host star is to be characterised in detail. In conclusion, the authors say that they also need to determine whether the star has other planets orbiting it at a smaller distance. New observations with SPHERE at ESO’s Very Large Telescope are already planned for this purpose. “Even though it is currently still unclear exactly how YSES 2b was formed, we are confident that new observations will help solve this mystery,” explains principal investigator Alexander Bohn, a PhD student at Leiden University (the Netherlands). “In this way, we hope to gain new insights into the formation of gas giants, which could also be relevant to our solar system.”
Profile of YSES 2b and its host star YSES 2
YSES stands for “Young Suns Exoplanet Survey” and it is the title of an observational project at the European Southern Observatory’s Paranal Observatory in Chile. In YSES, planets are searched for around young, Sun-like stars in the Scorpius-Centaurus Association and their properties are characterised. YSES 2, the new planet host star, is almost 14 million years “young” and a member of the Lower Centaurus-Crux stellar association. The newly discovered planet YSES 2b is about 115 times further away from its host star than the Earth is from the Sun. Its surface temperature is between 900 and 1100 °C and it has about five to eight times the mass of the planet Jupiter in our solar system. YSES 2b thus belongs to the class known as “far gas giants”, of which only a few have so far been detected around young stars. YSES 2b is also one of the lowest-mass directly imaged exoplanets to have been found so far. Notably, the exoplanet has only 0.5 per cent of the mass of its host star, the lowest mass ratio of any directly imaged exoplanet in orbit around a Sun-like star to date.
The young, Sun-like star YSES 2, imaged on 08.12.2020 in the Ks-band (approx. 2.2µm) with SPHERE at the Nasmyth focus of Melipal, one of the four 8.2m telescopes of the Very Large Telescope at the European Southern Observatory’s Paranal Observatory in Chile. The star is located in the centre of the image behind a coronagraph mask (grey circular area, radius of 0.1 arcsec). In this SPHERE image, the newly discovered exoplanet YSES 2b can be seen about 1 arcsecond south of its host star.Picture: Markus Mugrauer/Universität Jena