Thanks to the SPHERE instrument, installed in the VLT (Very Large Telescope) of the ESO (European Southern Observatory) in Chile, a team of astronomers has captured an image of a planet located in a wide orbit within a triple star system . Similar systems had already been discovered, and as a result of those earlier sightings it was assumed that the orbit of such a planet should be unstable, as if it were soon to be ejected from the system. But this new planet remains in a very stable orbit, suggesting that these systems may be more common and complex than previously thought.
A hypothetical inhabitant of this exotic planet would experience a life under constant daylight and could enjoy three sunrises and sunsets per day, depending on the seasons, which would be longer than a human lifetime. The new world has been discovered by a team of astronomers led by the University of Arizona through direct imaging at the VLT, in Chile. The planet, named HD 131399Ab , is unlike any other known world and its orbit around the brightest of the three suns is the widest ever discovered within a multiple star system. As we already said, these orbits are usually unstable due to the complex and changing gravitational attraction of the other two stars in the system.
Located about 320 light-years from Earth in the constellation Centaurus , HD 131399Ab is about 16 million years old , also making it one of the youngest exoplanets ever found and one of the few to a direct image has been obtained. With a temperature of 580 ° C and an estimated mass of four times that of Jupiter, it is also one of the coldest and least massive exoplanets ever imaged. “During about half of its orbit, which lasts 550 Earth years, three stars can be seen in the sky; the two fainter ones are always much closer to each other and their separation from the brightest star varies as long as possible. throughout the year, “says Kevin Wagner, lead author of the Science paper and discoverer of HD 131399Ab.
It is also the first exoplanet discovered with SPHERE , an instrument sensitive to infrared light , allowing it to detect the signatures or heat traces of young planets. It also has sophisticated functions that correct atmospheric disturbances and block the light from the stars in the system . If not, its luminosity would blind us. Although more long-term observations will be necessary to accurately determine the path of HD 131399Ab between its host stars, observations and simulations seem to suggest the following hypothesis: the brightest star, HD 131399A, is 80% more massive than the Sun and it is orbited by the other two less massive stars, HD 131399B and HD 131399C, located at about 300 ua (1 ua or astronomical unit is equal to the average distance between the Earth and the Sun). At the same time, B and C rotate around each other, separated by a distance similar to that between the Sun and Saturn. In this assumption, the planet HD 131399Ab travels around star A in an orbit twice that of Pluto, compared to the Solar System, and puts the planet at a distance of one third of the separation between star A and the star pair B / C. Knowing the long-term stability of the system will require planned follow-up observations that will establish the planet’s orbit with greater precision.
“If the planet were further from the most massive star in the system, it would be expelled from the system,” explains one of the astronomers who have participated in the research. “Our computer simulations have shown that this type of orbit can be stable, but if you change the surrounding conditions, even just a little, they are likely to quickly become unstable.” Planets in multiple star systems are of special interest to astronomers and planetary scientists, as they provide an example of how the planetary formation mechanism works in these more extreme environments. Although to us, who live in our orbit around our lonely star, multiple star systems seem exotic, they are actually systems as common as those of individual stars.
“We still don’t know what could have happened so that this planet ended up rotating in such a wide orbit, but it is proof that there is more variety than was previously considered possible. What we do know is that, although planets in multiple systems Stellars have been studied much less, they are potentially as numerous as planets in individual star systems, “concludes Kevin Wagner.
Caption: Illustration shows triple star system HD 131399 viewed from a position close to the giant planet HD 131399Ab (left, in orange) orbiting the system. In the center, the largest star HD 131399A. In the background, on the right, the other two suns HD 131399B and HD 131399C
Credit: ESO / L. Calçada
