The big, wide universe has a lot of stars. The exact figure is unknown, although estimates put the number somewhere in the region of 70 sextillion. Some of those stars, just like our own sun, have their own planetary satellites. Yet, according to the NASA Exoplanet Archive, astronomers have only confirmed the existence of 1,832 planets outside of the solar system.
And most of these confirmed planets we haven't even seen. They are too far away, and don't emit any light of their own, making them very difficult to spot. Their existence is proven by the behaviour of the star's light. A disappearing, reappearing star might mean, for instance, that a planet is passing between it and our telescopes, eclipsing it briefly.
So a directly imaged planet is a rare and wonderful thing.
Say hello, then, to VHS 1256b, a giant planet just 40 light-years from Earth, the closest for which a direct image and spectrum have been obtained to date. A team of researchers from the Instituto de Astrofísica de Canarias, the Polytechnic University of Cartagena and the Centre for Astrobiology collaborated on the project.
The star, M-class, or red dwarf VHS J125601.92-125723.9 and its planetary system are very young, somewhere between 150 million and 300 million years old. Our solar system, in comparison, is 4.6 billion years old. VHS 1256b orbits the star at a distance around 100 times greater than the distance between the Earth and the sun.
This, combined with its closeness to Earth, is what made the planet -- photographed by the ESO's VISTA telescope, which has a 4.1-metre reflector, and the 10.4-metre Gran Telescopio Canarias -- easier to detect than other exoplanets.
"It is a gas giant planet with a size similar to that of Jupiter, but 11 times more massive," explained Bartosz Gauza, first author on the paper, currently available via arXiv.
"As it is young, its atmosphere is still relatively warm, around 1,200 degrees Celsius, and it is still sufficiently luminous for us to be able to detect it with the VISTA telescope of the European Southern Observatory."
The planet was discovered by correlating two enormous surveys: the 2 Micron All Sky Survey, which shows a catalogue of the entire sky in infrared, and the VISTA Hemisphere Survey, which is currently underway, cataloguing the sky of the southern hemisphere also in infrared. Special software developed by the University of Cartagena compared the two surveys, picking out objects that move in the sky and those that have companions with a proper common motion. This identified the planet.
In order to capture VHS 1256b's spectrum, though, something a little more heavy-duty was required.
"In its atmosphere, we have found traces of water vapour and of alkali metals, which are normal for this type of planet, but not of methane, which is also expected at these temperatures," said Victor Sánchez Béjar, IAC researcher and paper co-author.
"Due to its youth and proximity, we have been able to obtain for the first time in great detail the spectrum of an exoplanet in the visible. We needed to use a large diameter telescope: the Gran Telescopio Canarias, with the OSIRIS [spectrograph] instrument."
Its brightness and distance from its star means that VHS 1256b can even be used to study phenomena very difficult to see on other planetary systems, such as comet impacts, or even the detection of moons comparable in size to Earth. And its the planet that allowed the team to give the system an accurate age.
"The study of the red dwarf, a star on the borderline between low mass stars and brown dwarfs, has allowed us to determine the distance and the age of the system with great accuracy, and VHS 1256b is one of the few exoplanets for which those parameters are known," said study co-author María Rosa Zapatero Osorio, researcher at the Centre for Astrobiology.