Astronomers using the Very Large Telescope (VLT) have obtained the first direct spectrum of one of three Jupiter-like planets orbiting the young star HR 8799.

The three planets in the 130 light year distant system were discovered in 2008, and have masses between seven and ten times that of Jupiter. The planets orbit the central star, which has a mass 1.5 times that of the Sun, at distances between 20 and 70 times the Earth-Sun separation. The system also contains two belts of debris analogous to the Asteroid Belt and Kupier Belt of our own Solar System.

Now, for the first time, astronomers have obtained the first spectrum of an exoplanet orbiting an almost Sun-like star, providing information on the planet's formation and composition.

“The spectrum of a planet is like a fingerprint. It provides key information about the chemical elements in the planet’s atmosphere,” says Markus Janson, lead author of a new paper describing the results presented in the Astrophysical Journal. “With this information, we can better understand how the planet formed and, in the future, we might even be able to find tell-tale signs of the presence of life.” The planet chosen for the study was the middle planet of the system, ten times the mass of Jupiter and with a temperature of 800 degrees Celsius. “After more than five hours of exposure time, we were able to tease out the planet's spectrum from the host star's much brighter light,” says team member Carolina Bergfors.

The observation was made possible by the infrared instrument NACO on the VLT, and its adaptive optics system that counteracts image distortion normally caused by ground-based observing through the Earth's atmosphere. Since the star is several thousand times brighter than the planet, this is a remarkable technological achievement – it is like trying to see what a candle is made of by observing it from a distance of two kilometres when it’s next to a blindingly bright 300 Watt lamp. Until now, spectra were obtained by space-based observatories watching as an exoplanet passes directly behind its parent star, and comparing the light of the star before and after, a technique that relies on a favourable orientation of the planet to the star. In contrast, the ground-based observations are not limited by the orientation of the planet's orbit.

In the case of HR 8799c, the team found that the spectrum differed from existing theoretical models, suggesting that phenomena such as cloud structure or dust contained in the planet's atmosphere need to be treated in more detail in the models.

“The features observed in the spectrum are not compatible with current theoretical models,” says co-author Wolfgang Brandner. “We need to take into account a more detailed description of the atmospheric dust clouds, or accept that the atmosphere has a different chemical composition from that previously assumed.”

The astronomers will next try and obtain spectra for the two other planets in the system, in order to make comparisons between the planets, and shed light on the processes that formed the planetary system, which may be similar to how our own Solar System formed.