Astronomers have obtained a direct spectrum of the exoplanet HR 8799 c, about 130 light years from Earth, and if you watch your definitions, it’s possible to call this the first ‘direct spectrum’ of such a world. I throw in the qualifier because way back in 2004, astronomers using the ESO’s Very Large Telescope and the infrared instrument NACO obtained an image and a spectrum of a planet of about five Jupiter masses around a brown dwarf. The question then involved how the two objects formed — did they form together, like a stellar binary, or did the smaller object form out of the disk around the brown dwarf?
Whatever the case, the new work on HR 8799, also conducted with the VLT and NACO, takes us into interesting territory. Up until now, the way we’ve obtained a spectrum from an exoplanet has been to observe the planet moving directly behind its host star. The spectrum was then derived by comparing the light from the star before and after this event. That method relies, of course, on the orbital plane of the planet being aligned along our line of sight so that the ‘exoplanetary eclipse’ is observable from Earth, a significant limitation on our observations.
Direct observations like this one don’t rely on the planet’s orbital orientation, but they do count on remarkable adaptive optics for such ground-based work as astronomers tease out a planetary signal that is thousands of times dimmer than the star. The spectral information thus derived should offer clues to the planet’s formation and composition, according to Markus Janson, the lead author on the paper reporting the findings. Says Janson:
“The spectrum of a planet is like a fingerprint. It provides key information about the chemical elements in the planet’s atmosphere. 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 planetary system around HR 8799 is an intriguing one. The star is an A5-class about 1.5 times as massive as the Sun. Three planets are known here. HR 8799 b, c and d have estimated masses of 7, 10 and 10 Jupiter masses respectively, and separations of 68 AU, 38 AU and 24 AU. Moreover, we have evidence of a debris belt analogous to the Edgeworth/Kuiper belt outside of HR 8799 b, and a second debris belt similar to our own asteroid belt outside of HR 8799 d. The paper on this work refers to a ‘scaled-up version of our own Solar System,’ but it’s also a young one at an estimated 60 million years.
“Our target was the middle planet of the three, which is roughly ten times more massive than Jupiter and has a temperature of about 800 degrees Celsius,” says team member Carolina Bergfors. “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.”
Image: By studying a triple planetary system that resembles a scaled-up version of our own Sun’s family of planets, astronomers have been able to obtain the first direct spectrum of a planet around a star, thus bringing new insights into its formation and composition. The spectrum is that of a giant exoplanet, orbiting around the bright and very young star HR 8799, about 130 light-years away. This montage shows the image and the spectrum of the star and the planet as seen with the NACO adaptive optics instrument on ESO’s Very Large Telescope. As the host star is several thousand times brighter than the planet, this is a remarkable achievement at the border of what is technically possible. According to the scientists 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. Despite the power of the VLT’s extraordinary adaptive optics system, the spectrum of the planet appears very faint, but still contains enough information for the astronomers to characterise the object. In the spectrum, several artefacts from the instrument are seen, such as internal reflections, or “ghosts”, and diffraction rings. Credit: ESO/M. Janson.
Yes, and it’s a spectrum that’s not in agreement with current theoretical models, a result that may be explained by a more detailed explanation of dust clouds in the atmosphere, or else a chemical composition different from what had been assumed. From the paper:
The results therefore imply that a more detailed treatment of dust in the models is necessary – or perhaps, that non-equilibrium chemistry is involved… Non-equilibrium models are worth to explore as they predict large differences in the spectrum as function of metallicity. Thus, a better understanding of the spectral behavior in this wavelength range might lead to a determination of whether or not the planet is metal-enhanced or not, and thereby provide further clues to its formation.
And this is interesting:
In the future, further observations with NACO can yield a spectrum also of HR 8799 b and maybe d, and yield a broader coverage of HR 8799 c by optimizing the observation procedure. This will provide the opportunity for comparative exoplanetology within a single system.
Advances in spectroscopy have obvious implications for astrobiology as we try to understand the atmospheres of distant planets and look, eventually, for chemical biomarkers. Thus the better we become at spatially resolving a planetary spectral signal from that of its star, the closer we are to reaching our long-term goal of identifying life elsewhere in the universe. The paper is Janson et al., “Spatially resolved spectroscopy of the exoplanet HR 8799 c,” in press at Astrophysical Journal Letters (available online).