Astronomers have obtained the first direct spectrum – a “chemical fingerprint” – of a planet orbiting a distant, Sun-like star, providing direct data about the composition of the planet’s atmosphere. An international team of researchers studied the planetary system around HR 8799 a bright, young star with 1.5 times the mass of our Sun, and focused on one of three planets orbiting the star. While the results were unusual and pose a challenge to current models of the exoplanet’s atmosphere, the accomplishment represents a milestone in the search for life elsewhere in the Universe.
The planetary system resembles a scaled-up version of our own Solar System and includes three giant planets, which had been detected in 2008 in another study. “Our target was the middle planet of the three,” said team member and PhD student Carolina Bergfors, from the Max Planck Institute for Astronomy, (MPIA), “which is roughly ten times more massive than Jupiter and has a temperature of about 800 degrees Celsius,”
The researchers recorded the spectrum using the NACO instrument ion the Very Large Telescope (VLT) in Chile.
As the host star is several thousand times brighter than the planet, and the two are very close, obtaining such a spectrum is an immense feat.
“It’s like trying to see what a candle is made of, by observing it next to a blinding 300 Watt lamp – from a distance of 2 kilometres [1.3 miles],” said Markus Janson of the University of Toronto, lead author of the paper.
Bergfors added, “It took more than five hours of exposure time, but we were able to tease out the planet’s spectrum from the host star’s much brighter light.”
However, the spectra of the exoplanet’s atmosphere shows a clear deviation between the observed spectral shape and what is predicted by the current standard models. “The features observed in the spectrum are not compatible with current theoretical models,” said MPIA’s Wolfgang Brandner, a co-author of the study.
The models assume chemical equilibrium between the different chemical elements present in the atmosphere, and a continuous temperature profile (hotter layers below colder layers). At longer wavelengths (above 4 micrometres), the planet is significantly fainter than expected, which points to molecular absorption spectrum in its atmosphere. The simplest explanation is that the atmosphere contains less methane and more carbon monoxide than previously assumed.
“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 than previously assumed,” Brandner said.
In time, the astronomers hope that this technique will help them gain a better understanding of how planets form. Next, they hope to record the spectra of the two other giant planets orbiting HR 8799 – which would represent the first time that astronomers would be able to compare the spectra of three exoplanets that form part of one and the same system. As a much more distant goal, the technique will allow astronomers to examine exoplanets for habitability, or even signs of life.