The composition of exoplanets can be studied via their spectra – those rainbows of information contained in starlight – but these chemical fingerprints can be difficult to wrestle out of the data.



Spectra of exoplanets are slowly dribbling in, but for the first time astronomers have attained spectra of multiple planets in a system, and the results are not quite as expected.

Improvements in technology and data analysis are enabling astronomers to gradually tease out the secrets of these alien worlds. The latest advance in technology has combined direct imaging with spectroscopy to unveil details of the atmospheres of four planets around the young star HR 8799; the first time multiple planets outside our Solar System have been studied like this.

HR 8799 lies approximately 130 light years from Earth and it is an A-type star, meaning that it is hotter than the Sun. The first three exoplanets belonging to this star were discovered in 2008, with the more elusive fourth planet remaining hidden until 2010. What is remarkable about their detection is that they are among the handful of planets that can be imaged directly.

Ordinarily, a star’s luminosity overwhelms any light emitted or reflected by a nearby planet, making the planets invisible. This is why most of the 850 or so exoplanets discovered to date have been found using indirect methods such as measuring how a planet makes a star wobble. Nonetheless, a young planet that is glowing brightly with the heat generated during its formation can be imaged if it is situated far enough from the parent star.

These new measurements come from “Project 1640,” which is aiming to install exoplanet imaging equipment and spectrographs on various ground based telescopes. This equipment includes a coronograph, which blocks the light of the star to help pinpoint the planets, as well as an adaptive optics system which corrects for turbulence in the atmosphere. The end results of these upgrades is that it only took one hour for the planetary spectra to be obtained.

The four planets orbiting HR 8799 are gas giants with roughly the same temperature, and are thus expected to have similar compositions. “If you take a gas with the chemical composition of today’s universe, chemistry predicts what molecules form as that gas is cooled down,” explains Ben Oppenheimer, lead author of the study. “Since these planets are all believed to be roughly around 1000 degrees Kelvin, this ‘thermochemistry’ predicts that methane should be in abundance, assuming the gas in the planets’ atmospheres are in equilibrium.”

So there is plenty of methane in all four planets, right? Not so, as only planets d and e are playing ball and displaying methane in their spectra. Planets b and c have little methane, instead being consisting mainly of ammonia, and possibly acetylene.

“The lack of methane in some of them is surprising,” Oppenheimer tells Skymania News. “However, there may be processes at work that we don’t understand that modify the predictions of ‘thermochemical equilibrium’.”

Not only that, but the spectra are like nothing seen before. Each spectrum is unique, and they are different from other low temperature objects, such as brown dwarfs. Three of the planets bear no similarities to the gas giants in our own Solar System, although planet e does seem to be somewhat similar to models of Saturn’s night side.

There are also tentative signs of variability in the c planet, possibly due to the high levels of UV radiation from the star. HR 8799 belts out 1000 times more UV radiation than the Sun, and this radiation can wreak havoc with planetary atmospheres.

The direct imaging technique is currently limited to gas giant planets at large distances from their parent stars, well away from the coveted “habitable zone” where life might exist on rocky planets. As the technique improves it is expected the boundaries will be pushed so that smaller, older gas giants can also be imaged. The James Webb Space Telescope, due to launch around 2018, should be able to image Saturn sized planets. The ultimate goal will of course be to take a snapshot of another Earth, but a dedicated space mission will be needed to achieve this ambitious goal.

The paper has been published in the Astrophysical Journal, and the preprint can be found here.

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