Royal Society Invites Bernath to Present His Innovations in Probes for Life on Exoplanets

March 18, 2013

Peter Bernath

If there is life in the universe beyond our solar system, Peter Bernath, the Old Dominion University chemist and expert in molecular spectroscopy, could be one of the scientists who points the way to this discovery.

He has helped provide laboratory data needed to analyze the chemical composition of exoplanets - the planets that orbit stars outside our solar system. Exoplanets are a hot topic just now because advances in detection techniques have turned up hundreds of them in the past few years. At last count, about 800 have been identified.

Bernath's expertise in this field of study was recognized by the Royal Society, London, when he was invited there last week to present his research at a scientific discussion meeting, "Characterising Exoplanets: Detection, Formation, Interiors, Atmospheres and Habitability."

One of 23 scientists invited to speak at the meeting March 11-12, Bernath gave a presentation on "Molecular Opacities for Exoplanets." Although his topic resists a concise explanation, it depends on the essential fact that a spectrometer can identify molecules by how much light of particular wavelengths a substance absorbs or emits.

One of the most intriguing aspects of exoplanet research is the possibility that one - or some - of these planets will be just the right distance from a star to have a temperature and other conditions similar to Earth's. This could allow life to exist. "Exoplanets have captured the imagination of scientists and the general public, in part because they are first steps in the search for life on other worlds," Bernath said.

But how do we even determine the existence of an exoplanet, or describe it and its atmosphere, when the exoplanet is many light years away from Earth and too dim for us to see through our most powerful telescopes?

The star that an exoplanet orbits is often so bright that light from the much smaller exoplanet cannot be seen, and this forces scientists to use a clever scheme to collect information. If a planetary system is oriented in such a way that an exoplanet's orbit makes it "transit" across a parent star, telescopes can detect a dip in total light coming from the star whenever the exoplanet orbits between the star and Earth. A significant periodic dip in a star's light is good evidence for an exoplanet's existence. Another technique measures small shifts in the wavelengths of the star light caused by the motion of the planet and star due to the gravitational attraction between them.

An exoplanet dubbed HD 209458b, which is about the size of our solar system's Jupiter but much hotter, was the first transiting planet observed by a dip in light intensity. That was about 15 years ago, when scientists discovered that this exoplanet blocked about 1.5 percent of its parent star's light. This type of precision photometry is best done from orbit, for example using NASA's Hubble and Spitzer space telescopes. NASA's Kepler mission, which is currently searching for Earth-like planets by their transits, is credited with many of the recent exoplanet discoveries.

Even though the discovery of exoplanets is fascinating, Bernath says the research that has generated the most interest is the characterization of exoplanets by spectroscopy. This, a field in which he works, aims at determining what we would find if we could land on an exoplanet.

Recording the spectrum of a celestial body involves the separation of the light coming from it into individual wavelengths or colors, and this can reveal the chemical makeup of the body giving off the light. A common example is the use of a piece of glass to split sunlight into bands of colors.

So when a powerful telescope detects an exoplanet in transit across a star, or being eclipsed by the star, there is a change in the spectrum coming from that location. If the amount of the change at a particular wavelength is measured very carefully, it tells us a lot about the chemical composition of the exoplanet and its atmosphere.

"Very carefully," said Bernath, is the key term. "Spectroscopy of exoplanets is extraordinarily difficult because of the overwhelming radiation from the parent star. The measurements require an unprecedented degree of photometric stability and correction of systematic errors. Not unexpectedly, some of the work has been greeted with skepticism."

To find evidence for the presence of molecules such as water, carbon dioxide, ammonia or methane, scientists have to enhance the information they are getting from distant planetary systems. This process might be compared to computational enhancement of a blurry photo. The scientists use theoretical calculations and laboratory experiments to try to match the observed exoplanet spectrum, and this is where the skepticism can come in.

Bernath's recent contributions, and the subject of a paper he wrote for the Royal Society, London, involve work he has done with hot ammonia and methane. Calculating the spectra of these substances has proven to be difficult. But Bernath said he and colleagues have "implemented an entirely experimental approach for hot methane and ammonia" that utilizes spectroscopic evidence gathered right here on Earth.

"The basic idea is straightforward. We record infrared and near infrared emission spectra with a Fourier transform spectrometer for a wide range of temperatures that match the expected exoplanet temperatures," he said. This leads to laboratory data that can be used directly for exoplanet spectroscopy.

Bernath, who is the chair of ODU's Department of Chemistry and Biochemistry, has an international reputation for his innovative accomplishments in making chemical detections and measurements in the Earth's atmosphere and in outer space.

He was among a group of scientists who reported last year the first direct evidence that emissions of carbon dioxide caused by human activity on Earth are propagating upward to the highest regions of the atmosphere. The findings, published by the journal Nature Geoscience, resulted in Bernath and his colleagues from the U.S. Naval Research Laboratory's Space Science Division receiving a top paper award at the 45th Annual Alan Berman Research Publication and Edison Patent Awards dinner March 15, sponsored by the Naval Research Laboratory.