The study of exoplanets — worlds around other stars — has come a long way in just a few decades. With nothing but guesses and imagination about such extrasolar planets as recently as the early 1990s, scientists have now confirmed the existence of more than 5,000 such worlds.
And the detailed study of exoplanets, particularly the molecular composition of their atmospheres, is even more recent. While Hubble and the new James Webb Space Telescope have now discovered organic compounds on multiple planets, the first detection of organics on an exoplanet was just 15 years ago. Using the Hubble Space Telescope, NASA Jet Propulsion Laboratory scientist Mark Swain made the discovery and published a paper 15 years ago today in Nature, on March 20, 2008.
Methane and other organic compounds are important because they are essential to prebiotic chemistry, at least when it comes to life as we know it. Detecting methane on a distant exoplanet isn’t a sure sign of life, but it helps scientists get a better handle on what worlds out there might be habitable, and which may not.
A Synthesis of Ideas
The story detection of methane on HD 189733b is one of intellectual meandering and reflection, an idea borne out of an unplanned but fortuitous synthesis of ideas. As a graduate student, Swain was interested in radio astronomy, doing his thesis work on the Very Large Array, a radio interferometer.
“An interferometer is where you connect separate telescopes together in a special way to make them act as one big telescope,” Swain tells Inverse.
After graduation, Swain worked on Cornell’s South Pole Imaging Fabry-Perot Interferometer (SPIFI) instrument for observing the star-forming regions of distant galaxies.
From there, Swain got a job at NASA’s Jet Propulsion Laboratory — as an optical engineer rather than a scientist — in the early 2000s. Given his instrumentation work on SPIFI and his doctoral work on radio astronomy interferometers, he was sent to Hawaii to help on the Keck Interferometer project, connecting the two Keck telescopes on Mauna Kea to work as one big telescope.
The Keck Interferometer was built to look at exoplanets, and it was exposure to that work that gave Swain the kernel of the idea that would grow into the observations of methane on HD 189733b.
“One of the things about interferometers is that they work best if the atmosphere is really, really stable,” he says. He was aware of work going on in Antarctica suggesting the atmosphere was very stable there, “so I had this idea that if we took the Keck interferometer technology that we were building, at NASA, and put it in shipping containers,” Swain says, and sent it to the bottom of the world, astronomers could get a better view of other worlds.
Swain proposed the idea to the French astronomy community, which had a base where such an instrument could be placed. He was invited to do a visiting appointment in France at the Observatoire de Grenoble, to start working on this idea. But as he began to contemplate what was shaping up to be a €50 to €100 million project, he realized someone needed to conduct some precursory studies, and it would have to be him.
Limits of Detection
Over the winter of 2005 to 2006, Swain wrote several exoplanet observing proposals, including an observation of HD 189733b using the Hubble Space Telescope. Swain proposed that Hubble conduct infrared transit spectroscopy observations of HD 189733b, a fancy way of saying Hubble would look at the light passing through HD 189733b’s atmosphere as it passed in front of its star — infrared wavelength light — and look for signatures of different chemicals.
A spectrograph relies on the fact that different chemicals either absorb light or let it pass by, depending on the light's wavelength, obtaining a pattern that looks like squiggly lines called a spectrum.
“When the planet goes in front of the star, it blocks starlight, but the planet appears slightly bigger. Because the atmosphere, the opaque part of the atmosphere, is literally larger at wavelengths where a molecule like water or methane absorbs,” Swain says. “That is a fingerprint of the molecular signature, and that's how we use the spectrum to figure out what's in the exoplanet's atmosphere.”
HD 189733b is a hot Jupiter-type planet, according to Swain. Roughly the mass of Jupiter, but a little puffier due to orbiting much closer to its star than most planets. Daytime temperatures can exceed 2,000 degrees Fahrenheit.
Because of this temperature, Swain and JPL Astronomer Gautam Vasisht, who joined Swain in his Hubble project around 2007, didn’t expect to find methane on HD 189733b. When you mix different elements such as hydrogen, oxygen, and carbon at different temperatures and pressures, different structures tend to result.
“The carbon at some temperatures prefers to make methane and at other temperatures it prefers to make carbon monoxide,” Swain says. And based on prior work on gas giants, he says, “We thought we should see [carbon monoxide]” on HD 189733b.
But they didn’t see carbon monoxide. When they got the Hubble spectrum of HD 189733b, “we could see the absorption of water vapor in the planet's atmosphere, but there was something else,” Swain says. “And we weren't sure what it was.”
Assembling the team
As Swain and Vasisht puzzled over their mystery feature in HD 189733b’s spectrum, they read a paper published in Nature that year by Italian physicist Giovanna Tinetti. She used NASA’s Spitzer infrared telescope to observe HD 189733b and published the first finding of water vapor in the planet’s atmosphere.
Swain ran into Tinetti at a conference a few weeks after she published her paper. He congratulated her on what he said was a big result, then asked if she had thoughts on his own work.
“‘This is awesome, let me show you our spectrum,’” Swain told Vasisht. “Because we've got a spectrum with 15, 16 data points that shows absolutely your water feature, but we've got something else we don't know what it is,” Swain recalls asking Tinetti. “She looks at it and says, ‘I think you have methane.’”
So Tinetti joined the team. The subsequent paper showing the first detection of methane on another world was published in Nature in 2008, with Swain, Vashist, and Tinetti as co-authors.
The future of exoplanet spectroscopy
One of the immediate legacies of the 2008 paper was that it unlocked infrared spectroscopy with the Hubble telescope.
“There had been some earlier work in the visible [light spectrum], but for whatever reasons had not really taken off,” Swain says.
It also led scientists to consider what they could do with newer instruments than Hubble. Tinetti is now the primary investigator for a European Space Agency mission called Ariel, scheduled to launch in 2029, which will conduct transit spectroscopy of a sample of 1,000 planets, according to Swain. And Swain, for his part, is the primary investigator of NASA’s contribution to ARIEL, the Contribution to ARIEL Spectroscopy of Exoplanets (CASE), a near-infrared light detection instrument.
Of course, scientists don’t have to wait for ARIEL to launch to perform cutting-edge infrared spectroscopy studies. JWST, less than two years into its operational lifespan, is already churning out detailed observations of exoplanets.
“James Webb is going to transform this field, there's no question in my mind about that. It's got the combination of a lot greater sensitivity, because of the big mirror, it's got a higher spectral resolution instrument,” Swain says. “You can really use that, for bright stars and bright planets.”
Reflecting on how far exoplanet science has grown since his 2008 detection of methane on a hot gas giant planet, Swain sees the future, the near future, as one where researchers regularly study a wide range of exoplanets, from Neptune-sized bodies, to super-Earths and rocky, terrestrial planets more similar to those in our own inner Solar System.
”The complexity and the range of processes that can occur on small planets is really fascinating to me, and I think that we're gonna be able to start studying that with James Webb,” he says. “That's tremendously exciting.”
