Though exoplanet spectroscopy has come a long way from its early days, much more needs to be done to gather complete knowledge about exoplanets, Jeff Hecht learns from researchers
The trickle of discoveries has become a torrent. Little more than two decades after the first planets were found orbiting other stars, improved instruments on the ground and in space have sent the count soaring: It is now past 2,000. The finds include “hot Jupiters,” “super-Earths” and other bodies with no counterpart in our solar system — and have forced astronomers to radically rethink their theories of how planetary systems form and evolve.
Yet, discovery is just the beginning. Astronomers are aggressively moving into a crucial phase in exoplanet research: finding out what these worlds are like. Most exoplanet-finding techniques reveal very little apart from the planet’s mass, size and orbit. But is it rocky like Earth or a gas giant like Jupiter? Is it blisteringly hot or in deep-freeze? What is its atmosphere made of? And does that atmosphere contain molecules such as water, methane and oxygen in odd, unstable proportions that might be a signature of life?
The only reliable tool that astronomers can use to tackle such questions is spectroscopy: a technique that analyses the wavelengths of light coming directly from a planet’s surface, or passing through its atmosphere. Each element or molecule produces a characteristic pattern of “lines” — spikes of light emission or dips of absorption at known wavelengths — so observers can look at a distant object’s spectrum to read off what substances are present. “Without spectroscopy, you are to some extent guessing what you see,” says Ian Crossfield, an astronomer at the University of Arizona in Tucson.
But spectroscopy has conventionally required a clear view of the object, which is generally not available for exoplanets. Most new worlds show up only as an infinitesimal dimming of a star as the otherwise invisible planet passes across its face; others are known only from the slight wobble of a star being tugged back and forth by the gravity of an unseen companion. Astronomers often say that trying to study such an object is like staring into a far-off searchlight (the star) and trying to see a firefly (the planet) hovering nearby. In recent years, however, observers have begun to make headway. Some have extracted the spectra of light passing through the atmospheres of exoplanets as they cross the face of their parent stars — the equivalent of measuring the colour of the firefly’s wings as it flits through the searchlight beam. Others have blocked the light of the parent star so that they can see exoplanets in distant orbits and record their spectra directly.
Custom-built
In the past two years, astronomers have begun to record spectra from a new generation of custom-built instruments such as the Gemini Planet Imager on the 8.1- metre Gemini South telescope at the summit of Cerro Pachon in Chile. Exoplanet spectroscopy will be a priority for several spacecraft and ground-based telescopes that are now in development. And astronomers are waiting eagerly for NASA’s James Webb Space Telescope, which will bring unprecedented light-gathering power and sensitivity to the task when it gets launched in 2018.
These are heady times for those hoping to get a deep understanding of newfound worlds, says Thayne Currie, an astronomer at Japan’s Subaru Telescope on Mauna Kea, Hawaii. “We are on the cusp of a revolution.” The first exoplanet in orbit around a sun-like star was discovered in 1995, when astronomers Michel Mayor and Didier Queloz of the Geneva Observatory in Switzerland detected a regular, back-and-forth wobble in the movement of star 51 Pegasi. They concluded that it was caused by the gravity of a planet at least 150 times the mass of Earth — roughly half the mass of Jupiter — orbiting the star every four days or so. Other discoveries followed as exoplanet fever took hold, and led telescope managers to make more observing time available for planet-hunting.
Transit spectroscopy
The list of finds soon sparked an idea for astronomer David Charbonneau of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. He reasoned that when a planet “transits,” or passes in front of a star, molecules in its atmosphere will absorb some of the starlight, and leave their spectroscopic fingerprints in it. Might it be possible to detect those fingerprints?
To find out, David decided to look for sodium. “It’s not particularly abundant,” he says, “but sodium has very clear spectroscopic features” — excited molecules of it emit two very strong lines of light, which give sodium streetlights their familiar yellow-orange colour. When the sodium is backlit, the light that floods through it has dark bands at the same points of the spectrum, and David hoped that these would be comparatively easy to spot. They were: In 2002, David and his co-workers announced that they had used the Hubble Space Telescope to detect a sodium signal from a Jupiter-sized exoplanet transiting HD 209458, a star about 47 parsecs (150 light years) from Earth. It was both the first detection and the first spectroscopic measurement of an exoplanet atmosphere. Within a few years, space-based transit observations were recording more complete spectra, and detecting gases such as carbon monoxide and water vapor.
Using this technique means looking for very tiny changes in a star’s spectrum, says David — maybe 1 part in 10,000. Hubble was and is observers’ first choice of instrument: it does not have to contend with absorption of light by gases in Earth’s atmosphere, so its spectra are very clean and easy to interpret. But competition for observing time is intense, so astronomers also use ground-based telescopes.
These do have to deal with atmospheric interference, but can overcome it by collecting more light than Hubble can. This allows them to detect fainter objects and to separate individual spectral features more clearly. That pays off because most exoplanets are in star systems that are moving relative to Earth. “So their wavelengths are Doppler-shifted,” says David, meaning that the radiation coming from them is stretched or squeezed by their movement, displacing the spectral lines slightly from the corresponding lines in Earth’s atmosphere. Because the two sets of spectral lines no longer overlap, observers can know for sure how much of the signal comes from the exoplanet. Using this method, astronomers have been able to detect gases making up as little as 1 part in 100,000 of a planet’s atmosphere.
Transit spectroscopy does have its limitations. Some exoplanets have nearly featureless spectra characteristic of clouds, which consist of droplets or fine dust particles that do not leave their imprint on the spectrum in the same way as isolated molecules. The clouds are a big headache, says David. “We don’t have any direct measurement of what the clouds are made of. We just know they block the light.” They aren’t necessarily made of water vapor. David points out that the cloud-shrouded super-Earth GJ 1214b, 12 parsecs from Earth, is so hot that its clouds could be made of zinc sulfide and potassium chloride. On still hotter worlds, the clouds could contain droplets of iron or rock.
Exoplanet spectroscopy has come a long way from its early days, when practitioners were struggling to extract extremely faint signals from noisy environments. The first results were often problematic. Now, Ian says, “for the most part what we are finding holds up and is repeatable”.
The New York Times