Astronomers have discovered water vapor in the atmospheres of five planets beyond the solar system – results that help signal an important change taking place in the hunt for Earth-like planets orbiting other stars.
It heralds a expansion from merely hunting for planets and putting them into bins by mass or distance from their stars to studying their atmospheres. The new research could help scientists refine how they look for key signatures that a planet could sustain life, as well as improve our understanding of where and how the planets formed.
Armed with a fresh approach to using the Hubble Space Telescope, astronomers are opening “the era of characterization,” says Avi Mandell, a planetary scientist at NASA's Goddard Space Flight Center in Greenbelt, Md., and the lead author of one of two research papers reporting the results.
In the current issue of the Astrophysical Journal, his team reports discovering water vapor in the atmospheres of three gas-giant planets orbiting close to their host stars, which are 815 to 1,200 light-years from Earth. That followed results published in September, which reported discovering water vapor in the atmospheres of two other "hot Jupiters" in star systems much closer to Earth.
While these planets are not candidates for life, water vapor is of keen interest for biological reasons, since water is a necessary ingredient for organic life. Moreover, water, especially in conjunction with measurements of carbon monoxide, can yield clues about whether a planet formed close to its star or out beyond what often is called the snow line. There, temperatures are so cold that water becomes ice and gets incorporated into a planet's core.
But to make advances in the search, researchers have to know what to look for and how hard it will be to spot water vapor in a planet's atmosphere. In some ways, hot Jupiters are a useful place to start. Researchers have long suggested water vapor would dominate the spectroscopic signature of molecules in hot Jupiters' atmospheres.
Astronomers detect the molecules when a star's light passes through a planet's atmosphere as the planet passes in front of the star. The spectrum in the starlight takes on added features as it passes through the planet's atmosphere, allowing astronomers to back out of the combined spectrum the signatures of molecules in the planet's atmosphere.
Both reports noted that the signatures water vapor gave were weaker than models had predicted. It's a phenomenon researchers have noted since they first detected a chemical element in the atmosphere of an extrasolar planet – sodium in 2002. Researchers attribute the lower-than-expected readings to clouds or haze in the atmospheres.
Other researchers are on the verge of publishing yet more results from observations of extrasolar planet atmospheres, Dr. Mandell says. Among the targets is a smaller, Neptune-scale planet.
“As we move forward, and as we build better instruments and have a better idea of what we expect to see, we're only going to discover new and unexpected things,” he says.
Neither team represents the first researchers to claim to have detected water vapor in the atmosphere of an alien world. But earlier results were contested because of uncertainties introduced by the quirks and limited capabilities of the detectors they were using, Mandell explains.
That changed with the addition of a new camera to the Hubble Space Telescope in 2009. The Wide Field Camera 3 (WFC3) wasn't designed with extrasolar planet atmospheres as its main research goal, but it turned out to be better suited for the task than other tools at astronomers' disposal.
It took time, however, for astronomers to figure out how to use the WFC3 to greatest advantage.
Mandell's team acknowledges that its detections were weaker and less precise than the detections by the other team, led by University of Maryland astronomer Drake Deming.
The difference was partly due to the fact that the planets observed by Dr. Deming's team are much closer than those studied by Mandell.
More important, however, was the way Deming's team used the WFC3. Under the old approach, Hubble stared at a nearby star, but only briefly; it was designed to observe faint objects and couldn't remain trained on a bright planet-hosting star for long because elements in the telescope would become overloaded with light, Deming explains.
The newer approach in effect sweeps the light across the camera until all of the individual elements in the detector have had their fill of light.
“This greatly increases the efficiency and the light gathering,” Deming says, leading to far-more precise measurements. With that precision and a deeper understanding of the camera's quirks and how to adjust for them comes greater confidence in the measurements it makes.