In a big step forward for exoplanet research, two teams of astronomers using the Hubble Space Telescope were able to conclusively identify traces of water in the atmospheres of five planets outside of our Solar System.
Since the discovery of the first exoplanet 20 years ago, the search for other planetary systems has been constantly yielding impressive results at an accelerated pace. At first, even the simple discovery of an exoplanet was regarded as a major accomplishment. But in recent years, with detection techniques refined and better instruments used, the research has advanced from the stage of mere planetary detection to that of planetary characterisation and classification.Yet one of its ultimate goals has always been the detection of Earth-like worlds, harboring atmospheres that could indicate the presence of a habitable environment. Two recent studies made by two independent teams of astronomers represent yet another step toward that goal.
The first team, led by Dr. Avi M. Mandell, a planetary scientist at the Planetary Systems Laboratory at NASA’s Goddard Space Flight Center, published the results of its study of exoplanets WASP-12b, WASP-17b, and WASP19b. All three planets are “hot Jupiters,” gas giants located in very close orbits around Sun-like stars. Worthy of note is WASP-19b, an exoplanet that has the shortest orbital period of any other “hot Jupiter” ever discovered, completing an orbit approximately every 19 hours! Due to these very close orbits and proximity to their stars, these planets have been characterised as being “inflated,” due to the considerable expansion of their atmospheres caused by the intense stellar radiation coming from their parent stars. In the case of WASP-12b, its atmosphere is slowly “consumed” by its star. Another planet in the study, WASP-17b, is also a peculiar one, revolving in a retrograde orbit, opposite to its star’s rotation. This is very similar to Triton’s retrograde orbit around Neptune in our Solar System.
Although all three planets had been discovered a few years earlier and had been the objects of previous studies, Dr. Mandell’s team was the first to detect clear traces of water in their atmospheres. The team used the Wide Field Camera 3 (or WFC3) on the Hubble Space Telescope to conduct its observations, using a method called “transmission spectroscopy.” Astronomers use transmission spectroscopy to analyse the spectrum of a planet’s light during a transit, when the planet crosses the face of its star. When that happens, the light of the star passes through the many different layers of the planet’s atmosphere. Different chemical elements in the atmosphere will absorb different wavelengths of light and allow others to pass right through. The resulting spectrum of the planet will display dark absorption lines in certain wavelengths, with each line representing the distinct “fingerprint” of a specific chemical element, thus helping astronomers to determine the chemical makeup of the atmosphere.
Dr. Mandell’s team utilised the very high-resolution infrared capabilities of Hubble’s WFC3 camera to analyse and study all three planets’ spectra during one transit for each. The WFC3 is capable of near-infrared observations at wavelengths between 0.8 to 1.17 μm, which is the range of wavelengths where water absorption occurs. The team was able to find very clear water absorption lines in the spectra of all three exoplanets. “We’re very confident that we see a water signature for multiple planets,” says Dr. Mandell. “This work really opens the door for comparing how much water is present in atmospheres on different kinds of exoplanets, for example hotter versus cooler ones.”
A second team of astronomers led by Dr. Leo Drake Deming, a professor at the University of Maryland’s Astronomy Department, conducted similar observations with the Hubble Telescope on another set of exoplanets. Dr. Deming’s team focused on HD 209458b and XO-1b, two “hot Jupiters” that are also orbiting Sun-like stars. HD 209458b is somewhat historic in exoplanet research, being the first exoplanet ever to be detected by the transit method, the first one discovered having an atmosphere, and the first one ever to be spectroscopically observed, revealing signs of carbon and oxygen in its atmosphere. Now, the results of the study by Dr. Deming’s team confirm the presence of water traces in the atmospheres of both planets. The team achieved high levels of precision in their research by using a method called “spatial scan,” which increases the time and resolution of the observations, while eliminating the saturation of the telescope’s detectors by the stars’ very bright light. “To actually detect the atmosphere of an exoplanet is extraordinarily difficult. But we were able to pull out a very clear signal, and it is water,” says Dr. Deming.
One thing that surprised both teams was that the spectra showed lower levers of water absorption than what has been hinted at by earlier observations obtained with the Spitzer space telescope. Previous studies had also revealed the presence of sodium in HD 209458b’s atmosphere—again in lower concentrations than those expected. The most probable explanation, according to both teams, is that these exoplanets must have opaque atmospheres covered with global layers of haze or dust, producing the weakened absorption lines observed in their spectra. “These studies, combined with other Hubble observations, are showing us that there are a surprisingly large number of systems for which the signal of water is either attenuated or completely absent,” says Heather Knutson, an assistant professor in the Division of Geological and Planetary Sciences at the California Institute of Technology and co-author of Dr. Deming’s study. “This suggests that cloudy or hazy atmospheres may in fact be rather common for hot Jupiters.”
Although the water traces were found in the atmospheres of inhospitable, scorching-hot gas giant planets, that doesn’t take away from the discoveries’ importance. As the authors of Dr.Deming’s team note in their study, “Successful transmission spectroscopy of giant exoplanetary atmospheres, is a crucial first step toward eventual spectroscopy of a nearby habitable super-Earth, using the James Webb Space Telescope.”
Video Credit: NASA Goddard Space Flight Center
With detection methods constantly refined and more powerful telescopes used, astronomers are constantly advancing their searches, discovering more and more Earth-type planets with the passage of time, while also learning how to detect the necessary chemical elements in their atmospheres that would indicate a habitable environment. It may not be too far into the future when the first truly Earth-analog planet outside of our Solar System will be observed for the first time, and clear bio-signatures will be detected in its atmosphere. The day this discovery happens will see a major paradigm shift in our thinking as a human species, and will be the beginning of a new era.