Sunny and hot all year-round, with no clouds on the horizon … That’s not a weather forecast only for the Maldive Islands here on Earth, but also for exoplanet HAT-P-11b, according to the latest findings by an international team of astronomers. But don’t start packing for that holiday package just yet, for HAT-P-11b is a steamy Neptune-sized world located so close to its host star that average temperatures there reach a scortching 1,120 degrees Fahrenheit.
Exoplanet research has transitioned in recent years from the simple discovery of planets around other stars to that of their detailed characterization. Following the exciting findings of thousands of extrasolar worlds during the last two decades, which have established that planetary formation is a common occurrence in the galaxy, astronomers around the world are now striving to understand the overall evolution of these distant worlds, by studying their properties, bulk composition, and internal structure. To that end, astronomers are using one of the best tools at their disposal, which is called transmission spectroscopy. More specifically, when an extrasolar planet happens to cross or transit the face of its star as seen by our line of sight here on Earth, it causes a small dip in the star’s brightness which is proportional to the size of the exoplanet itself. If that planet also happens to have an atmosphere, the latter will absorb some of the star’s light in certain wavelengths as it transits, resulting in a wavelength-dependent transit depth, better known as a transmission spectrum. By studying this spectrum of the combined star-planet light, astronomers can extract detailed information about the planet’s atmosphere, like its chemical composition, temperature, density, and overall dynamics.
With the use of transmission spectroscopy, astronomers have managed to obtain the detailed spectra of many dozens of extrasolar worlds, revealing more of their true nature than what could otherwise be known through the determination of their mass and size alone, which is the hallmark of exoplanet detection techniques like the radial velocity and transit methods. Nevertheless, researchers had only been able up to now to obtain the spectra of hot-Jupiters—large and massive, scorching gas giant planets that lie very close to their respective stars in orbits that are considerably smaller than that of Mercury around the Sun. The reason for this is because hot-Jupiters lie so close to their star, they can more easily be detected because of their size and their feeble light can be more easily distinguished from the overwhelming glare of their host stars. Yet, even though hot-Jupiters are an interesting class of exoplanets in their own right, the overarching goal of astronomers is to ultimately study the atmospheres of smaller, more Earth-like worlds that could be capable of supporting life.
An important milestone toward that goal was recently accomplished by an international team of astronomers, led by Jonathan Fraine, a joint PH.D. Fellow at the University of Maryland’s Department of Astronomy. In a new study published on the 25 September issue of the journal Nature, the researchers presented the results of their observations of exoplanet HAT-P-11b, a Neptune-sized world four times bigger than the Earth and 26 times more massive, which is located approximately 123 light-years away in the constellation of Cygnus. Orbiting its host metal-rich orange dwarf star at a mean distance of approximately 0.053 Astronomical Units (1 A.U is the mean Sun-Earth distance), HAT-P-11b can be characterized as a hot-Neptune, completing one orbit every 4.8 days. The exoplanet was discovered in 2009 by the ground-based Hungarian Automated Telescope Network (or HATNet), making it the smallest such world to be discovered by ground-based telescopes at the time and the first of its size to be detected with the transit method. Using previous radial velocity and transit photometry measurements, astronomers speculated that HAT-P-11b was most probably a gas giant planet, with a huge atmosphere which laid on top of a rocky core. But unlike the gas giants in our Solar System, HAT-P-11b’s close proximity to its host star results in the latter exhibiting an infernal equilibrium temperature of 1,120 degrees Fahrenheit.
Utilising the combined observing capabilities of NASA’s Hubble, Spitzer, and Kepler space telescopes, Fraine’s team managed to illuminate more of the planet’s enigmatic nature, by obtaining the transmission spectrum of HAT-P-11b in unprecedented detail, marking an important first in the atmospheric study of Neptune-sized exoplanets. More specifically, the researchers used Hubble’s Wide Field Camera 3 and Spitzer’s Infrared Array Camera to conduct spectroscopic observations of the host star’s light as it travelled through HAT-P-11b’s atmosphere at wavelengths between 1.1–1.7 mm and 3.6–4.5 mm respectively, during a series of six observations between July 2011 and December 2012. The Spitzer observations were then combined with precision optical photometry measurements by the Kepler space telescope, which also observed HAT-P-11b during 208 transits across its host star, since the latter was also located within the telescope’s fixed field of view.
The observations revealed a very strong water absorption signature at a wavelength of 1.4 micrometres in HAT-P-11b’s spectrum, indicating the clear presence of traces of water vapor in the planet’s atmosphere. In addition, the latter was also found to be rich in hydrogen while also appearing quite transparent, with no apparent obscuring clouds or haze layers. These results were in very good agreement with planetary formation models as well, according to which Neptune-sized exoplanets should have a similar chemical composition to that of the gas giants in our own Solar System. “The amplitude of the water absorption (approximately 250 parts per million) indicates that the planetary atmosphere is predominantly clear down to an altitude corresponding to about 1 millibar, and sufficiently rich in hydrogen to have a large-scale height,” write the researchers in their study. “The spectrum is indicative of a planetary atmosphere in which the abundance of heavy elements is no greater than about 700 times the solar value. This is in good agreement with the core-accretion theory of planet formation, in which a gas giant planet acquires its atmosphere by accreting hydrogen-rich gas directly from the protoplanetary nebula onto a large rocky or icy core.”
Since HAT-P-11b’s host star is an active one, Fraine’s team wanted to further validate their findings by examining if the water absorption signature on the planet’s spectrum was due to the presence of star spots on the host star instead, which can also contain water vapor. By combining the infrared observations made by Spitzer with those by Kepler at optical wavelengths, the researchers were able to confirm that the water vapor signature was coming from the planet itself. “Because HAT-P-11 is an active planet-hosting star, we show that star spots on the stellar surface are not sufficiently cool, nor sufficiently prevalent, to mimic the effect of water vapour absorption in the planet,” writes Fraine’s team. “Our simultaneous Spitzer and Kepler photometry was critical to defining the temperature of the star spots that could otherwise, potentially mimic the effect of water vapour absorption in the planetary atmosphere.” “When astronomers go observing at night with telescopes, they say ‘clear skies’ to mean good luck,” commented Fraine in a press release. “In this case, we found clear skies on a distant planet. That’s lucky for us because it means clouds didn’t block our view of water molecules.”
These results are of great importance to astronomers, in their quest to study and characterize the nature of both Neptune-sized exoplanets and super-Earths alike, which are the most prevalent types of extrasolar worlds in the galaxy, as revealed by the Kepler space telescope. “This discovery is a significant milepost on the road to eventually analyzing the atmospheric composition of smaller, rocky planets more like Earth,” says John Grunsfeld, Associate Administrator of NASA’s Science Mission Directorate in Washington, D.C. “Such achievements are only possible today with the combined capabilities of these unique and powerful observatories.”
The significance of this achievement is also heightened by the fact that similar spectroscopic studies of a handful of exo-Neptunes in the past had come up empty, with the latter’s spectra appearing featureless and flat indicating the presence of heavy clouds and thick haze layers in their atmospheres. The recent discovery of a cloud-free atmosphere on a Neptune-sized exoplanet gives astronomers more hope that smaller, potentially life-friendly extrasolar worlds located inside their star’s habitable zone, which are today beyond the observing capabilities of current instruments, can similarly be probed in the future by the next generation of telescopes like NASA’s James Webb Space Telescope, which is scheduled for launch in 2018. “The work we are doing now is important for future studies of super-Earths and even smaller planets, because we want to be able to pick out in advance the planets with clear atmospheres that will let us detect molecules,” says Heather Knutson Assistant Professor of Planetary Science at the California Institute of Technology in Pasadena and co-author of the study.
If the results by Fraine’s team are any indication, then the first detailed atmospheric observations of a super-Earth located in a more life-friendly orbit around a distant star somewhere in the Milky Way galaxy might indeed be a few years in the future.