Artist's Illustration of Clear Skies on Exoplanet HAT-P-11b
Scientists were excited to discover clear skies on a relatively small
planet, about the size of Neptune, using the combined power of NASA's
Hubble, Spitzer, and Kepler space telescopes. The view from this planet
— were it possible to fly a spaceship into its gaseous layers — is
illustrated at right. Before now, all of the planets observed in this
size range had been found to have high cloud layers that blocked the
ability to detect molecules in the planet's atmosphere (illustrated at
left).
The clear planet, called HAT-P-11b, is gaseous with a rocky core,
much like our own Neptune. Its atmosphere may have clouds deeper down,
but the new observations show that the upper region is cloud-free. This
good visibility enabled scientists to detect water vapor molecules in
the planet's atmosphere. Illustration Credit: NASA, ESA, and R. Hurt (JPL-Caltech)
Artist's Illustration of Exoplanet HAT-P-11b
A Neptune-size planet with a clear atmosphere is shown crossing in
front of its star in this artist's depiction. Such crossings, or
transits, are observed by telescopes like NASA's Hubble and Spitzer to
glean information about planets' atmospheres. As starlight passes
through a planet's atmosphere, atoms and molecules absorb light at
certain wavelengths, blocking it from the telescope's view. The more
light a planet blocks, the larger the planet appears. By analyzing the
amount of light blocked by the planet at different wavelengths,
researchers can determine which molecules make up the atmosphere.
The problem with this technique is that sometimes planets have thick
clouds that block any light from coming through, hiding the signature
of the molecules in the atmosphere. This is particularly true of the
handful of Neptune-sized and super-Earth planets examined to date, all
of which appear to be cloudy.
As a result, astronomers were elated to find clear skies on a
Neptune-sized planet called HAT-P-11b, as illustrated here. Without
clouds to block their view, they were able to identify water vapor
molecules in the planet's atmosphere. The blue rim of the planet in
this image is due to scattered light, while the orange rim on the part
of the planet in front of the star indicates the region where water
vapor was detected. Illustration Credit: NASA, ESA, and R. Hurt (JPL-Caltech)
Artist's Illustration of HAT-P-11b Transmission Spectrum Plot
A plot of the transmission spectrum for exoplanet HAT-P-11b, with
Kepler, Hubble WFC3, and Spitzer transits combined. The results show a
robust detection of water absorption in the WFC3 data. Transmission
spectra of selected atmospheric models are plotted for comparison. Credit: NASA, ESA, and A. Feild (STScI)
Astronomers using data from three of NASA's space telescopes —
Hubble, Spitzer, and Kepler — have discovered clear skies and steamy
water vapor on a gaseous planet outside our solar system. The planet is
about the size of Neptune, making it the smallest for which molecules
of any kind have been detected.
"This discovery is a significant milepost on the road to eventually
analyzing the atmospheric composition of smaller, rocky planets more
like Earth," said John Grunsfeld, assistant administrator of NASA's
Science Mission Directorate in Washington. "Such achievements are only
possible today with the combined capabilities of these unique and
powerful observatories."
Clouds in the atmospheres of planets can block the view to underlying
molecules that reveal information about the planets' compositions and
histories. Finding clear skies on a Neptune-size planet is a good sign
that smaller planets might have similarly good visibility.
"When astronomers go observing at night with telescopes, they say
'clear skies' to mean good luck," said Jonathan Fraine of the
University of Maryland, College Park, lead author of a new study
appearing in Nature. "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."
The planet, HAT-P-11b, is a so-called exo-Neptune — a Neptune-size
planet that orbits another star. It is located 120 light-years away in
the constellation Cygnus. Unlike our Neptune, this planet orbits closer
to its star, making one lap roughly every five days. It is a warm
world thought to have a rocky core and gaseous atmosphere. Not much else
was known about the composition of the planet, or other exo-Neptunes
like it, until now.
Part of the challenge in analyzing the atmospheres of planets like
this is their size. Larger, Jupiter-like planets are easier to see
because of their impressive girth and relatively puffy atmospheres. In
fact, researchers have already been able to detect water vapor in those
planets. Smaller planets are more difficult to probe, and what's more,
the ones observed to date all appeared to be cloudy.
In the new study, astronomers set out to look at the atmosphere of
HAT-P-11b, not knowing if its weather would call for clouds or not.
They used Hubble's Wide Field Camera 3, and a technique called
transmission spectroscopy, in which a planet is observed as it crosses
in front of its parent star. Starlight filters through the rim of the
planet's atmosphere and into a telescope's lens. If molecules like water
vapor are present, they absorb some of the starlight, leaving distinct
signatures in the light that reaches our telescopes.
Using this strategy, Hubble was able to detect water vapor in
HAT-P-11b. This technique indicates the planet did not have clouds
blocking the view, a hopeful sign that more cloudless planets can be
located and analyzed in the future.
But before the team could celebrate clear skies on the exo-Neptune,
they had to show that starspots — cooler "freckles" on the face of
stars — were not the real sources of water vapor. Cool starspots on the
parent star can contain water vapor that might appear erroneously to
be from the planet. That's when the team turned to Kepler and Spitzer.
Kepler had been observing one patch of sky for years, and HAT-P-11b
happens to lie in the field. Those visible-light data were combined
with targeted Spitzer observations taken at infrared wavelengths. By
comparing these observations, the astronomers figured out that the
starspots were too hot to have any steam.
It was at that point the team could celebrate detecting water vapor
on a world unlike any in our solar system. "We think that exo-Neptunes
may have diverse compositions, which reflect their formation
histories," said Heather Knutson of the California Institute of
Technology, Pasadena, co-author of the study. "Now with data like these,
we can begin to piece together a narrative for the origin of these
distant worlds."
The results from all three telescopes demonstrate that HAT-P-11b is
blanketed in water vapor, hydrogen gas, and likely other
yet-to-be-identified molecules. Theorists will be drawing up new models
to explain the planet's makeup and origins.
"We are working our way down the line, from hot Jupiters to
exo-Neptunes," said Drake Deming, a co-author of the study also from
University of Maryland, College Park. "We want to expand our knowledge
to a diverse range of exoplanets."
The astronomers plan to examine more exo-Neptunes in the future, and
hope to apply the same method to smaller super-Earths — the massive,
rocky cousins to our home world with up to 10 times the mass. Our solar
system doesn't have a super-Earth, but NASA's Kepler mission is
finding them around other stars in droves. NASA's James Webb Space
Telescope, scheduled to launch in 2018, will search super-Earths for
signs of water vapor and other molecules; however, finding signs of
oceans and potentially habitable worlds is likely a ways off.
"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," said Knutson.
CONTACT
Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4514
villard@stsci.edu
Whitney Clavin
Jet Propulsion Laboratory, Pasadena, California
818-354-4673
whitney.clavin@jpl.nasa.gov
