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. Image Credit: NASA/JPL-Caltech
Using the combined power of NASA’s Spitzer, Hubble and Kepler
space telescopes, researchers have discovered clear skies and water
vapor on a Neptune-size gaseous exoplanet – the smallest planet from
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 a planet’s atmosphere can block the view to underlying
molecules that reveal information about the planet’s composition and
history. 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 categorized as an exo-Neptune — a
Neptune-sized planet that orbits the star HAT-P-11. It is located 120
light-years away in the constellation Cygnus. This planet orbits closer
to its star than does our Neptune, 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.
A
plot of the transmission spectrum for exoplanet HAT-P-11b, with data
from NASA’s Kepler, Hubble and Spitzer observatories combined. The
results show a robust detection of water absorption in the Hubble data.
Transmission spectra of selected atmospheric models are plotted for
comparison. Image Credit: NASA/ESA/STScI
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 inflated atmospheres.
In fact, researchers already have detected water vapor in the
atmospheres of those planets. The handful of smaller planets observed
previously had proved more difficult to probe partially because they 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. 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; 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. 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 erroneously
appear to be from the planet.
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. This discovery
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.
“We think that exo-Neptunes may have diverse compositions, which
reflect their formation histories,” said study co-author Heather Knutson
of the California Institute of Technology in Pasadena. “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. “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 super-Earths — massive, rocky cousins
to our home world with up to 10 times the mass. Although our solar
system doesn’t have a super-Earth, NASA’s Kepler mission is finding them
in droves around other stars. 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.
Once again, astronomers will be crossing their fingers for clear skies.
Publication: Jonathan Fraine, et al., “Water vapour
absorption in the clear atmosphere of a Neptune-sized exoplanet,” Nature
513, 526–529 (25 September 2014); doi:10.1038/nature13785
Source: Felicia Chou, NASA
Images: NASA/JPL-Caltech; NASA/ESA/STScI,src http://scitechdaily.com/
