Science Credit: NASA, ESA, L. Kreidberg and J. Bean (University of Chicago),
and H. Knutson (California Institute of Technology)
Highest-quality download options
and H. Knutson (California Institute of Technology)
Highest-quality download options
Weather forecasters on exoplanet GJ 1214b would have an easy job.
Today's forecast: cloudy. Tomorrow: overcast. Extended outlook: more
clouds.
Two teams of scientists using NASA's Hubble Space Telescope report
they have characterized the atmospheres of a pair of planets with
masses intermediate between gas giants, like Jupiter, and smaller,
rockier planets, like Earth. A survey by NASA's Kepler space telescope
mission showed that objects in this size range are among the most
common type of planets in our Milky Way galaxy. The researchers
described their work as an important milestone on the road to
characterizing potentially habitable, Earth-like worlds beyond the solar
system.
The findings appear in separate papers in the January 2 issue of the journal Nature.
The two planets studied are known as GJ 436b and GJ 1214b. GJ 436b is
categorized as a "warm Neptune" because it is much closer to its star
than frigid Neptune is to our Sun. The planet is located 36 light-years
away in the constellation Leo.
GJ 1214b is known as a "super-Earth" type planet. Super-Earths are
planets with masses between that of Earth and Neptune. Because no such
planet exists in our solar system, the physical nature of super-Earths
is largely unknown. GJ1214b is located just 40 light-years from Earth,
in the constellation Ophiuchus.
Both GJ 436b and GJ 1214b can be observed passing in front of, or
transiting, their parent stars. This provides an opportunity to study
these planets in more detail as starlight filters through their
atmospheres.
An atmospheric study of GJ 436b based on such transit observations
with Hubble over the last year is presented in one of the papers, led
by Heather Knutson of the California Institute of Technology in
Pasadena, Calif. The news is about what they didn't find. The Hubble
spectra were featureless and revealed no chemical fingerprints
whatsoever in the planet's atmosphere. "Either this planet has a high
cloud layer obscuring the view, or it has a cloud-free atmosphere that
is deficient in hydrogen, which would make it very unlike Neptune,"
said Knutson. "Instead of hydrogen, it could have relatively large
amounts of heavier molecules such as water vapor, carbon monoxide, and
carbon dioxide, which would compress the atmosphere and make it hard
for us to detect any chemical signatures."
Observations similar to those obtained for GJ 436b had been
previously obtained for GJ 1214b. The first spectra of this planet were
also featureless and presented a similar puzzle: The planet's
atmosphere either was predominantly water vapor or hydrogen-dominated
with high-altitude clouds.
A team of astronomers led by Laura Kreidberg and Jacob Bean of the
University of Chicago used Hubble to obtain a deeper view of GJ 1214b
that revealed what they consider definitive evidence of high clouds
blanketing the planet. These clouds hide any information about the
composition and behavior of the lower atmosphere and surface. The new
Hubble spectra also revealed no chemical fingerprints whatsoever in the
planet's atmosphere, but the high precision of the new data enabled
them to rule out cloud-free compositions of water vapor, methane,
nitrogen, carbon monoxide, or carbon dioxide for the first time.
"Both planets are telling us something about the diversity of planet
types that occur outside of our own solar system; in this case we are
discovering that we may not know them as well as we thought," said
Knutson. "We'd really like to determine the size at which these planets
transition from looking like mini-gas giants to something more like a
water world or a rocky, scaled-up version of the Earth. Both of these
observations are fundamentally trying to answer that question."
Models of GJ 436b and GJ 1214b predict clouds that could be made out
of potassium chloride or zinc sulfide at the scorching temperatures of
several hundred degrees Fahrenheit predicted to be found in these
atmospheres. "You would expect very different kinds of clouds to form
on these planets than you would find, say, on Earth," said Kreidberg.
The Chicago team had to make a big effort to conclusively determine
the nature of GJ 1214b's cloudy atmosphere. Kreidberg explained, "We
really pushed the limits of what is possible with Hubble to make this
measurement — our work devoted more Hubble time to a single exoplanet
than ever before. This advance lays the foundation for characterizing
other Earths with similar techniques." Added Bean, "I think it's very
exciting that we can use a telescope like Hubble that was never
designed with this in mind, do these kinds of observations with such
exquisite precision, and really nail down some aspect of a super-Earth
atmosphere."
Knutson continued, "For exoplanets, clouds are incredibly frustrating
because they can hide the bulk composition of the atmosphere that we
want to measure." However, more will be learned with the launch of the
James Webb Space Telescope later this decade. Said Kreidberg, "Looking
forward, the James Webb Space Telescope will be transformative. The new
capabilities of this telescope will allow us to peer through the
clouds on GJ 1214b and similar exoplanets."
CONTACT
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Space Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
