Is it a case of nature versus nurture when it comes to two "cousin"
exoplanets? In a unique experiment, scientists used NASA's Hubble Space
Telescope to study two "hot Jupiter" exoplanets. Because these planets
are virtually the same size and temperature, and orbit around nearly
identical stars at the same distance, the team hypothesized that their
atmospheres should be alike. What they found surprised them.
Lead researcher Giovanni Bruno of the Space Telescope Science
Institute in Baltimore, Maryland, explained, "What we're seeing in
looking at the two atmospheres is that they're not the same. One
planet—WASP-67 b—is cloudier than the other—HAT-P-38 b. We don't see
what we're expecting, and we need to understand why we find this
difference."
The team used Hubble's Wide Field Camera 3 to look at the planets'
spectral fingerprints, which measure chemical composition. "The effect
that clouds have on the spectral signature of water allows us to measure
the amount of clouds in the atmosphere," Bruno said. "More clouds mean
that the water feature is reduced." The teams found that for WASP-67 b
there are more clouds at the altitudes probed by these measurements.
"This tells us that there had to be something in their past that is changing the way these planets look," said Bruno.
Today the planets whirl around their yellow dwarf stars once every
4.5 Earth days, tightly orbiting their stars closer than Mercury orbits
our sun. But in the past, the planets probably migrated inward toward
the star from the locations where they formed.
Perhaps one planet formed differently than the other, under a
different set of circumstances. "You can say it's nature versus
nurture," explains co-investigator Kevin Stevenson. "Right now, they
appear to have the same physical properties. So, if their measured
composition is defined by their current state, then it should be the
same for both planets. But that's not the case. Instead, it looks like
their formation histories could be playing an important role."
The clouds on these hot, Jupiter-like gas giants are nothing like
those on Earth. Instead, they are probably alkali clouds, composed of
molecules such as sodium sulfide and potassium chloride. The average
temperature on each planet is more than 1,300 degrees Fahrenheit.
The exoplanets are tidally locked, with the same side always facing
the parent star. This means they have a very hot day-side and a cooler
night-side. Instead of sporting multiple cloud bands like Jupiter does,
each probably has just one broad equatorial band that slowly moves the
heat around from the day-side to the night-side.
The team is just beginning to learn what factors are important in
making some exoplanets cloudy and some clear. To better understand what
the planets' pasts may have been, scientists will need future
observations with Hubble and the soon-to-be-launched James Webb Space
Telescope.
The team's results were presented on June 5 at the 230th meeting of the American Astronomical Society in Austin, Texas.
The Hubble Space Telescope is a project of international cooperation
between NASA and ESA (European Space Agency). NASA's Goddard Space
Flight Center in Greenbelt, Maryland, manages the telescope. The Space
Telescope Science Institute (STScI) in Baltimore conducts Hubble science
operations. STScI is operated for NASA by the Association of
Universities for Research in Astronomy, Inc., in Washington, D.C.
For more information about NASA's Hubble Space Telescope, visit: www.nasa.gov/hubble
For additional information, visit: http://hubblesite.org/news_release/news/2017-22
Editor: Sara Blumberg
