This artist’s concept depicts a planetary system. Credit: NASA/JPL-Caltech
HAT-P-26b is an inflated, Neptune-mass exoplanet 437 light years from Earth, which orbits its parent star HAT-P-26 every 4.23 days. The planet has a relatively low surface gravity that is well suited for atmospheric characterization. To determine composition of HAT-P-26b’s atmosphere and finding traces of water vapor in it, the scientists made use of data provided by the Low Dispersion Survey Spectrograph 3C (LDSS-3C) instrument and NASA’s Spitzer Space Telescope. LDSS-3C is an optical imager and (multi-) slit spectrograph on the 6.5m Magellan II (Clay) telescope, Chile.
They observed the primary transit of HAT-P-26b with LDSS-3C on Apr. 16, 2015 for nearly 5 hours, using the multi-object technique developed to probe exoplanet atmospheres. They acquired 502 science frames using 20s integrations. Thanks to Spitzer, the scientists were able to observe 2 transits of the planet in 2013. The science observations using NASA’s spacecraft lasted approximately 4.5 hours and acquired 8,128 frames, with 2s frame times. HAT-P-26b was compared to other exoplanets with similar temperatures and surface gravities like HAT-P-12b and HAT-P-19b.
Stevenson et al found the evidence of H2O and lack of potassium in HAT-P-26b’s atmosphere. They also found out that the planet’s gas layer could be cloud-free with a high-metallicity. However, it could also have a solar metallicity atmosphere.
“We conclude that HAT-P-26b is likely to have a high-metallicity, cloud-free atmosphere or a solar metallicity atmosphere with cloud deck at about 10 mbar. Although more high-precision data are needed to break this degeneracy, a 100 times solar-metallicity atmosphere is consistent with expectations based on Uranus and Neptune’s similar metallicities. Alternatively, since HATP-26b has a similar equilibrium temperature and surface gravity to that of HAT-P-12b, it is conceivable that the smaller HAT-P-26b simply has lower altitude clouds and, thus, a metallicity that is closer to solar,” the researchers said.
The prevalence of clouds and hazes is one of the major outstanding issues in exoplanetary atmospheres. Temperature and surface gravity are thought to play a significant role in the production of clouds and hazes, but much work is needed to fully understand this mechanism. To advance knowledge, they need precise transmission spectra of targets that have a variety of physical characteristics. Besides HAT-P-26b’s atmosphere, the research also updated the planet’s transit ephemeris and orbital period.
The astronomers believe that pending Hubble Space Telescope observations and future studies of HAT-P-26b will provide the necessary precision to make a definitive detection and potentially distinguish between a high-metallicity or a solar-metallicity atmosphere. They recommend performing follow-up observations of this curious exoplanet using one or more instruments at overlapping wavelengths.
http://arxiv.org/abs/1511.08226 http://phys.org/news/2016-01-atmosphere-neptune-mass-exoplanet-hat-p-26b.html
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