The distant exoplanet WASP-127b is of great interest to researchers because of
its unusual characteristics, including a hypothetical thick cloud layer.
Recently, an international team of astronomers has not only detected clouds on
the distant exoplanet WASP-127b, but also measured their altitude with
unprecedented precision. A presentation by Dr. Romain Allart at the Europlanet
Science Congress (EPSC) 2021 shows how, by combining data from a space- and a
ground-based telescope, the team has been able to reveal the upper structure
of the planet's atmosphere. This paves the way for similar studies of many
other faraway worlds.
WASP-127b, located more than 525 light-years away, is a "hot Saturn"—a giant
planet similar in mass to Saturn that orbits very close to its sun. The team
observed the planet passing in front of its host star to detect patterns that
become embedded in the starlight as it is filtered through the planet's
atmosphere and altered by the chemical constituents. By combining infrared
observations from the ESA/NASA Hubble Space Telescope (HST) and visible light
measurements from the ESPRESSO spectrograph at the European Southern
Observatory's Very Large Telescope in Chile, the researchers were able to
probe different regions of the atmosphere. The results brought a few
surprises.
'First, as found before in this type of planet, we detected the presence of
sodium, but at a much lower altitude than we were expecting. Second, there
were strong water vapor signals in the infrared but none at all at visible
wavelengths. This implies that water-vapor at lower levels is being screened
by clouds that are opaque at visible wavelengths but transparent in the
infrared,' said Allart, of the iREx/Université de Montréal and Université de
Genève, who led the study.
The combined data from the two instruments enabled the researchers to narrow
down the altitude of the clouds to an atmospheric layer with a pressure
ranging between 0.3 and 0.5 millibars.
'We don't yet know the composition of the clouds, except that they are not
composed of water droplets like on Earth,' said Allart. 'We are also puzzled
about why the sodium is found in an unexpected place on this planet. Future
studies will help us understand not only more about the atmospheric structure,
but about WASP-127b, which is proving to be a fascinating place.'
With a full orbit around its star occurring in about four days, WASP-127b
receives 600 times more irradiation than the Earth and experiences
temperatures up to 1100 degrees Celsius. This puffs the planet up to a radius
1.3 times larger than Jupiter, with just a fifth of the mass, making it one of
the least dense or "fluffiest" exoplanets ever discovered.
The extended nature of fluffy exoplanets makes them easier to observe, and
thus WASP-127b is an ideal candidate for researchers working on atmospheric
characterisation.
The team's observations with the ESPRESSO instrument also suggests that,
unlike planets in our Solar System, WASP-127b orbits not only in the opposite
direction than its star but also in a different plane than the equatorial one.
'Such alignment is unexpected for a hot Saturn in an old stellar system and
might be caused by an unknown companion,' said Allart. 'All these unique
characteristics make WASP-127b a planet that will be very intensely studied in
the future.'
The Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic
Observations (ESPRESSO) is the world's most precise spectrograph for radial
velocity measurements, a method enabling to detect exoplanets.
The authors would like to acknowledge Dr. Jessica Spake and her team for
releasing the refined HST data used in this work.
Reference:
Romain Allart et al, WASP-127b: a misaligned planet with a partly cloudy
atmosphereand tenuous sodium signature seen by ESPRESSO, (2021).
DOI: 10.5194/epsc2021-438
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