Exoplanet surveys over the past 25 years have revealed that the "sub-Neptune planets" are the most common type of astronomical bodies in the Milky Way galaxy. These are planets that are intermediate in size between the Earth and Neptune and orbit closer to their host stars than Mercury does the Sun.
Many questions remain unanswered regarding the nature of sub-Neptune planets, but transmission spectroscopy with the Hubble Telescope provides more details. For instance, observation of the transiting exoplanet named Gliese 9827 d offers an opportunity to characterize the atmosphere of a warm sub-Neptune.
What is Gliese 9827d?
Gliese 9827 d is a sub-Neptune planet that belongs to the planetary system around the star GJ 9827, a bright K-type red dwarf located in the constellation of Pisces. This small exoplanet is the outermost planet of a trio of transiting massive exoplanets discovered by NASA's Kepler/ K2 mission.
Also known as GJ 9827 d, this planet is only 97 light years away from us and completes its revolution around its star in just 6.2 days.
GJ 9827 d was discovered in 2017 by the defunct Kepler space telescope. Its radius is only twice that of the Earth but smaller than Neptune, even though its mass would be more significant. It is also believed to be overheated due to its average temperature of 430 degrees Celsius.
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Water-Rich Atmosphere
Led by scientists from the University of Montreal, part of the atmosphere's composition of GJ 9827 d was determined during planetary transits. As explained in the study "Water Absorption in the Transmission Spectrum of the Water World Candidate GJ 9827 d", GJ 9827 d could be a new type of planet whose atmosphere would be composed mainly of water vapor.
To confirm the presence of water vapor in the atmosphere of GJ 9827 d, the Hubble telescope was used for three years to monitor 11exoplanet transits in front of its star. The light that passed through the atmosphere of this exoplanet generated lines of absorption, which acted like barcodes of the chemical compounds present.
However, scientists are still unsure whether the small amount of water vapor in an extended, hydrogen-rich atmosphere or the gaseous blanket is mostly water. Two possible scenarios were proposed regarding the origin of GJ 9827, which could have led to the emergence of water vapor in its atmosphere.
In the first scenario, the exoplanet initially resembled Neptune with a high ice content surrounded by an atmosphere of hydrogen and helium. Then, it could have migrated, leading to the initial atmosphere's evaporation and melting ice. In the second scenario, the atmosphere of GJ 9827 d contains only traces of water vapor because its closeness to its host star resulted in a low water content.
According to Björn Benneke, this study is the first time that scientists can directly demonstrate, through atmospheric detection, that planets with water-rich atmospheres exist around other stars. This could be an essential step towards a better understanding of the diversity of atmospheres of rocky planets.
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