Surprise: No methane on the night side of exoplanet WASP-43b
The night side of exoplanet WASP-43b, to the surprise of astronomers, does not appear to contain methane. It is likely that extreme winds do not allow enough time for methane to form in detectable amounts. This is the conclusion of an international team of scientists, with Leiden and Amsterdam contributions, in Nature Astronomy. They followed the exoplanet and its host star for an entire planetary year (19.5 hours) with the James Webb Space Telescope.
The exoplanet WASP-43b is similar in size and mass to Jupiter, but it is a very different type of world. The planet orbits its star extremely close (27 times closer than the distance between Mercury and the Sun). As a result, the planet is tidally locked. This means that it does not rotate on its own axis and that it is permanently day on one side and permanently night on the other. A year on the planet - the time it takes to orbit its star once - lasts only about 19.5 hours.
Temperature map
Thanks to the sharp view and high stability of the MIRI instrument on the James Webb Space Telescope, the researchers were able to record the brightness of the star and planet for an entire planetary year, a so-called phase curve observation. In that way, they could extract the very dim emission from the planet. From this, the researchers constructed a temperature map of the planet's atmosphere.
The permanently exposed day side of WASP-43b turns out to be scorching hot at 1,250 degrees Celsius. The night side is still hot at 600 degrees, but is cooler than expected. The astronomers explain that this is due to the presence of clouds that block radiations from deeper in the planet's atmosphere.
One year on exoplanet WASP-43b
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Watch the video on the original website orOne year on exoplanet WASP-43b. Researchers followed exoplanet WASP-43b for more than a planet's year (19.5 hours). This allowed them to create a temperature map of the planet's atmosphere. On the permanent day side of the planet, the temperature is 1,250 degrees Celsius. On the night side, it is 600 degrees Celsius. (c) Taylor J. Bell et al. (c) T. Müller/MPIA/HdA
Water vapor everywhere
In addition to temperature, astronomers also measured the chemical composition of the planet's atmosphere. They found that water vapour is present in the atmosphere on both the day and night sides of the planet. Previously, the Hubble Space Telescope had found water on the day side of the planet, but the night side was too dark for Hubble.
No methane on the dark side
The team was surprised to find no methane on the night side of the planet. It was already known from the day side that no methane could form there because it is too hot. But the night side, at 600 degrees, is theoretically cool enough for methane production. The researchers propose that extreme winds of up to 7,500 kilometers per hour race across the planet. These do not allow enough time for methane to form in detectable amounts.
This is the first time that researchers have so clearly mapped the temperature and composition of an exoplanet’s atmosphere. In the future, they plan to do this for more exoplanets. Eventually, they hope to use yet-to-be-built telescopes to study the atmospheres of exoplanets that are more like Earth.
Broader view
It's amazing that we can now probe the cold, dark side of exoplanets,’ says Leiden coauthor Nicolas Crouzet. ‘That gives us a much broader view of their atmospheres and really helps understand what is going on there. We have prepared this observation for many years and we are thrilled by all we can learn from it!’
For coauthor Jean-Michel Désert (University of Amsterdam) the research paper marks a culmination of anticipation and wonder. ‘It's been such a journey from when we first designed the phase curve observation to the breathtaking 3D map of this planet.’ According to Désert, studying phase curves in the mid infrared is a gateway to better understanding the energy budget of exoplanets, a fundamental criterion for atmospheres. ‘So in my opinion, this observation signifies a new era in our exploration of exoplanet atmospheres, he concludes.
The MIRI instrument
The MIRI instrument on the James Webb Space Telescope was built through a partnership between Europe and the US. The Dutch Research School for Astronomy (NOVA) was responsible for the main optics of the MIRI spectrometer, with ASTRON and TNO as subcontractors and with contributions from SRON. The Dutch research funder NWO provided financial support. Many scientists from Leiden University also contributed to MIRI.
Scientific paper
Nightside clouds and disequilibrium chemistry on the hot Jupiter WASP-43b. Door: Taylor J. Bell, Nicolas Crouzet, et al. In: Nature Astronomy, 30 April 2024. [original | preprint]
This article appeared as a press release on astronomie.nl