Taylor J. Bell

Image Credit: NASA, ESA, CSA, Ralf Crawford (STScI)

Introduction

In a paper recently published in Nature Astronomy, the JWST Transiting Exoplanet Early Release Science (JTEC-ERS) team observed the entire orbit of WASP-43b using the Low-Resolution Spectrometer on NASA's JWST's Mid-Infrared Instrument (MIRI). These "phase curve" observations were used to measure the temperature across the entire planet and indicate that the planet's dayside reaches an intense 1250°C (2285°F), as hot as a blacksmith's forge, while the planet's nightside is still unbearably hot at 600°C (1115°F). The team also found water throughout the planet's atmosphere, a surprising lack of methane on the planet's nightside, and a thick layer of clouds covering the planet's nightside. Dr. Taylor Bell from the Bay Area Environmental Research Institute (BAERI) and Dr. Joanna Barstow from The Open University tell us more about the significance of these exciting discoveries and how these new Webb observations enabled them.

Background

WASP-43b is similar in size and mass to Jupiter, but it is a very different kind of world. Its host star, WASP-43, is much cooler and redder than our Sun and is ~280 lightyears away from the Earth in the constellation Sextans (near Leo). WASP-43b orbits extremely close to its star (70 times closer than the Earth orbits the Sun and 27 times closer than Mercury does), which means that the planet's year is only ~19.5 hours long; that's about the same length as the combined length of the Lord of the Rings Extended Trilogy and the Hobbit Trilogy (which sum to 19hr 16min). The planet's close proximity to its star resulted in the day and year of the planet becoming synchronized, so one half of the planet is permanently illuminated (called the planet's dayside) and very hot, while one half is permanently shadowed (called the nightside) and much colder.

The small separation between the planet and star as well as their large distance from us means that the planet cannot be visually separated from its star, and instead astronomers need to use indirect methods of studying the planet. The most popular observing method is called the transit method, where the planet passes in front of its star from our perspective, causing a faint dimming in the starlight we receive from the system. Some small fraction of the light coming from the system also comes from the planet itself; at short wavelengths of light the planet may reflect some of the starlight, while at longer wavelengths the planet emits some of its own infrared radiation. A common way of studying the light coming from the planet is the secondary eclipse method where the planet passes behind its star from our perspective, allowing us to compare the total amount of light with the star and the planet to the light just coming from the star when the planet isn't visible. Finally, the method used by the JTEC-ERS team is called the phase curve method where astronomers observe a system for an entire orbit of the planet which allows the astronomers to study the amount of light coming from all different parts of the planet. Since the planet's dayside is much hotter than its nightside, phase curve observations typically look like a sine wave with a peak in brightness when the planet's dayside is pointed towards us and a trough in brightness when the planet's nightside is pointed towards us.

Figure 1: Measured MIRI Phase Curve

Illustration Credits: NASA, ESA, CSA, Ralf Crawford (STScI)

Science Credits: Taylor Bell (BAERI), Joanna Barstow (The Open University), Michael Roman (University of Leicester)

All visible matter in the universe emits some amount of light that depends on the object's temperature. Hotter objects glow bright (like stars, hot metal, etc.) while colder objects may only emit lower-energy infrared light (like the Earth and even humans). Many readers will be familiar with the non-contact thermometers that became widespread during the COVID-19 pandemic which measure the infrared radiation emitted from your forehead to determine whether you have a fever. These non-contact thermometers measure your body temperature using light with a wavelength of 8–14 microns. Similarly, Webb's MIRI/LRS instrument is sensitive to light with wavelengths between 5–12 microns. This basically makes Webb a gigantic non-contact infrared thermometer which can remotely measure the temperature of planets tens or hundreds of lightyears away. Additionally, by studying the infrared radiation as a function of wavelength, MIRI/LRS allows astronomers to study molecules in the distant planet's atmosphere which absorb light in specific regions of the detector's wavelength range. Previous phase curve observations using the Spitzer Space Telescope (e.g., Murphy+2023) and the Hubble Space Telescope (e.g., Stevenson+2014) also used a similar method, although Spitzer's phase curves were not spectroscopic (making their interpretation more challenging) and Hubble's phase curves had much less wavelength coverage and were at much shorter wavelengths where reflected stellar light can be a complicating factor.

Results

The JTEC-ERS team measured WASP-43b's temperature throughout the planet's entire orbit in order to study the planet's climate. At any one point in time, an entire half of the planet is visible from our perspective, with the portion of the planet visible to us changing as the planet goes around its star. A plot of the measured temperature of the hemisphere facing Earth is shown at the bottom of Figure 1. The team was then able to use those observations to reconstruct a coarse map of the temperature across the planet. The team found that WASP-43b's permanently-illuminated dayside is as hot as a blacksmith's forge at 1250°C (2285°F). Meanwhile, the planet's permanently-shadowed nightside is still very hot at 600°C (1115°F). Since the planet's nightside doesn't receive any heat directly from its star, all of that heat has been brought from the planet's dayside through extremely strong winds. These winds blow across the face of the planet toward the East, pushing the hottest part of the atmosphere slightly eastward of the most heavily illuminated part of the planet. Comparisons of the planet's temperature map with complex, 3-dimensional atmospheric models demonstrate that the temperature contrast between the dayside and nightside is stronger than would be expected for a cloud-free atmosphere; a similar conclusion was previously made using data from the Hubble and Spitzer Space Telescopes. These models clearly suggest that the planet's nightside is shrouded in a thick layer of clouds which blocks much of the infrared radiation we would otherwise see. The exact type of clouds is still unknown, but they will not be water clouds like those on Earth since the planet is far too hot for water to condense; instead, clouds made of rocks and minerals are expected at these temperatures.

Figure 2: Inferred Temperature Maps

Illustration Credits: NASA, ESA, CSA, Ralf Crawford (STScI)

Science Credits: Taylor Bell (BAERI), Joanna Barstow (The Open University), Michael Roman (University of Leicester)

In addition to measuring the temperature across the planet, spectroscopic data can also be used to measure the chemical composition of the planet's atmosphere. Previous observations from Hubble found water on the planet's dayside, but the planet's nightside was too dark for Hubble to clearly detect water in those observations; meanwhile, previous Spitzer observations were not spectroscopic and could not meaningfully probe the planet's composition. Using these new Webb observations, the JTEC-ERS team found that there were clear signals of water vapor in the planet's emission spectrum at all points across the planet which is consistent with model predictions for a planet of this size and temperature. Interestingly, the fact that water vapor is still visible on the planet's nightside despite the thick layer of clouds allows the team to estimate the height and thickness of the clouds for the first time, finding that they are very high and thick compared to typical clouds on Earth.

Figure 3: Inferred Disequilibrium Chemistry

Background Illustration Credits: NASA, ESA, CSA, Ralf Crawford (STScI)

Foreground Diagram Credits: Joanna Barstow (The Open University)

Finally, the JTEC-ERS team also found a surprising lack of methane on the WASP-43b's nightside. The planet's dayside is too hot for methane to form — instead, most of the carbon in the atmosphere is paired with oxygen to form carbon monoxide. However, the nightside is cold enough that we would expect to see methane features. The fact that we don't see methane tells us something important; WASP-43b must be very windy, with wind speeds reaching something like 8000 km/hr (5000 mph). If winds move the gas in the planet's atmosphere around from the dayside to the nightside and back again fast enough, then there won't be enough time for the expected chemical reaction converting carbon monoxide to methane to take place. This wind-driven mixing (an example of "chemical disequilibrium") means that the atmospheric chemistry is the same all around the planet, something that we couldn't tell from past work with Hubble and Spitzer.

Additional NIRSpec/G395H observations of WASP-43b by another team led by Dr. Stephan Birkmann from the European Space Agency (ESA) will probe the planet at wavelengths between 2.87–5.10 microns. Together with the JTEC-ERS team's MIRI/LRS observations, this will give remarkably broad wavelength coverage which will increase the fidelity of the inferred temperature map and the chemical composition of the planet's atmosphere. In particular, Dr. Birkmann's NIRSpec observations will be sensitive to carbon monoxide which these MIRI observations suggest should be prevalent throughout the atmosphere. Broader wavelength coverage may also give more insights into the cloud distribution and composition. Future work will also combine the new information gained from these Webb observations with those of previous observations from the Hubble and Spitzer Space Telescopes which will also add to the wavelength coverage. Finally, JWST phase curve observations of many other planets have also already been approved and are gradually being published (e.g. WASP-121b, GJ 1214b, GJ 367b), and it's clear these observations will provide exciting new insights into the dynamics and chemistry occurring in the atmospheres of worlds far from our own.

About the authors

• Dr. Taylor Bell is a postdoctoral research scientist at the Bay Area Environmental Research Institute (BAERI), working at the NASA Ames Research Center in Mountain View, California.
• Dr. Joanna Barstow is a STFC Ernest Rutherford Fellow at The Open University in Milton Keynes, UK.

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