JWST quest to study two exoplanets
Combined image of two extrasolar planets observed with JWST
In July 2024, astronomers published astonishing results about two extrasolar planets observed with the James Webb Space Telescope (JWST).
WASP-39b
The first result, published in the article entitled: “Inhomogeneous terminators on the exoplanet WASP-39 b,” and published in Nature, talks about the boundary region between day and night in an exoplanet [1].
In 2011, scientists discovered WASP-39b, an exoplanet 700 light-years from Earth. Its inflated and puffy atmosphere, filters the strong signal from the starlight through the gas, allowing astronomers to do transmission spectroscopy to determine its composition.
These properties put WASP-39b on the list of Early Release Science Program (ERS), making it the first exoplanet observed by this powerful telescope. WASP-39b was the first exoplanet observed by this powerful telescope. The initial images, released in August 2022, used transmission spectroscopy to confirm the presence and amount of CO2 in the atmosphere of this giant [2].
Artist concept of WASP-39b. Credit: NASA, ESA, CSA, Joseph Olmsted (STScI)
However, transmission spectroscopy assumes the planet has a uniform atmosphere, which might not be the case for WASP-39b. From previous observations, we know that the planet orbits very close to WASP-39 and is tidally locked, much like the Moon is to Earth. This configuration puts one side always facing the star, heating the gas to much larger temperatures than on the night side. The pressure difference on both sides must lead to large instabilities in the atmosphere, putting into question the predictions from transmission spectroscopy.
A group of astronomers, led by Dr. Nestor Espinoza, decided to check this. Using the same ERS observations, they focused on the study of the gas around the distinct and fixed boundary between the dayside and nightside, known as “the terminator.”
Using the orbital parameters of this exoplanet and 3D General Circulation Models, similar to those used to predict weather patterns on Earth, the team determined, with considerable precision, the temperature on both sides of “the terminator.” The results indicate that the evening border is 1,450 oF, hotter than the morning edge, with only 1,150 o F.
Astronomers explain that the apparent flip in temperatures results from the movement of the gas from one side to the other via a powerful equatorial jet stream. These arise from the air pressure difference in the hot day and cold night, producing winds that reach thousands of miles an hour. The jets circulate the whole planet and cross both Terminators, resulting in a cooler edge on the morning side. In other words, the morning limb gets slammed with air cooled on the night side, and the evening limb is hit by air heated on the dayside [3].
Although the observations show that the Terminator has a dynamic atmosphere, the composition and the metallicity still agree with the ratios found previously via the transmission models. This result is reassuring to astronomers.
Epsilon Indi Ab
The second result appeared in the Nature paper: “A temperate super-Jupiter imaged with JWST in the mid-infrared.” It tells us about the direct observation of one of the coldest exoplanets observed to date, Epsilon Indi Ab[4].
Epsilon Indi A is the host star to this exoplanet and is a K-type star, similar in age to our Sun but slightly cooler. The star grabbed the attention of the astronomical community after they found it had a wobble. They concluded that the wobble indicated the existence of a planet orbiting the star and disrupting its movement. In 2018, a group of astronomers confirmed this trend, determining that the orbiting planet was a gas giant about 3 times the mass of Jupiter. They also calculated a nearly circular orbit with a semi-major axis of 12 AU and an orbital period of about 53 years [5].
Because a planet like this emits most of its light in the infrared, a group of astronomers proposed to use JWST to observe it. They obtained a direct image Using the choronagraph of the Mid-Infrared Instrument (MIRI), and confirmed previous findings. The team confirmed the planet had several times the mass of Jupiter and an estimated temperature of 35 oF. This is one of the coldest exoplanets detected, colder than any observed planet beyond our solar system [2A].
Epsilon Indi Ab observed with JWST’s MIRI coronagraph. Credit: NASA, ESA, CSA, STScI, Elisabeth Matthews (MPIA)
From the observations, the team infers a very cloudy atmosphere for this planet. They also estimate that its composition is mainly methane, carbon monoxide, and carbon dioxide.
However, there are still unanswered questions, in particular about the differences in the predicted properties. For example, the observed planet is twice as massive, a little farther from its star, and has a different orbit than expected. Its atmosphere also appears to be somewhat different than the model predicted.
Astronomers expect that with more observations they can confirm its composition and answer other questions astronomers still have about the Epsilon Indi system.
More information about WASP-39b
WASP-39b is an exoplanet that orbits a Sun-like star roughly 700 light-years away in the constellation of Virgo. It orbits extremely close to its star (less than 1/20th the distance between Earth and the Sun) and completes one orbit in just over 4 Earth days. Its mass is 0.28 times that of Jupiter (0.94 times Saturn) and has a diameter 1.3 times greater than Jupiter. Its parent star, WASP-39, is roughly the same size, mass, temperature, and color as the Sun[2A].
More information about Epsilon Indi A and Ab
Epsilon Indi A is a K5V star about 3.6pc (or about 12 light-years) from the Sun and has a binary companion. A direct image taken in 2004 with HST/NICMOS, shows this pair, two brown dwarfs known as Eps Ind Ba/Bb. The planet Epsilon Indi Ab, discovered in 2019, is one of the coldest exoplanets observed to date. With a mass 3.25 times the mass of Jupiter and at 11.55 AU from its star, it takes 45.2 years to complete one orbit around it.
What is a transmission spectrum?
A transmission spectrum is the difference between the spectrum from the starlight and the exoplanet. When the exoplanet passes in front of the star, it lets the starlight pass through its atmosphere. When the exoplanet is at the side of the star, we detect only the exoplanet’s atmosphere. Making that comparison provides information about the temperature, composition, and other properties of the exoplanet’s atmosphere.
How do coronagraphic observations work?
In these observations, the star is behind a mask, usually built on the detector. The mask, also known as a coronagraph, covers the star completely and blocks its intense light. This allows the dimmer light from a surrounding planet to stick out against the background
References
[1] Espinoza, N., Steinrueck, M.E., Kirk, J. et al. Inhomogeneous terminators on the exoplanet WASP-39 b. Nature (2024). https://doi.org/10.1038/s41586-024-07768-4
[2] JWST Transiting Exoplanet Community Early Release Science Team. Identification of carbon dioxide in an exoplanet atmosphere. Nature 614, 649–652 (2023). https://doi.org/10.1038/s41586-022-05269-w
[4] Matthews, E.C., Carter, A.L., Pathak, P. et al. A temperate super-Jupiter imaged with JWST in the mid-infrared. Nature (2024). https://doi.org/10.1038/s41586-024-07837-8
[5] Feng, F. et al. 2019, MNRAS,.490,.5002F