The first science paper about a transiting exoplanet observed by JWST reports the detection of carbon dioxide in the spectrum of WASP-39b.
NASA’s press release on the “Early Release Science” result gives this image:
In addition they have produced this graphic of how transit spectroscopy works:
Among the first 5 images released from the James Webb Space Telescope is its first spectrum of an exoplanet, WASP-96. This shows the clear detection of water vapour, and is the first taster of many such results to come:
Here is the transit light-curve from which the spectrum derives:
The JWST results have been reported in a large fraction of the world’s media outlets. This is the first time the spectrum of an exoplanet has lit up New York’s Times Square:
Of numerous media articles on WASP-96b, here is a typical example from The Atlantic.
As you’ll likely know from flying in an aircraft above the weather, clouds are bright, they reflect a lot of sunlight. This means that if a hot-Jupiter exoplanet has a cloudy atmosphere, then it should also be relatively bright, and so we should be able to detect a discernible drop in light when it it eclipsed behind its host star.
A new paper by Jonathan Fraine et al analyses data obtained with the Hubble Space Telescope WFCS/UVIS instrument to look for the eclipse of WASP-43b. Here is the result (with the data compared to model eclipse profiles):
The authors find no significant eclipse, deriving only an upper limit to any drop in light of 67 parts-per-million, which means that the dayside face of the planet is reflecting less than 6% of the illuminating starlight. And that means “that we can rule out a high altitude, bright, uniform cloud layer”.
Fraine et al remark that “Because of its observational and atmospheric viability for spectroscopic detections, WASP-43b has become a benchmark planet for current and future hot Jupiter observations. Upcoming … JWST observations [will] map the thermal structure and chemical composition of this exoplanet with exquisite detail … We expect that no other exoplanet has or will be observed with this much precision and wavelength coverage for many years to come.”
The importance of cloud-free skies is that one can then see atomic and molecular spectral features much more readily, and so learn much more about the atmosphere’s composition.
James Webb’s “Continuous Viewing Zone” is the patch of sky where the satellite can point continuously at a target and so observe it most efficiently. Exoplanets within the CVZ that are suitable for atmospheric characterisation are thus of high importance, and so far WASP-62b is the only gas giant known within the CVZ.
Munazza Alam et al have now pointed the Hubble and Spitzer space telescopes at WASP-62b to see what its atmosphere looks like. Importantly, they find that WASP-62b has clear skies. This matters since cloudy or haze-filled atmospheres tend to produce flat spectra lacking any spectral features, and so don’t tell us much.
Here, Alam et al plot the spectrum near the sodium (Na) line, showing that it has a broad base, akin to that in the clear-skied planet WASP-96b. The broad base of the line means that it is being widened by “pressure broadening”, and that can only happen deep in the planet’s atmosphere where the pressure is high. And we can only see deep into the atmosphere if it is clear rather than cloudy.
Clear skies mean that spectral features produced by the molecules in the atmosphere should be readily detectable with JWST. Here Alam et al simulate what we expect to see with JWST, showing that Na, H2O, NH3, FeH, SiH, CO, CO2, and CH4 can all be detected.
They conclude by saying that: “As the only transiting giant planet currently known in the JWST Continuous Viewing Zone, WASP-62b could prove a benchmark giant exoplanet for detailed atmospheric characterization in the James Webb era.
The bloated hot-Jupiter WASP-79b has been selected as an Early Release Science target for the James Webb Space Telescope, so is being studied with current facilities such as HST and Spitzer.
Here is a simulation of what the spectrum of WASP-79b might look like when observed with JWST, taken from a new paper by Kristin Sotzen et al.
Sotzen et al have collected together data from HST, Spitzer and the Magellan telescope in order to model the atmosphere of the planet and use that to predict the results of the JWST observations. The different coloured symbols are for different instruments of JWST, namely NIRSpec, NIRCam and NIRISS. The main spectral features are caused by water and carbon dioxide molecules. With a partially cloudy atmosphere and detectable water features, Sotzen et al confirm that WASP-79b is a prime target for JWST.
NASA have written a publicity page on JWST’s plans to study the atmospheres of gas-giant exoplanets, including an animation on how this is done. Since the prime targets for the “Early Release Science” program are three WASP-discovered planets, WASP-18b, WASP-43b and WASP-79b, we “re-blog” the piece here:
“In April 2018, NASA launched the Transiting Exoplanet Survey Satellite (TESS). Its main goal is to locate Earth-sized planets and larger “super-Earths” orbiting nearby stars for further study. One of the most powerful tools that will examine the atmospheres of some planets that TESS discovers will be NASA’s James Webb Space Telescope. Since observing small exoplanets with thin atmospheres like Earth will be challenging for Webb, astronomers will target easier, gas giant exoplanets first.”
With the TESS launch scheduled for this very day, I wrote the following popular-level piece for The Conversation (which has also been re-published by the BBC Focus Magazine).
Previous generations have looked up at the stars in the night sky and wondered whether they are also orbited by planets. Our generation is the first to find out the answer. We now know that nearly all stars have planets around them, and as our technology improves we keep finding more. NASA’s newest satellite, TESS (the Transiting Exoplanet Survey Satellite), scheduled for launch on April 16, 2018, will extend the hunt for small, rocky planets around nearby, bright stars.
NASA’s TESS planet hunter (artist’s impression)
We want to know how big such planets are, what kind of orbits they have and how they formed and evolved. Do they have atmospheres, are they clear or cloudy, and what are they made of? Over the coming decades, we will find Earth-like planets at the right distance from their star for water to be liquid. It’s conceivable that one will have an atmosphere containing molecules such as free oxygen that indicate biological activity. TESS is a major step towards this long-term goal.
Planets are so faint and tiny compared to their host stars that it is remarkable we can detect them at all, let alone study their atmospheres. Yet planets can, from our viewpoint, appear to travel or “transit” across the face of their star as they orbit, blocking a small fraction of the star’s light. TESS will monitor 200,000 bright stars in the solar neighbourhood, looking for tiny dips in their brightness that reveal a transiting planet.
To understand the atmospheres of exoplanets, we have to examine how they interact with starlight. As a planet transits across a star, the thin smear of its atmosphere is backlit by starlight. Some wavelengths of the starlight will be absorbed by molecules in the atmosphere while other wavelengths will shine straight through. So looking at which wavelengths reach us and which don’t can reveal what the atmosphere is made of.
The spectrum of starlight passing through a planet’s atmosphere can tell us what the atmosphere is made of. Credit: Christine Daniloff/MIT, Julien de Wit
Such observations are right at the limit of current capabilities, requiring the James Webb Space Telescope (JWST), the $8 billion successor to Hubble scheduled for launch in 2020. With a 6.5-metre-wide mirror, collecting much more light than Hubble ever could, and with specially designed instruments, JWST has been built to study exoplanet atmospheres.
In order to use JWST most effectively, we first need to know which stars host the best transiting exoplanets to study, and that’s why we need TESS. Its predecessor spacecraft, Kepler, surveyed 150,000 stars in a patch of sky near the constellation Cygnus, and found over a thousand planets ranging from gaseous giants like Jupiter to rocky planets as small as Mercury. But Kepler covered only a small patch of sky containing few stars bright enough for us to study their planets.
In contrast, ground-based telescopes have searched wider swathes of the sky looking at many more brighter stars for transiting exoplanets. The most successful has been the UK-led Wide Angle Search for Planets (WASP) project, of which I am a member. Using an array of camera lenses, WASP has spent the last decade monitoring a million stars every clear night looking for transit dips, and has found nearly 200 exoplanets, some of which have now been chosen as targets for JWST.
But ground-based transit surveys have one big limitation: they look through Earth’s atmosphere and that severely limits the data quality. They can detect brightness dips as small as 1%, which is sufficient to find giant gaseous planets that are like our own Jupiter and Saturn. But smaller, rocky planets block out far less light. Our Earth would produce a dip of only 0.01% if seen projected against our sun.
The JWST is currently being readied for launch. Credit: NASA/Chris Gunn
TESS will combine the best of both these approaches, observing bright stars over the whole sky with the advantage of doing so from space. It should find the small, rocky planets that Kepler proved are abundant but find them orbiting stars that are bright enough for us to study their atmospheres with JWST.
TESS will typically observe each region of sky for 30 days. This means that it will detect planets that don’t take long to orbit their stars and so will produce several transits while TESS is looking at them. Planets with short orbits are located close to their stars, meaning that most planets TESS finds will be too hot for liquid water. But planets orbiting dimmer, cooler red dwarf stars might be at the right temperature for life even if they are so close. The dwarf star TRAPPIST-1 is 1,000 times dimmer than our sun, and is known to host seven closely orbiting planets.
While TESS looks for planets orbiting dwarf stars from space, the SPECULOOS survey will be looking at even smaller and dimmer stars from the ground. Any planets it finds will be prime targets for JWST.
This exploration is a step towards finding rocky planets in the habitable zone of stars like our sun. In 2026, The European Space Agency is expected to launch PLATO, a satellite with the potential to discover rocky planets in Earth-like orbits with periods of a year. The race will then begin to find biomarker molecules, such as free oxygen, in the atmosphere of an Earth-like exoplanet.
With the launch of the James Webb Space Telescope only a year away the exoplanet community is gearing up to exploit its capability for characterising exoplanet atmospheres. A new paper by Yu et al contains a plot of the best targets, giving the expected “signal to noise” for each planet as a function of the planet’s mass. The higher the S/N the better, enabling more atmospheric features to be discerned.
It is notable that most of the best targets do not come from Kepler (which had a relatively small field of view, and so looked at mainly fainter stars), but instead from the ground-based transit surveys (which focus mainly on brighter stars, which are thus better targets for follow-up). WASP features strongly, supplying half of the best targets.
The focus of the Yu et al paper, however, is the discovery of two very good targets from the K2 phase of Kepler‘s mission. K2 is observing more fields for less time than the original Kepler, and so covers more bright stars.
HD 89345b (labelled in red above) is only 10% of Jupiter’s mass but is bloated to 0.6 Jupiter radii. Transiting a bright star of V = 9.4 makes it a prime target.
The transit depth of only 0.15% means that it is too shallow to have been detected by WASP (which can do 0.2–0.3% at best), especially given the 11.8-day orbit, which means that it produces fewer transits than shorter-period planets.
The other new discovery, HD 286123b (which had also been independently found by Brahm et al), is a larger and more massive planet producing a 0.8% dip. This one should have been within the reach of the WASP survey, but happens to lie in a region of the Northern sky where SuperWASP-North has only limited data.
Studying the atmospheres of exoplanets is one of the main goals of the James Webb Space Telescope, now scheduled for launch in mid 2019. The mission recently asked for proposals for “Early Release Science”, observations to test out the instruments, show what JWST can so, and supply the community with data to start analysing.
Of 13 ERS proposals accepted, the “The Transiting Exoplanet Community ERS Program”, led by Kepler lead-scientist Natalie Batalha, got all the time it asked for.
WASP planets feature heavily in the ERS program, since many transit relatively bright stars. Large, puffy gaseous planets will also give the strongest and clearest signals of atmospheric features, and so are optimum early targets. While JWST will want to look also at atmospheres of smaller, rocky planets, “Astronomers initially will train their gaze onto gaseous Jupiter-sized worlds like WASP-39b and WASP-43b because they are easier targets on which to [look for the chemical fingerprints of the atmosphere’s gases]”.
The target list for the ERS proposal is currently being finalised in the light of the recent delay in JWST launch from 2018 to 2019, though an earlier draft of the proposal featured 7 WASP planets out of 12 targets.
Further, the four GTO teams have also selected WASP planets for early JWST observations. GTO time (“Guaranteed Time Observations”) is time allocated to the teams who built the JWST instruments as a reward and incentive. All four instrument teams have picked WASP planets, including WASP-17b, WASP-52b, WASP-43b, WASP-69b, WASP-77Ab, WASP-80b, WASP-107b and WASP-121b.
Meanwhile, Kevin Heng, of the University of Bern, has written a popular-level account for American Scientist of how JWST is expected to revolutionise the study of exoplanet atmospheres.
WASP Planets 