From Astrobites And The NASA/ESA/CSA James Webb Space Telescope: “Webb’s First Directly Imaged Exoplanet”

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From Astrobites

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The NASA/ESA/CSA James Webb Space Telescope

9.1.22
Briley Lewis

Title: JWST Early Release Science: High Contrast Imaging of the Exoplanet HIP 65426 b from 2−16 µm

Authors: A. L. Carter, S. Hinkley, J. Kammerer, et al.

First author’s institution: The University of California-Santa Cruz

Status: Open access

This has been the summer of JWST. NASA’s newest major space telescope — a huge step up in size and capabilities from past observatories — finally began its science observations in June after a lengthy commissioning process. So far, we’ve seen stunning vistas of star forming nebulae and an incredible deep field showing lensed galaxies in JWST’s first images. Now, we’re starting to get exciting results from the Early Release Science (ERS) teams, who proposed creative ways to use the telescope in its first few months of observations, such as insight into some of the oldest galaxies we’ve seen.

Although JWST is somewhat known for its galaxy-hunting capabilities since mid-infrared observations allow us to see the distant redshifted parts of the Universe, it’s also going to be incredible for exoplanet science. One of the first “images” released was a transit spectrum of WASP-96b’s steam-filled atmosphere, and a recent ERS result made the first unambiguous detection of CO2 in its atmosphere, too.

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NASA’s James Webb Space Telescope has captured the distinct signature of water, along with evidence for clouds and haze, in the atmosphere surrounding a hot, puffy gas giant planet orbiting a distant Sun-like star.

The observation, which reveals the presence of specific gas molecules based on tiny decreases in the brightness of precise colors of light, is the most detailed of its kind to date, demonstrating Webb’s unprecedented ability to analyze atmospheres hundreds of light-years away.

While the Hubble Space Telescope has analyzed numerous exoplanet atmospheres over the past two decades, capturing the first clear detection of water in 2013, Webb’s immediate and more detailed observation marks a giant leap forward in the quest to characterize potentially habitable planets beyond Earth.

WASP-96 b is one of more than 5,000 confirmed exoplanets in the Milky Way. Located roughly 1,150 light-years away in the southern-sky constellation Phoenix, it represents a type of gas giant that has no direct analog in our solar system. With a mass less than half that of Jupiter and a diameter 1.2 times greater, WASP-96 b is much puffier than any planet orbiting our Sun. And with a temperature greater than 1000°F, it is significantly hotter. WASP-96 b orbits extremely close to its Sun-like star, just one-ninth of the distance between Mercury and the Sun, completing one circuit every 3½ Earth-days.

The combination of large size, short orbital period, puffy atmosphere, and lack of contaminating light from objects nearby in the sky makes WASP-96 b an ideal target for atmospheric observations.

On June 21, Webb’s Near-Infrared Imager and Slitless Spectrograph (NIRISS)[below] measured light from the WASP-96 system for 6.4 hours as the planet moved across the star. The result is a light curve showing the overall dimming of starlight during the transit, and a transmission spectrum revealing the brightness change of individual wavelengths of infrared light between 0.6 and 2.8 microns.

While the light curve confirms properties of the planet that had already been determined from other observations – the existence, size, and orbit of the planet – the transmission spectrum reveals previously hidden details of the atmosphere: the unambiguous signature of water, indications of haze, and evidence of clouds that were thought not to exist based on prior observations.

A transmission spectrum is made by comparing starlight filtered through a planet’s atmosphere as it moves across the star to the unfiltered starlight detected when the planet is beside the star. Researchers are able to detect and measure the abundances of key gases in a planet’s atmosphere based on the absorption pattern – the locations and heights of peaks on the graph. In the same way that people have distinctive fingerprints and DNA sequences, atoms and molecules have characteristic patterns of wavelengths that they absorb.

The spectrum of WASP-96 b captured by NIRISS is not only the most detailed near-infrared transmission spectrum of an exoplanet atmosphere captured to date, but it also covers a remarkably wide range of wavelengths, including visible red light and a portion of the spectrum that has not previously been accessible from other telescopes (wavelengths longer than 1.6 microns). This part of the spectrum is particularly sensitive to water as well as other key molecules like oxygen, methane, and carbon dioxide, which are not immediately obvious in the WASP-96 b spectrum but which should be detectable in other exoplanets planned for observation by Webb.

Researchers will be able to use the spectrum to measure the amount of water vapor in the atmosphere, constrain the abundance of various elements like carbon and oxygen, and estimate the temperature of the atmosphere with depth. They can then use this information to make inferences about the overall make-up of the planet, as well as how, when, and where it formed. The blue line on the graph is a best-fit model that takes into account the data, the known properties of WASP-96 b and its star (e.g., size, mass, temperature), and assumed characteristics of the atmosphere.

The exceptional detail and clarity of these measurements is possible because of Webb’s state-of-the-art design. Its 270-square-foot gold-coated mirror collects infrared light efficiently. Its precision spectrographs spread light out into rainbows of thousands of infrared colors. And its sensitive infrared detectors measure extremely subtle differences in brightness. NIRISS is able to detect color differences of only about one thousandth of a micron (the difference between green and yellow is about 50 thousandths of a micron), and differences in the brightness between those colors of a few hundred parts per million.

In addition, Webb’s extreme stability and its orbital location around Lagrange Point 2 roughly a million miles away from the contaminating effects of Earth’s atmosphere makes for an uninterrupted view and clean data that can be analyzed relatively quickly.

The extraordinarily detailed spectrum – made by simultaneously analyzing 280 individual spectra captured over the observation – provides just a hint of what Webb has in store for exoplanet research. Over the coming year, researchers will use spectroscopy to analyze the surfaces and atmospheres of several dozen exoplanets, from small rocky planets to gas- and ice-rich giants. Nearly one-quarter of Webb’s Cycle 1 observation time is allocated to studying exoplanets and the materials that form them.

This NIRISS observation demonstrates that Webb has the power to characterize the atmospheres of exoplanets—including those of potentially habitable planets—in exquisite detail.

JWST is also going to do wonders for direct imaging — the notoriously tricky way of detecting exoplanets, where we actually resolve light from the exoplanet itself instead of observing the host star. The NIRCam [below] and MIRI [below] instruments have coronagraphs, small optics that block the light from a bright host star so you can see the comparatively faint planets orbiting them. NIRISS [below] also has the ability to do a cutting-edge technique called non-redundant aperture masking and NIRSpec [below] and MIRI have a type of instrument called an Integral Field Unit (IFU), all of which can help directly detect planets. Lucky for us (or more accurately, thanks to the folks who planned this mission), it’s also easier to directly detect exoplanets in the infrared, making JWST extremely well-suited to this task.

The JWST Direct Imaging ERS team (ERS-1386) is now testing out these direct imaging capabilities, with the goal of determining how well these modes of observing are performing and coming up with advice for observers who want to use these modes in the future. The expectation is that JWST will be able to image planets smaller than Jupiter — which is a big deal! So far, we haven’t been able to spot a planet smaller than ~2 Jupiter masses from the ground.

In today’s bite, we share a hot-off-the-presses result from the JWST Direct Imaging ERS Team: the first directly imaged exoplanet observed with JWST, HIP 65426 b. This is also the first direct detection (ever!) of an exoplanet at wavelengths longer than 5 microns.

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Newly released image of HIP 65426 b in multiple wavelength bands, as seen by JWST NIRCam and MIRI. Image from A. Carter (UCSC) NASA/ESA/CSA, , the JWST ERS 1386 team, and A. Pagan (STScI).

HIP 65426 b is a Super-Jupiter sized planet that was already known to us originally discovered with ground-based observations around 2017. It’s part of the Lower Centaurus-Crux association, a grouping of stars that were all born near each other, including the relatively famous PDS 70 b. Moving group associations like this make it possible for us to estimate stellar ages — for example, HIP 65426 is around 14 million years old. This star and its planetary companion were chosen as a relatively easy target to test out JWST’s capabilities, and see what more we could learn about this planet! The team observed it with NIRCam, which covers 2 to 5 microns across five different filters, and MIRI, covering 11-16 microns over two filters — and it was detected in all seven filters, as seen below.

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HIP 65426 b as seen by NIRCam and MIRI in seven different filters, after data processing and PSF subtraction using ADI+RDI KLIP. Figure 8 from the paper.

The “hamburger-like” shape in some of the wavelengths is an expected effect, just an artifact from the Lyot stop (part of the coronagraph). There aren’t actually multiple real sources there, unfortunately! In addition to the images, the team presented astrometry, photometry, model fits, and more for this observation.

Since JWST has such a large span of wavelength coverage in the infrared, they were able to constrain this exoplanet’s bolometric luminosity (its energy output across all wavelengths) like never before. No matter what model atmosphere they used, their result was the same thanks to the incredible data they had on hand! Their photometry of the planet was also incredibly precise — 7% precision for NIRCam and 16% for MIRI, compared to the ground-based observations of this star at 13-32% precision. The team also investigated the planet’s spectral energy distribution (SED) using the new photometry, alongside old measurements, shown below.

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SED for HIP 65436 b including existing data from VLT/SPHERE and VLT/NACO, plus the new data from JWST presented in this work. An atmospheric model fit is shown in blue, with the residual error between the best fit model and the data on the bottom. Figure 9 from the paper.

Using models of the planet’s thermal evolution and atmospheric models, they derived the mass (7.1 Jupiter masses), temperature (1282 K), and radius (1.45 Jupiter radii) of HIP 65426 b. The constraint on radius is ~3x more precise than before the JWST data! They also fit for the planet’s orbit using the package orbitize! and found a semi-major axis of 87 AU and inclination of 100 degrees, which agrees with but doesn’t significantly improve on past measurements.

To further quantify just how well JWST is doing things, they computed contrast curves for these observations, shown below. Contrast curves show the contrast (how faint of a planet you can detect, compared to its star, at 5 sigma confidence) versus the separation (how far the planet is from the star). So far, JWST appears to be outperforming expectations by about a factor of 10!

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Contrast curves from JWST observations of HIP 65426 b, shown as the black lines. Different line styles represent different data processing (ADI, RDI, ADI+RDI). The vertical dashed red line shows the inner working angle defined by the coronagraphic mask, and the blue lines are previous predictions of JWST’s capabilities using a package called PanCAKE. Excitingly, JWST is outperforming the predictions! Figure 5 from the paper.

The team also estimated what kinds of planets JWST would be able to detect around this star. From their calculations, NIRCam can easily find a sub-Jupiter mass planet between 150-2000 AU from its star, and might be able to find something as small as 0.4 Jupiter masses. That’s still about 120x bigger than an Earth-like planet, but it’s significantly smaller than what we’ve been able to find with direct imaging before! MIRI is a little less sensitive, able to detect 1-2 Jupiter mass planets from 150-2000 AU.

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Diagrams (often called “tongue plots”) showing the detectable masses and semi-major axes of planets with NIRCam (top) and MIRI (bottom), for the most sensitive filter according to these observations. Figure 6 from the paper.

Since one of the goals of the ERS program is to make recommendations for best use of the instruments going forward, we’re going to describe a few of these technical pieces of advice here, too. If technical details of coronagraphs are not your jam, feel free to skip this next paragraph!

One of the tricky parts of direct imaging is always aligning the coronagraph to perfectly cover the star, and they report that coronagraph alignment is still being perfected for NIRCam, whereas the MIRI procedure is further refined, accurate down to 0.1 pixels. The other notorious step of direct imaging is data processing, and this team tried both reference star differential imaging (RDI) and angular differential imaging (ADI) from different “rolls” of the spacecraft. They ended up using a combo of ADI and RDI for this paper, but suggest that future observations can just pick one or the other depending on their needs. RDI worked better in this case, but ADI may be better for wider separations. To do the data processing, they used spaceKLIP, an adaptation of pyKLIP, an image processing algorithm widely used for direct imaging work.

There is so much more to come with JWST observations, especially in the realm of direct imaging. For example, the authors suggest that for fainter M-type stars, JWST may be able to image even smaller planets than estimated in this investigation. They say in the paper, “It will be possible to detect Uranus and Neptune mass objects beyond 100−200 AU, and Saturn mass objects beyond ∼10 AU [around M dwarfs].” They also mention that JWST’s incredibly precise infrared data might allow measurements of CH4 and CO, providing a fascinating window into the complex chemistry of giant planet atmospheres.

This result is clearly a harbinger (and a thrilling one at that!) of much more to come — the team even mentions other ERS results coming soon, including a different look into HIP 65426 b, observations of a circumstellar disk, and spectroscopy of another substellar object. Direct imaging, welcome to the era of JWST!

See the full article here .


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National Aeronautics Space Agency/European Space Agency [La Agencia Espacial Europea] [Agence spatiale européenne][Europäische Weltraumorganisation](EU)/ Canadian Space Agency [Agence Spatiale Canadienne](CA) James Webb Infrared Space Telescope annotated, finally launched December 25, 2021, ten years late.

The NASA/ESA/CSA James Webb Space Telescope is a large infrared telescope with a 6.5-meter primary mirror. Webb was finally launched December 25, 2021, ten years late. The James Webb Space Telescope will be the premier observatory of the next decade, serving thousands of astronomers worldwide. It will study every phase in the history of our Universe, ranging from the first luminous glows after the Big Bang, to the formation of solar systems capable of supporting life on planets like Earth, to the evolution of our own Solar System.

The James Webb Space Telescope is the world’s largest, most powerful, and most complex space science telescope ever built. Webb will solve mysteries in our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it.

Webb telescope will be the premier observatory of the next decade, serving thousands of astronomers worldwide. It will study every phase in the history of our Universe, ranging from the first luminous glows after the Big Bang, to the formation of solar systems capable of supporting life on planets like Earth, to the evolution of our own Solar System.

Webb telescope was formerly known as the “Next Generation Space Telescope” (NGST); it was renamed in Sept. 2002 after a former NASA administrator, James Webb.

Webb is an international collaboration between National Aeronautics and Space Administration, the European Space Agency (ESA), and the Canadian Space Agency (CSA). The NASA Goddard Space Flight Center managed the development effort. The main industrial partner is Northrop Grumman; the Space Telescope Science Institute will operate Webb after launch.

Several innovative technologies have been developed for Webb. These include a folding, segmented primary mirror, adjusted to shape after launch; ultra-lightweight beryllium optics; detectors able to record extremely weak signals, microshutters that enable programmable object selection for the spectrograph; and a cryocooler for cooling the mid-IR detectors to 7K.

There are four science instruments on Webb: The Near InfraRed Camera (NIRCam), The Near InfraRed Spectrograph (NIRspec), The Mid-InfraRed Instrument (MIRI), and The Fine Guidance Sensor/ Near InfraRed Imager and Slitless Spectrograph (FGS-NIRISS). Webb’s instruments are designed to work primarily in the infrared range of the electromagnetic spectrum, with some capability in the visible range. It will be sensitive to light from 0.6 to 28 micrometers in wavelength.
National Aeronautics Space Agency Webb NIRCam.

The European Space Agency [La Agencia Espacial Europea] [Agence spatiale européenne][Europäische Weltraumorganisation](EU) Webb MIRI schematic.

Webb Fine Guidance Sensor-Near InfraRed Imager and Slitless Spectrograph FGS/NIRISS.

Webb has four main science themes: The End of the Dark Ages: First Light and Reionization, The Assembly of Galaxies, The Birth of Stars and Protoplanetary Systems, and Planetary Systems and the Origins of Life.

Launch was December 25, 2021 on an Ariane 5 rocket. The launch was from Arianespace’s ELA-3 launch complex at European Spaceport located near Kourou, French Guiana. Webb is located at the second Lagrange point, about a million miles from the Earth.

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