AAS Nova

Spotting an Exoplanet in the Mid-Infrared

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A photograph of a masked-out star surrounded by a halo of light leaking around the edge of the mask. Surrounding the halo are smaller star-like dots, one of which is annotated with an arrow. This is the directly imaged planet, and the other dots are faint background stars.
A directly imaged exoplanet circling a masked-out Sun-like star. [ESO/Bohn et al.; CC BY 4.0]

Given JWST’s unprecedented capabilities, the first time it tries out a new instrument setting, it often sees something never before observed. Recently, that trend continued when JWST deployed its coronagraphic masks and snapped the first direct picture of an exoplanet in the mid-infrared.

New Settings

As has been noted repeatedly in AAS publications, JWST is a telescope like no other. For more than a year, scientists and the public alike have been spoiled with nearly weekly discoveries enabled by its unparalleled sensitivity. In exoplanet science especially, it has made good on its promise to revolutionize the field. Now, as JWST becomes a toddler and celebrates almost 20 months in space, astronomers are finishing up analyzing the last of its initial studies with each instrument and setting.

One of these studies, led by Aarynn L. Carter (University of California, Santa Cruz), describes JWST’s first attempts at high-resolution direct imaging of an exoplanet using its coronagraphic masks.

A Challenge, Even for the Best

Directly imaging an exoplanet is not an easy task: even planets that circle their host stars on wide, 100+ au orbits appear right next to them from our faraway vantage within the solar system. Additionally, planets are also much smaller, cooler, and therefore fainter than their hosts. Putting these two issues together, finding exoplanets by direct imaging is often compared to trying to see a firefly next to a floodlight from miles away.

Initial (leftmost column) and fully processed (right columns) images of HIP 65426b. Each row shows the planet through a different wavelength filter, and each column shows the result of a different processing technique. [Carter et al. 2023]

Luckily, JWST and other telescopes have a trick to deal with such domineering stars. A few of its instruments, including its Near Infrared Camera and Mid-Infrared Instrument, have tiny coronagraphic masks that can block most of a star’s light without shielding the area around it. Just as someone might block the Sun with their hand when trying to spot a plane flying overhead, by suppressing the parent star’s glare, JWST can see fainter nearby planets more clearly.

Even still, these masks cannot perfectly block out all of a star’s light, so image processing must be carried out back on the ground to cleanly extract any shy, small planets.

Impressive Performance 

So how did JWST do? Even better than its high expectations. Although its target, a super-Jupiter named HIP 65426b, is more than 1,000 times fainter than the very nearby parent star, JWST could easily disentangle the two in all seven filters the team observed through. Some of those filters only let through mid-infrared wavelengths of light, making this the first direct picture of an exoplanet taken beyond 5 microns (1 micron = 10-6 meter).

Three histograms describing the likelihood of the mass, effective temperature, and radius of HIP 65426b. The median values are 7.1 Jupiter masses, 1283 Kelvin, and 1.44 Jupiter radii, respectively.

Properties of HIP 65426b calculated by comparing JWST observations to evolutionary models. Click to enlarge. [Carter et al. 2023]

By comparing their observations to different models, the team measured the size and temperature of HIP 65426b more accurately than ever before. Possibly more exciting than their results about this specific planet, however, were the implications for studies to come. Since JWST barely broke a sweat finding this wide-orbiting super-Jupiter planet, Carter and collaborators estimate that under the right circumstances, JWST might be able to detect planets smaller than Saturn and within 10 au of their host star.

Planets this small and this close to their stars have never before been directly imaged. The fact that the next few years could include pictures of exoplanets similar to those in our solar system is a thrilling and surprising possibility, one brought about directly through the work that went into designing, building, and operating this miracle of a telescope.

Citation

“The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems I: High-contrast Imaging of the Exoplanet HIP 65426 b from 2 to 16 μm,” Aarynn L. Carter et al 2023 ApJL 951 L20. doi:10.3847/2041-8213/acd93e