The James Webb Space Telescope – Revealing New Worlds
16th December, 2021
Niall Deacon describes the James Webb Space Telescope, and what it means for our understanding of the universe.
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The past 30 years has seen the discovery and characterization of thousands of planets orbiting other stars. Later this month a space telescope will launch that will push the boundaries of our understanding of these worlds, providing images of smaller planets, and possibly providing the first tentative evidence of habitability.
Almost all of the exoplanets (planets orbiting other stars) astronomers have found so far have been discovered using indirect techniques. However a handful of these worlds have had direct images taken of them.
Looking up at the sky we are familiar with the idea of stars twinkling. This is caused by starlight passing through countless, constantly shifting pockets of air in the Earth’s atmosphere. When taking images of stars, these air pockets cause stellar images to dance around on the image, smearing them out. Planets are much fainter than their stars and the vast distances between Earth and other star systems means that these planets will appear very close on the sky to their host star. This relative faintness and closeness to bright stars is made worse by the blurring of stellar images by the Earth’s atmosphere.
In the first few years of this century, new instruments came online at some of the biggest telescopes in the world. These instruments had a clever bit of kit included in them, a deformable mirror. When the telescope is observing a star that might host a planet, the mirror constantly flexes, correcting for the subtle shifts caused by the air pockets in the atmosphere. This makes the stellar image more stable and reduces the smearing, leading to sharper stellar images.
This technique allowed astronomers to take pictures of a handful of planets, imaged not from reflected light from their star, but from the planets’ own glow. All were found around young stars. This is because planets around young stars are typically hot and relatively bright. Planets form in discs around stars and gather up gas and dust from these maelstroms. All of this material slamming on to a young planet heats it up. This creates a glow, mostly in light too red for our eyes to see, that fades over millions of years. More massive planets are also brighter so this combination has meant that the planets we have direct images of so far are young planets more massive than Jupiter.
This month will see the launch of the James Webb Space Telescope (JWST). This is the most important space observatory since the launch of the Hubble Space Telescope in the early 90s. JWST has been jointly developed by NASA, ESA and the Canadian Space Agency and will observe in infrared light, beyond the limit the human eye can see. As a space telescope, JWST will not need to deal with the blurring effects of the Earth’s atmosphere. This means it will be able to observe smaller planets and planets closer to their host star. Among the youngest nearby stars, JWST may be able to observe planets with masses a third of the mass of Saturn. However JWST will not be able to take images of planets similar to the Earth.
There is however another technique to study planets similar to the Earth. When a planet’s orbit around its star is aligned with the direction we on Earth view that planet from, the planet can sometimes pass between us and its star, blocking a little of the star’s light and causing it to dim. Many small rocky worlds of similar size to the Earth have been discovered using this technique.
These dips in a planet host star’s brightness also contain a small amount of information about the planet’s atmosphere. While most of the light will be blocked by the solid part of the planet, a small amount will pass through the planet’s atmosphere on its way to Earth. This light will be imprinted with the spectral absorption features of the atoms and molecules in the planet’s atmosphere. This means that astronomers can look for the spectral fingerprints of important molecules for life such as oxygen, ozone or methane.
Measurements like this are pretty hard to make as many stars have spotty surfaces with colder, darker patches meaning that the star’s brightness and spectrum change subtly as it rotates and different patches come in and out of view. The Earth’s atmosphere also introduces noise to these measurements. JWST will not face the atmospheric noise ground-based telescopes face when making these measurements. It may be possible to detect ozone in the atmosphere of rocky planets around other stars using JWST, and possibly other molecules, but again, these are tricky measurements and noise from spotty stars may make them even trickier.
JWST promises to advance entire fields of astronomy, not just exoplanet science. This is the result of the work of hundreds of astronomers, engineers and other staff at institutions across the globe. We can only wait to see what new worlds this gargantuan effort will reveal.
– Niall Deacon
To read more about this fascinating subject, order Twenty Worlds from our online shop now.
Niall Deacon is an astronomy researcher and writer, and lives in Heidelberg, Germany. His research focuses on failed stars called brown dwarfs and giant planets orbiting other stars.
References
Barstow & Irwin, 2016, Monthly Notices of the Royal Astronomical Society: Letters, Volume 461, Issue 1, p.L92-L96
https://arxiv.org/pdf/1605.07352.pdf
Rackcham, Apai and Giampapa, 2018, The Astrophysical Journal, Volume 853, Issue 2, article id. 122, 18pp.
https://arxiv.org/abs/1711.05691
Cater et al., 2021, Monthly Notices of the Royal Astronomical Society, Volume 501, Issue 2, pp.1999-2016
https://arxiv.org/pdf/2011.07075.pdf
Image caption:
The James Webb Space Telescope is tested before launch. Credit: NASA/Chris Gunn. Licensed under CC-BY-2.0
