Physics, Space

How JWST can see exoplanets and ‘a bee on the moon’

If you’ve paid attention to any science news lately, you’ll know that the long awaited James Webb Space Telescope was probably this year’s best christmas present for anyone in the space community, with a successful launch yesterday (25/12/21) just before UK readers were tucking into christmas lunch! And when you hear that this mission is a long time coming, it’s no exaggeration: first proposed in 1996, it had a very difficult journey to get to where it is today, with lots of things going wrong, which just goes to show how insanely complicated this extraordinary feat of technology really is!

The mirror is made of 18 segments of gold coated beryllium slotted together into a primary mirror 6.25x larger (in area) than Hubble, meaning it can collect more light to see fainter objects. Although the mirror is 6.5m across (about 2 storeys/ 4 people/ half a climbing wall tall), it’s incredibly thin, so only needed 48g of gold to cover the whole 25m2 mirror, in a layer a thousandth the width of a human hair! This mirror reflects and focuses the light up into the secondary mirror, which reflects it back down to the tertiary mirror, and is redirected by another set of mirrors into the required instrument. 

One of my favourite facts about JWST is that it is sensitive enough to detect a bumblebee, on the moon! How? When I first heard this, I tried a few calculations, initially considering its angular resolution.  

Say a bumblebee is 2cm wide, then using trigonometry, you can work out what its angular width (how big it appears to be in relation to the whole sky- for example the moon is about half a degree wide) would be if it was on the moon- which turns out to be about 0.00001 arcseconds… 10,000 times smaller than JWST’s angular resolution!

So how can it be true? Well JWST isn’t exactly like a typical telescope like Hubble, because it doesn’t observe visible wavelengths, it actually looks for infra-red light. This is light which has a longer wavelength than what we can see, and is emitted by any hot object, producing a heat signature, which can be detected- like by night vision/thermal cameras! So instead of the bumblebee’s size, we need to consider how much heat it produces, which, according to JWST senior project scientist Dr John Mather, is about 50 milliwatts, with a wavelength of say 2 micrometres, which radiates in all directions, so spread out over a sphere with radius 400,000 km, the intensity is 2.5 x 10-20 W/m2, which when you convert to Janskys (the weird unit telescope sensitivity is measured in) does come out at roughly the sensitivity of the James Webb Space Telescope, provided my maths is correct! You can see my method below, and scroll to the end for my (failed) attempt at following the method in the video.

Despite that, and not due to the small problem that there are no bumblebees on the moon, JWST won’t be focussing it’s observations on the moon- in fact, it can’t focus on the moon, for 2 reasons:

  • It is orbiting a point in space called Lagrange Point 2, one of 5 places where there is a unique balance between the gravitational forces of two masses, and centripetal force, balance out, so spacecraft are able to stay in a relatively stable orbit around this point with little fuel usage. Lagrange 2 is directly in line with the Earth and Sun, on the opposite side of the earth to the sun. This allows it to face the depths of space to do its observations, and have its shielding, solar panels and communications instruments on the back… meaning it couldn’t point its mirrors back at the moon without a) causing the instruments to overheat, and b) having no way to get power or communicate!
  • It’s primary mission is to observe deep space, so it can only focus on objects at a very far distance, from Mars (just- it will be very bright!) and beyond. 

What it will focus on (one of many!), however, is exoplanets, particularly exoplanet characterisation. Another quick calculation shows that unfortunately JWST won’t be able to resolve features on the surfaces of exoplanets, but it can analyse the light detected from exoplanets using spectroscopy.

Every molecule absorbs a specific wavelength, or pattern of wavelengths, of light, so when JWST observes an exoplanet passing in front of its star (a transit), it can detect the light which has passed through the exoplanet’s atmosphere, and look for missing wavelengths to determine what the atmosphere is made of.

This is called transmission spectroscopy, but JWST will also be able to take some direct images of exoplanets, and analyse them using reflectance spectroscopy, using light that hasn’t passed through, but has been reflected or emitted by the planet. This could allow for observations similar to those taken by earth observation satellites, such as to analyse its surface composition: its geology and potentially even vegetation!

What makes JWST so useful for exoplanet characterisation is its use of infrared, because a lot of the interesting spectral features (like absorption lines) are found in this part of the spectrum, including a lot of the basic atmospheric gases like carbon dioxide, oxygen, methane and water.

Absorption Spectra of different molecules
http://irina.eas.gatech.edu/EAS8803_Fall2009/Lec6.pdf

The infrared is also fundamentally associated with heat, so observing the composition and temperature of planets, as well as how this changes over the course of observations, could help build up a picture of what these worlds are really like, weather and all!

Water absorbs a huge range of wavelengths in IR, which is one of the reasons infra-red telescopes really ought to be space-based, and the few which are ground-based have to be at the very tops of mountains like Mauna Kea in order to be above any clouds and reduce the amount of atmosphere it has to look through. Although clouds on exoplanets can be made out of anything from water to iron, depending on the temperature, water is a key molecule in the search for life, and many of the first proposals, including this one which will focus on the Trappist-1 system.

TRAPPIST-1: How Flat Can a Planetary System Get?

Some other cool exoplanets that are potential targets for JWST include

  • HR 4796A: a a young planetary system with a debris ring and planetesimals (baby planets)
  •  HIP 65426b: a huge planet 6-12 times the mass of Jupiter, orbiting at 92 AU
  • VHS 1256b: this may either be a very large planet or perhaps a brown dwarf, orbiting at 100 AU
  • Proxima Centauri b: The closest exoplanet to earth, just 4 light years away, is a small rocky exoplanet orbiting in the habitable zone!
  • TRAPPIST-1: a system of 7 rocky planets orbiting a red dwarf star, including 1e and f which are in its habitable zone
  • HD 189733 b: one of the first exoplanets to be discovered- a typical hot jupiter with a dry, carbon dioxide rich atmosphere, its hoped JWST will reveal more about wind and weather on the planet
  • 55 Cancri e: a ‘super earth’ sized rocky planet orbiting so close to its star that its thought that its surface is mostly molten
  • HD 80606 b: An extremely hot jupiter type planet with an incredibly elliptical orbit meaning it sees huge temperature variations depending on where it is in it 111 day orbit, creating interesting and intense weather patterns!
  • LHS 3844 b: a rocky planet with no atmosphere, so JWST will be used to look for volcanic activity (how cool!?) and determine its surface composition

One of the goals of JWST’s exoplanet exploration is to analyse the reason for the difference between super-earths and mini-neptunes, which are often of a similar mass class, but super earth’s have a thinner, more earth-like atmosphere above a rocky surface, and mini-neptunes are more similar to ice/gas giants. We don’t know much about these planets because current technology is biased to finding large planets close to their star, but hopefully JWST’s increased sensitivity will provide a more representative sample of exoplanets in our galaxy!

As well as looking for clouds on exoplanets, the other useful property of Infrared is that it can look through dust clouds. These huge clouds of dust and gas are called nebulae, and the abundance of resources make them productive star forming regions. And they’re stunning- some of Hubble’s most famous photographs, like the “Pillars of Creation” seen below, are of these star forming regions… but the dust and gas keeps the newly formed stars mostly out of view. So in comes JWST, which will be able to observe these fledgling star systems to investigate stellar and planetary formation in a totally new way! 

Another source of infrared is from visible light emitted so long ago that as it travelled, the universe has expanded enough to stretch the light into the infrared, which will hopefully allow astronomers to see the first galaxies ever formed, just a few hundred million years after the big bang!

It may have taken 25 years and a sea voyage through the Panama Canal (not because it’s too heavy for planes to carry- it was flown from NASA Goddard, where the mirrors were assembled, in Maryland to the Johnson Space Centre in Texas for testing back in 2017- but because the bridges between the airport and spaceport in Kourou aren’t strong enough!) to get here, but with all this exciting science yet to come, it’s safe to say the launch livestream was the soundtrack to my Christmas day!

Sources

Further Reading

The method that went wrong!

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