James Webb Space Telescope: the new “Hubble”?

A major new tele­scope, the James Webb Space Tele­scope (JWST)¹, is sched­uled for launch a few days before Christ­mas this year. It will leave Earth on an Ariane 5 rock­et from the European Space Agency’s (ESA) spa­ce­port in Kour­ou, French Guiana. The long-awaited mis­sion is often described as being the suc­cessor of the renowned Hubble Space Telescope.

As the most advanced space obser­vat­ory ever built, the JWST will oper­ate primar­ily at near- and mid-infrared wavelengths rather than the vis­ible-spec­trum used by Hubble. As such, it will allow for the most detailed explor­a­tion yet of the very dis­tant and ancient galax­ies and stars. JWST will also study nearby celes­ti­al bod­ies, extra­sol­ar plan­ets and our own sol­ar sys­tem. It is a tele­scope that should revolu­tion­ise our under­stand­ing of exo­plan­ets and how the first stars and galax­ies formed in the Universe.

The JWST is the joint flag­ship pro­ject between NASA, ESA and the Cana­dian Space Agency. It boasts a 6.5‑metre-diameter seg­men­ted mir­ror – three times the size of Hubble’s – mak­ing it 400 times more sens­it­ive than cur­rent ground-based or space-based infrared tele­scopes. The mir­ror is so large that it has to be fol­ded in three and will be unfol­ded once the tele­scope has reached its destination.

It will span the long-wave vis­ible spec­trum and infrared wavelengths from 0.6 to 28 microns and will carry four sci­entif­ic instru­ments. Oper­at­ing for at least 5–10 years – hope­fully more – it will be sent to the Lag­range point L2, which is 1.5 mil­lion kilo­metres from Earth, behind the orbit of the Moon. Again, this makes it very dif­fer­ent from Hubble, which has remained in Earth’s orbit. The JWST is also equipped with a very large 22 x 10 m sun­shield to cool it down and pro­tect it from the Sun­’s infrared radiation.

In its first year of oper­a­tion, or “Cycle 1”, JWST will look for atmo­spheres on nearby rocky exo­plan­ets and probe the earli­est galax­ies in the Uni­verse – those that formed less than a bil­lion years after the Big Bang. These galax­ies are so faint that they could not be detec­ted by pre­vi­ous tele­scopes, with the excep­tion of a hand­ful dis­covered by Hubble. These new obser­va­tions will help us under­stand an import­ant part of the his­tory of the Uni­verse, known as the reion­isa­tion (or first light) epoch – a peri­od span­ning about 400,000 to 1 bil­lion years after the Big Bang, when the first stars and galax­ies emerged. It is pos­sible that reion­isa­tion did not occur every­where at once, but in pock­ets and bubbles. These bubbles are related to the ini­tial large-scale struc­tures of the Uni­verse, and JWST hopes to map this structure.

The JWST will be able to see much fur­ther back in time, to just 200 mil­lion years after the Big Bang, which occurred 13.8 bil­lion years ago. Until now, we have been able to go back as far as 400 to 500 mil­lion years after the Big Bang with exist­ing instru­ments, but JWST will be able to see the ‘first light’ of the Universe.

The total obser­va­tion time in Cycle 1 will be split into sev­er­al sub-cat­egor­ies: 32% for galaxy obser­va­tions, 23% for exo­plan­ets, 12% for stel­lar phys­ics and 6% for our own sol­ar sys­tem. With­in these pro­grams, there are small, medi­um, and large pro­grams, some of which are regarded as ‘treas­ur­ies’, expec­ted to provide huge amounts of data that will keep future gen­er­a­tions of research­ers busy for dec­ades to come.

JWST will also study the atmo­spheres of about ten of the thou­sands of exo­plan­ets dis­covered in recent years and observe these worlds as they ‘trans­it’ in front of their host stars. These obser­va­tions will allow astro­phys­i­cists to determ­ine wheth­er they have an atmo­sphere and to ana­lyse the com­pos­i­tion and basic struc­ture of any atmo­sphere present using spectroscopy.

The tar­geted exo­plan­ets will be between one and three times the size of Earth and are known as ‘super-Earths’ and ‘sub-Nep­tunes’. JWST could trans­form our under­stand­ing of these plan­ets. To be able to detect biosig­na­tures on poten­tially hab­it­able plan­ets, we first need to under­stand the full diversity of plan­ets that have been dis­covered to date. Super-Earths and sub-Nep­tunes appear to be the most com­mon types of plan­ets in the galaxy, even though we still don’t know what they actu­ally are.

The JWST is a ‘titan’, built to trans­form our view of the Uni­verse and to per­form ground-break­ing astro­nomy. It will shed light on the fur­thest reaches of space ever. But it will also take images to show the world beau­ti­ful objects for the sake of it. These images will kindle the ima­gin­a­tion and encour­age reflec­tion. What is our Uni­verse? What are we in the midst of all this? And that’s anoth­er reas­on why the JWST is so wonderful.