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.