Science - USA (2021-11-12)

Primary

mirror

Hubble (2.4 m)

Webb

(6.5 m)

Sunshield

Microshutter

Visible Infared

13.8 Billions of years after big bang 0

Modern

galaxies

Webb

(2021)

Hubble

(2016)

Protogalaxy

Wavelength (micrometers)

CO 2

CO 2

CH 4

H 2 O

More light absorbed

Star

Coronagraphic masks

Hubble Webb

O 2

CO 2

CO 2 O 3

O 3

O 2

H 2 O

H 2 O

N 2 O

CO 2

Ultraviolet

Exoplanet

Star

Computer and

controls

CH 4

Big

bang

First

stars

Ionization of

hydrogen begins

1 0.5 0.2

Ionization is

complete

2

Peak star

formation

Present-day

universe

10 nm 400 nm 700 nm 1 million nm

600 nm 28,500 nm

Neutral

hydrogen

Human (1.7 m)

Wavelength in nanometers (nm)

P

GalGalaxyGalaxyax MicMicMicrosssssshut plane c mask

C

0.6 0.8 1 2 3 4 5 6 8 1012

onag

809

A golden opportunity

On 18 December, NASA will launch the $10 billion James Webb

Space Telescope, a successor to the Hubble Space Telescope.

The segmented mirror–coated with a thin layer of gold–

will work in the infrared, looking back across the universe to

gather the light of the first stars and galaxies. It will also

bring into view the atmospheres of Earth-size exoplanets.

Graphic by Chris Bickel

Science instruments

Cooling systems chill

Webb’s four instruments

below –230°C, lest

their heat pollute the

infrared view.

Multitasking

An array of nearly 250,000

microshutters allows one

instrument to analyze light from

hundreds of galaxies at once.

Mask mandate

To gather the light of a

planet and image it

directly, Webb will mask

the glare of the star.

Honeycomb eye

Webb has more than five times the

light-gathering power of Hubble.

Lighting up a planet

Only a few big, hot exoplanets can be imaged directly. But astronomers can still

probe a planet’s atmosphere by performing transit spectroscopy, or by comparing

light when the star eclipses the planet with light gathered just before.

Peering into the past

Webb will reach farther out in space and further back in time than Hubble or ground-based telescopes. It could show how galaxies evolved from ungainly clumps to the elegant

spirals and ellipses seen today. It should also reveal whether starlight was sufficient to ionize the hydrogen gas that filled the universe, or whether black holes assisted.

Signs of life

If TRAPPIST-1e, an Earth-size exoplanet 40 light-years away, had an atmosphere like

Earth’s, its transit spectrum would look like the synthetic one below. Biosignatures

like oxygen and ozone are at the edge of detectability. Webb is more likely to sense

carbon dioxide and methane, which can have both living and nonliving sources.

Tr a n s i t

A portion of the star’s light

passes through the planet’s

atmosphere as it crosses

the face of the star, gathering

telltale absorption lines.

Eclipse

Subtracting an eclipse spectrum from

a “full-face” spectrum leaves just the planet’s.

Full face

Just before an eclipse, the

planet’s emitted light

makes a tiny contribution

to the total spectrum.

DATA: RYAN MACDONALD/CORNELL UNIVERSITY; NASA