Science - USA (2021-11-12)

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star—so-called hot Jupiters. With its height-

ened sensitivity, Webb will be able to sniff

out molecules in the thinner atmospheres of

rocky planets. “We will see the first chemical

fingerprints from Earth-sized planets,” says

Batalha, who has assembled a team of about

60 collaborators to test transit spectroscopy

in one of Webb’s largest early programs.

Few think Webb will have sharp enough

eyes to sense signs of life in an exoplanet’s

atmosphere, such as oxygen, ozone, or chlo-

rophyll. But it will go a long way toward un-

derstanding the variety of atmospheres and

how habitable they are. “Webb will give us

the demographics of atmospheres,” Batalha

says. “It’s like moving from silent movies in

black and white to color TV.”

SO INTENSE is the interest in Earth-size

planets and their suitability for life that in

2018 NASA launched a “spotter” for Webb:

a space mission to identify the best candi-

dates for atmospheric studies—rocky plan-

ets around dim red dwarf stars, which do

not swamp a planet’s light as much as other

stars. The Transiting Exoplanet Survey Sat-

ellite (TESS) has in its first 3 years found

159 confirmed planets and has another

4500 awaiting confirmation.

A first task for Batalha’s team will be to

solve a mystery posed by the planetary in-

ventory Kepler and TESS have assembled:

The most common exoplanets are a type

that doesn’t exist in the Solar System, with

a mass between that of Earth and Nep-

tune. “We don’t really understand what

they are,” Batalha says. Webb should help.

If the mystery planets are smaller versions

of Neptune, a gas giant, they should have

a thick atmosphere dominated by primor-

dial hydrogen mixed with water, carbon

dioxide, and carbon monoxide. If they are

rocky bodies—super-Earths—their atmo-

sphere should reflect geological processes,

like volcanism, that often spew molecules

such as methane or ammonia.

Transiting planets also periodically pass

behind their star; Webb will compare the

star’s light just before and during these

eclipses. Prior to the eclipse, the light in-

cludes both the star’s emissions and the

much fainter light from the planet, whereas

the eclipse itself blots out the planet’s light.

By comparing the two signals, astronomers

can infer the planet’s own glow, which be-

trays information about temperatures and

cloud cover.

The combined light of the star and orbit-

ing planet should change subtly as observ-

ers receive light from the planet’s dayside,

nightside, and varying mixtures of the two.

The resulting “phase curve” reveals the dy-

namics of an atmospheric system: how the

atmosphere shuffles heat from the hot day-

side to the cooler nightside, which in turn

affects winds and cloud movement. “We

will also be able to tease out 2D and 3D ef-

fects, such as clouds on one side and not

the other,” Lewis says. “It’ll keep us busy for

a long time.”

Webb will also take pictures of exoplanets

directly, with the help of a coronagraph, a

mask within the telescope that blocks the

overwhelming glare of the star so that the

tiny dim planets around it can be seen. The

largest telescopes on Earth can only dis-

cern the largest planets, more than twice

Jupiter’s mass, in wide orbits. Furthermore,

only young planets have been imaged, ones

glowing with the leftover heat of their for-

mation, not just the much dimmer reflected

light of the star.

Webb, designed to catch this infrared

glow without the blurring interference

of Earth’s atmosphere, will be able to see

smaller planets later in life, orbiting closer

to their stars. “Something like Jupiter

would be great,” Baraffe says; research-

ers wonder whether the composition and

temperature of an alien Jupiter will be any-

thing like the homegrown version. “Will it

share general characteristics or be very dif-

ferent because it formed in different condi-

tions?” Baraffe asks. But directly imaging

an Earth-like planet and searching for

signs of life will have to wait for an even

larger and more capable space telescope

(Science, 14 December 2018, p. 1230).

FOR MANY, the first data from Webb will

prompt a shift from one tension to an-

other: from anxiety over its launch and

deployment to pressure to make the most

of its limited life. Whereas Hubble could

be refueled and upgraded by visiting as-

tronauts, extending its life for decades, the

clock is ticking for Webb: It’s designed to

last 5 or 10 years, but beyond that nothing

is certain.

The limiting factor is propellant. Webb

needs its thrusters to maintain its orbit

around L2 and to occasionally dump an-

gular momentum from the reaction wheels

that point the telescope and keep the sun-

shield in position. Without propellant,

Webb will drift from L2 and eventually fall

into a fatal spin. It won’t be able to charge

its solar-powered batteries or communicate

with Earth. Webb will see no more.

Finkelstein will rue that day, but he

is confident he will be plenty busy in the

meantime. “I’ve basically cleared the next

decade from my schedule.” j

The James Webb Space Telescope will reach deeper in time than Hubble, which captured “deep field” images (left), revealing galaxies that shone 800 million years after the

big bang. It will also zoom in on gaps in disks of gas and dust around some young stars, seen in radio telescope images (right), which are thought to mark planetary orbits.

IMAGES:
LEFT TO RIGHT NASA/ESA/S. BECKWITH
STSCI AND THE HUDF TEAM; ALMA
NRAO/ESO/NAOJ

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