Astronomy - USA (2020-06)

Saturn

82

1 ST

PLACE

Neptune

14

Uranus

27

Mercury

0

Venus

0

Earth

1

Mars

2

Jupiter

79

WWW.ASTRONOMY.COM 11

On New Year’s Day 2019, the New

Horizons probe buzzed past a

small world now called Arrokoth,

which means “sky” in the Native

American Powhatan language.

More than a year after the

flyby, the data are still com-

ing in. The New Horizons team

presented their latest results

February 13 in a series of papers

published in Science, as well

as at the annual meeting of the

American Association for the

Advancement of Science in

Seattle. Their findings: Planet

formation may be a gentler pro-

cess than previously thought.

Our solar system started as

a cloud of gas and dust, which

evolved into the well-ordered

place we see today. However,

the steps in between are fuzzy.

Orbiting in the Kuiper Belt,

Arrokoth functions as a sort of

time capsule, providing a pristine

snapshot of the conditions shortly

after the solar system’s birth.

Arrokoth has a relatively

smooth surface with few signs of

impacts. Most importantly, its two

red-hued lobes appear to have

been moving in lockstep even

before they made contact, like

two lovers on a walk, gravitating

toward each other even before

reaching out to join hands.

Exactly how those lobes came

together reveals clues about

how our solar system assembled

itself. “For decades, there’s been

a war of computer models,” New

Horizons’ principal investigator,

Alan Stern, tells Astronomy. In

one model, called hierarchical

accretion, dust grains collided

to form pebbles, which formed

rocks, then boulders, and finally

large bodies, in a long and violent

game of bumper cars. The other

model, cloud collapse, says the

original nebula of dust and gas

swirled together and tenderly col-

lapsed directly into larger bodies.

Early data from New Horizons

hinted at a gentle crash between

Arrokoth’s two lobes — which is

actually evidence for the violent

collision theory. But, says Stern,

with 10 times as much data and

many months of computer model-

ing, the team now sees a different

story, in which Arrokoth formed

more placidly. And this story sup-

ports the cloud collapse model.

“Arrokoth has provided a deci-

sive test between the two,” Stern

said during the Seattle briefing.

“I believe this is a game changer.”

— KOREY HAYNES

Arrokoth flyby hints at how planets formed

328

The number

of days NASA

astronaut

Christina

Koch had

spent on the

International

Space Station

by the time

she returned

to Earth

February 6,

breaking the

record for

the longest

continual time

in space for

a woman.

A NEW CHAMPION EMERGES. On October 7,

2019, Saturn officially surpassed Jupiter

as king of the solar system — at least,

in terms of moons. Based on 20 newly

discovered moons, the ringed planet now

boasts the most moons of any planet

in the solar system. And astronomers

expect it to stay that way, estimating

Saturn may actually have nearly 20

more moons that have not yet been

discovered, due to their small size.

Regardless of future discoveries,

Saturn is now on top. Here’s

how it stacks up against

its contenders. — A.K.

The Carnegie Institution for Science recently

held a contest to name Saturn’s 20 new moons.

Entrants were required to choose names based

on Inuit, Norse, and Gallic mythology, in keeping

with current naming conventions.

THE MOST MOONS

FAST FACT

AST

RO

NO

MY

:^ RO

EN

KE

LLY

TWO BECOME

ONE. Arrokoth is

a contact binary

made of two

lobes. New data

indicate this

strange world

formed gently,

supporting the

cloud collapse

model of solar

system formation.

NASA/JHUAPL/SwRI