Time - USA (2022-04-11)

14 TIME April 11/April 18, 2022

GOOD QUESTION

How many planets have been found,

and how many more are out there?

TIME WAS, THERE WERE ONLY NINE

known planets in the entire universe—the

gaggle of worlds that orbit our sun. That

number was reduced to eight in 2006,

when the International Astronomical

Union busted Pluto down to a dwarf

planet. But even before Pluto was

pink-slipped, the planetary census far

deeper in space began to grow, with the

discovery, in 1992, of a planet orbiting

a rapidly spinning pulsar; and later, in

1995, of a Jupiter-like planet orbiting a

sunlike star. Since then

the planetary population

has exploded, and, as

NASA’s Jet Propulsion

Laboratory recently

reported, the oicial total

of known worlds beyond

our own has now topped

5,000.

The majority of the

discoveries were made

by the Kepler space

telescope. Launched

in 2009, it hunted for

planets using the so-

called transit method—

looking for the slight

dimming in light that

occurs when an orbiting

planet briely blocks the

light from the star. The

dimming is fantastically

subtle. Former Kepler

mission director Natalie

Batalha described it to

TIME as the equivalent

of removing a single

light bulb from a board

of 10,000 of them. And

Kepler studied only a

tiny portion of the sky, encompassing

just 150,000 stars. Still, in the 11 years

it operated, it conirmed the existence

of 2,709 exoplanets and has returned

data still being studied about a possible

2,057 more.

The newer Transiting Exoplanet Survey

Satellite (TESS), launched in 2018, also

uses the transit method, but is equipped

with multiple telescopic eyes, allowing it to

scan the entire bowl of the sky. In just the

short time it’s been operating,

it has conirmed the existence of 203

more exoplanets and has spotted another

possible 5,459 that astronomers are now

investigating.

The transit method is not the only

way to go looking for exoplanets. Other

telescopes—both space-based and

Earth-based—use what’s known as the

radial- velocity method. They study a star

looking for the slight wobble caused by the

gravity of a planet—or multiple planets—

tugging on it as they orbit.

The most celebrated

multiplanet system to

date is located just 39

light-years from Earth,

where seven planets orbit

the red dwarf known as

Trappist-1.

The planets that

have been discovered

so far range in size and

composition. There are

so-called hot Jupiters—

which, as their name

suggests, are gaseous

worlds that orbit close to

the ires of their parent

planet. Others are smaller

gas worlds, similar in size

to Neptune. Still others—

the most promising ones—

are compact, rocky planets

like Earth, some orbiting

in the habitable zone of

their star, a place where

temperatures are not too

hot and not too cold for

water, the sine qua non of

life as we know it, to exist

in a liquid state.

The mere fact that astronomers ind

planets pretty much everywhere they look

has led them to conclude that virtually

every star in the universe is orbited by

at least one planet—making for trillions

upon trillions of potential worlds. “Each

one of them is a brand-new planet,” said

NASA astronomer Jessie Christiansen in a

statement. “I get excited about every one

because we don’t know anything about

them.” —JEFFREY KLUGER

NEWS TICKER

President Donald Trump

“more likely than not”

committed felonies in

his efforts

banning

classroom instruction

about sexual

orientation or gender

identity

authorized a fourth

dose of Pfizer or

Moderna vaccine

THE BRIEF NEWS

HOW EXOPLANETS

ARE DETECTED

TRANSIT

Stars are slightly dimmed

as orbiting planets block

their light

WOBBLE

As a planet orbits,

its gravity tugs its

parent star slightly

Planet in

front of star

Orbit

Star

Planet

Star

STEPHEN VOSS—REDUX