Scientific American - February 2019

February 2019, ScientificAmerican.com 59

JAXA, ISAS, DARTS AND DAMIA BOUIC

All of this coincides with another stunning devel-

opment in astronomy: the discovery of thousands of

exoplanets in other solar systems, many of them

roughly the same size and distance from their stars as

Venus. Anything we learn about the planet next door

could teach us about these distant, inaccessible worlds.

In particular, if we can figure out whether and when

Venus may have had the conditions to host life, we will

know more about the likelihood of finding living

beings on the plethora of Venus-like bodies through-

out the Milky Way.

     A   A A    

MOST OF THE EOPLANETS discovered so far were found

using the transit method, in which astronomers watch

stars for telltale brightness fluctuations that occur as

orbiting worlds pass in front of them. With this tech-

nique, we can measure a distant planet’s size, but size

tells us only so much. After all, if an extraterrestrial

observer were to look at our solar system using the

transit method, Venus and Earth would appear almost

identical. Yet Venus is forbidding to life, whereas

Earth has been continually habitable for the past four

billion years.

We can further differentiate between similarly sized

planets by measuring their distances from their stars.

The “habitable zone” is the region around a star where

a rocky planet could have liquid water on its surface.

Earth, obviously, is in this zone. Venus, we think, used

to be in this zone—for quite a while, in fact. Yet the

VENUS’S

ATMOSPHERE ,

as seen in

this composite

image from

data taken by

Japan’s Akatsuki

spacecraft,

contains thick

clouds of

sul furic acid.

© 2019 Scientific American