The Astronomy Book

295

like Baby Bear’s porridge in the
fairy tale, “not too hot, not too cold.”
The size and locations of habitable
zones depend on the activity of
the host star. For example, if Earth
were orbiting a K-type star, an
orange dwarf that is considerably
cooler than the sun (the sun is a
G-type, or yellow dwarf), it would
need to orbit at about one-third its
current distance to receive the
same amount of warmth.
Of the thousands of exoplanets
that have been identified, only
a tiny proportion are candidates
orbiting in their star’s habitable
zones, with Earth-like conditions
for life—rocky surface with liquid
water. Typically, they are larger
than Earth, and very few have good
prospects for being Earth-like. If
and when Earth-like planets are
found, astrobiologists will look at

the atmospheric chemistry for signs

of life, such as the presence of

elevated levels of oxygen, produced

by photosynthesizing life-forms.

How life evolved from nonliving

material on Earth is still a mystery

but the study of Earth-like planets

THE TRIUMPH OF TECHNOLOGY

may throw light on that process.

Even if life is found, it is likely

that most extraterrestrial natural

histories will not have moved

beyond microorganisms. As

every step toward evolving more

complex life-forms becomes ever

more unlikely, so alien civilizations

that match humankind’s will be

a lot less common. However, if

only G-type stars, like the sun, are

counted, there are about 50 billion

in the galaxy. It is estimated that

22 percent of them have an Earth-

like planet in their habitable zones,

which equals 11 billion possible

Earths. Adding in other types

of stars such as orange and red

dwarfs, that number rises to 40

billion. Even if the probability of

civilizations evolving is one in

a billion, the chances are that

humankind is not alone. ■

The size of the habitable zone (green) depends on the size of the star. The red zone is too hot, while the
blue zone is too cold. The habitable zone is closer to cooler stars than it is to hotter stars. The size of a planet,
the shape of its orbit, and the speed of its rotation between night and day also affect its habitability.

If we keep working as well and

we keep being as enthusiastic

... the issue about life on other

planets will be solved.

Didier Queloz

A hotter star

The sun

A cooler star