Undreamed Shores - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 254

to suggest that, on average, each star in the Milky Way has at least one
'bound' planet. Planets, it seems, are a rule and not an exception in
our galaxy 163 —and presumably in other galaxies too.
Searching for Mirror Earths
If exoplanets are mainly dominated by super-Earths, hot Jupiters, and
giant Jupiters—what of ones that might resemble our Earth? Ones,
that is, that might not be too large to grow into gas giants, or too
small for oceans to be retained. Planets that might not be too hot, so
that any water just boils away, or too cold, so that the water just freezes
and becomes, in effect, a rock. We are searching here for those 'Goldi-
locks planets' that might, possibly, harbour water- and carbon-based
life as we know it.
Such planets seem rare. A plot made in 2011 of the 650 planets then
known 164 showed a cluster of hot Jupiters all closer to their stars than
we are to the Sun, a cluster of cold Jupiters orbiting farther out, and a
cluster of super-Earths all of which were larger than the Earth and
orbiting closer to their star. The Earth, shown on the same plot for
comparison, sat in splendid isolation in one corner of the graph.
One reason for this may be the difficulty in detecting such small,
distant objects. Alternatively, Earth-like planets might be relatively
rare in our galaxy. It is too early to tell. However, since that plot was
made Earth-sized planets have been discovered, unearthed by the
penetrating eye of the prolific Kepler space telescope. So—even if
rare—Earth-style bodies are at least present out there.
Typically, when one has been waiting so long to find Earth-sized
planets, two turn up at once, 165 in the same star system (much like
Venus and Earth around our Sun). Moreover, they orbit a Sun-like
star, now christened Kepler-20, which lies about 1,000 light years
away from our planet. There are at least five planets in the system:
two Earths and three super-Earths. They are not, though, at Venus-