Undreamed Shores - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 253

detected closely orbiting a star estimated at between just 8 and 10 mil-
lion years old, a star so young that there is still a circumplanetary disc
of gas and dust not yet blown away by the stellar wind. 159 There is, too,
the planetary system 'that should not exist': 160 HR 8799, with four
giant planets, each much bigger than Jupiter with orbits stretching
out to ten times that of Jupiter from our Sun. This is also difficult to
fit into planetary theory. Just as the moons of our own solar system
turned out to be much stranger and more diverse than originally
thought, so too are planetary systems far from ours. And there will be
more surprises to come, for sure. Greg Laughlin, an astronomer at the
University of California in Santa Cruz, thinks that the planetary theo-
rists will be revising their models for some time to come. Whatever
the next big thing in planetary science will be, he said, we won't see
it coming. 161
On the basis of what is known—and it is very early days yet—
how many planets might there be in our galaxy, the Milky Way? (It
is still quite impossible for us today to peer at planets in other galax-
ies.) The wobble and transit techniques, so eloquent in many ways,
are not very good at answering this question because they are biased
towards the discovery of planets that are relatively close to their stars.
Microlensing, on the other hand, although based on a rare phenom-
enon (two stars and the Earth being exactly in line) is not affected by
such a bias.
Microlensing can detect planets that are very far from their parent
stars and also planets that are 'unbound'—that is, planets that have
drifted off to float freely in outer space, as cold, dark, frozen, and cer-
tainly lifeless worlds. There turn out to be many of these distant or
'unbound planets' (one thinks of these objects—now forever dark and
cut off from the warmth of a sun—with a shudder, a little as one
might think of the undead in a horror film). They are as numerous, at
least, as are the stars in our galaxy. 162 Microlensing has also been used