28 ASTRONOMY • JUNE 2022
common sense, it doesn’t seem to hold
for planets smaller than Neptune. This
suggests that there is a fundamental
threshold that needs to be reached before
a giant planet can form.
The necessary conditions must be
reached more often in the outer proto-
planetary disk, where planet formation
can proceed rapidly. To reconcile these
hot Jupiters with a theory of planet for-
mation that says that such large worlds
must form out beyond the ice line where
water and other volatiles exist as ice, the
idea of planets migrating through the
disk as they form becomes essential. To
get to be a hot Jupiter, these worlds must
have traveled a significant distance,
plowing through the inner disk, gather-
ing up and expelling material through
their gravitational pull as they did so.
This is probably bad news for any small
rocky planets with ambitions to remain
in a nice sedate, stable orbit, but it also
raises a fundamental question — how do
you stop such a planet from falling into
the star?The answer is that most of the time,
you probably don’t. In many systems,
planets will form, migrate through the
disk, and disappear into fiery oblivion. But
in some cases, tidal interactions between
the migrating planet and its star force the
newly hot Jupiter to settle into the circular
— stable — orbits we see today.Tackling the gaps
What of the 99 percent of planets which
are not hot Jupiters? The most pro-
lific planet hunter was the Kepler Space
Telescope, designed specifically to detect
transits. Following launch in 2009, it
spent three years staring at a single patch
of the night sky, chosen because it is rich
in stars and yet devoid of any particularly
bright examples, on the border between
the constellations of Cygnus and Lyra in
the northern part of the sky. Kepler mea-
sured the brightness of each of 150,000
stars every 30 minutes, and that of a
smaller number of selected systems every
minute. Its intended quarry was planets
where life like our own might, perhaps,Our universe is a
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