mass of Jupiter) and which goes all the
way up to 13 times Jupiter’s mass. This
higher figure is most likely a funda-
mental limit; any more massive than
this, and the pressure and temperature
at the core will be high enough to
enable deuterium fusion, at which point
the body is at least a brown dwarf and
perhaps a full-blown star.
Building giants
This range of masses makes it clear
we’re not dealing with planets like our
own Mercury, the only world that lies
close enough to the Sun to complete an
orbit in less than 100 days — a range
that includes three-quarters of known
exoplanets. How did these strange
worlds come to be?
One part of the answer is that it is
sometimes easier to form planets effi-
ciently. Perhaps unsurprisingly, there’s
a correlation between the metallicity
of the star — how much material there
is in forms other than hydrogen and
helium — and the likelihood that giant
planets would exist. The more material
there is from which to form planets,
the more planets form! But, though this
relation sounds like nothing more than
Planets’ sizes
relative to each
other are shown
to scaleNu Planet radius (REarth)mb
e
r^
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f^ plan
ets^p
e
r^
star(
o
rbit
alpe
ri
o
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0
0
da
ys)1 1.3 1.8 2.4 3.5 4.5 20Planets from our solar system
Exoplanets discovered by Kepler68 1200.020.040.060.080.100.12Earth
UranusSaturn Jupiter
NeptuneStar– planet shadowStarPlanetFlu
x
TimeOccultationTransitStar^ +^ planet^
nightside
Star aloneStar + planet
daysideSIZING UP EXOPLANETS
ABOVE: When a planet crosses the face of a star,
there is a slight dimming in the brightness of that
star measured from Earth. This principle is used
to identify exoplanets orbiting other stars.
ASTRONOMY: ROEN KELLYRIGHT: Brian May observes the transit of Venus
in 2004 by projecting the Sun’s image onto a
white card. KATE SHEMILTTHE TRANSIT METHOD
The Kepler spacecraft’s mission was designed to look at a region
of the Milky Way and determine the fraction of the hundreds of
billions of stars in our galaxy that might host exoplanets. Kepler
discovered thousands of transiting exoplanets, getting a measure
of their radius relative to their stars. This diagram charts the
number of planets it discovered with orbits of less than 100 days,
showing there are a large number of worlds quite unlike those in
our solar system. ASTRONOMY: ROEN KELLY, AFTER WAKEFORD & DALBA, 2020