Sky and Telescope - March 2017 - 26

Small, Crazy Exoplanets

Sizes not seen in
our solar system

0.4

Average number of planets per star

0.35
0.3
0.25
0.2
0.15
0.1
0.05
0

0.35
0.3
0.25
0.2
0.15
0.1
0.05

Planet Size (Earth=1)

8-
11
11
-1
6
16
-2
3

5.
7-
8

-4

5.
7
4-

2.
8

2.
8
2-

-2
1.
4

8-
11
11
-1
6
16
-2
3

5.
7-
8

5.
7
4-

-4
2.
8

2.
8
2-

-2
1.
4

1-
1.
4

Fraction of observed planets

0.4

Planet Size (Earth=1)

p WHAT KEPLER SAW In its primary mission, Kepler discovered thousands of potential exoplanets orbiting within 1 astronomical unit of their host

Density Matters
Densities of Kepler Worlds
in Multi-Planet Systems

Planet radius (Earth radii)

1
Density of pure ice
Density of pure rock
Density of pure iron

0.1

1
10
100
Planet mass (Earth masses)

º Astronomers can surmise a planet's potential composition if they know both its radius and mass. Plotted
here are a few dozen worlds in multi-planet systems
discovered by Kepler for which that's true (with reasonable uncertainties). The contour lines designate densities for pure ice, rock, and iron. Planets above the ice
line are less dense than ice, suggesting that they have
thick gas envelopes. These worlds are all within a few
tenths of an astronomical unit of their host stars.

M A R C H 2 0 1 7 * SK Y & TELESCOPE

wasn't until 2016 when Brice-Olivier Demory (University of
Bern, Switzerland) and his colleagues created the ﬁrst heatmap that they realized what an oddity this world is.
Using 75 hours of observing time on NASA's infrared
Spitzer Space Telescope, the team mapped 55 Cancri e's thermal "phase curve." Because the planet is tidally locked to its
host star, like the Moon is to Earth, one hemisphere is eternally bathed in sunlight while the other is eternally dark. But
the same side isn't always pointed at Earth: seen from a distance, astronomers observe the planet pass through phases,
from new through crescent to full and back through crescent
to new again, just as we see with Venus. They can thus image
both hemispheres as the planet circles the star and search for
any temperature differences between the day- and nightsides.
Demory's team suspected that strong winds would carry
heat from the dayside to the nightside, meaning that they
would both radiate at similar temperatures. But instead, it
looks like 55 Cancri e's dayside is twice as scalding as its
nightside. "We were surprised to see a contrast similar to the
Moon, suggesting that there was no signiﬁcant atmosphere
surrounding it," Demory says.
But there was a contradiction in the team's measurements.
While there was clearly no atmosphere, the planet's hottest
point, which should reside directly beneath the star overhead,
had shifted - an effect that astronomers usually attribute to
heavy winds. Without an atmosphere, Demory and his colleagues think the movement is instead due to lava. A patch of
molten rock at the surface could circulate the heat away from
the planet's hottest point.
Although not all astronomers are convinced that 55 Cancri e is a lava world, it's pretty clear that the planet is far from

S&T LE A H TISCIONE, TOP SOURCE: N ATA LIE BATA LH A / N ASA A MES, PL A NE T
ICONS: MEOW U / ISTOCK; BOT TO M SOURCE: S. H A DDEN A ND Y. LITHWICK

stars. On the left is a graph of the confirmed worlds, divvied up by size. The y-axis is the fraction of planets that fall into each size category. On the
right is an extrapolation from that discovery population: astronomers' estimate for the average number of planets from each category per star, based
on various assumptions and data corrections. (Black, bracketed bars indicate uncertainty ranges.)

Sky and Telescope - March 2017

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