30 ASTRONOMY • JUNE 2022
around nearby stars, partly to correct
this problem. So far, what we know is
that mini-Neptunes and super-Earths are
both so common that it is odd that nei-
ther exists in our own planetary family.
If we take all the planets found by Kepler
on orbits less than 100 days and adjust
for the fact that larger, close-in planets
are easier to see, over half of the detected
planets still fall into this category.
Actually, this exercise reveals another
new truth about the planetary popula-
tion. There’s a distinct absence of planets
with sizes right in the middle of the gap
between Earth and Neptune; at 1.7 Earth
radii, in particular, there are few planets,
a feature that has become known as the
radius valley. It seems that planets really
are either a super-Earth or a mini-
Neptune, lying on one side of the valley
or the other. The explanation comes
from the time when planets are still
forming. As the star heats the disk, only
the most massive planets can hang onto
their atmospheres. If the forming planet
is not massive enough, its atmosphere is
lost and it will end up as a super-Earth,
while the more massive worlds can hang
onto the gas and remain mini-Neptunes.
The details of these processes, and
how they might have inf luenced our own
world’s formation, are still being worked
out, but the existence of this fundamen-
tal division between planetary types
which we were completely oblivious to
until a few years ago gives you an idea as
to how fast things are changing. Careful
observations of planets on both sides of
the radius valley, along with their stars,
will be a big part of astrophysics in the
next decade or so.On the move
A third big surprise from our exoplanet
discoveries is that one-third of known
worlds have orbits which are distinctly
eccentric. It’s true that Earth’s orbit
isn’t precisely a circle, but it’s prettyBELOW: Mini-Neptunes are gaseous planets smaller
than Neptune but larger than Earth. Our solar
system is also conspicuously missing this type of
planet, which astronomers are discovering are
common throughout the cosmos. Here, artist’s
illustrations of two such mini-Neptunes (TOI-421 b
and GJ 1214 b) are shown to scale with photos of
Earth and Neptune. NASA, ESA, CSA, DANI PLAYER (STSCI)EARTH
RADIAL VELOCITY METHOD
StarPlanetCenter
of massBlueshiftCenter
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100500-50-100
TimeThe star’s back-and-forth motion stretches
(redshifts) and squeezes (blueshifts) its light.Astronomers have several ways to find planets
circling other stars. One is the radial velocity
method, which measures the way a star
“wobbles” to and fro on the sky as an orbiting
planet tugs on its sun. This wobble affects the
wavelength, or color, of light we receive from
the star, making it appear redder or bluer,
depending on the direction of motion.ASTRONOMY:^
ROENK
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LY