WWW.ASTRONOMY.COM 31
of the Milky Way — can be found by
this technique, specifically via a form
known as microlensing, because the
changes are small. What’s more, the
amount the star brightens tells us about
the mass of the planet and several of
those detected have masses
comparable to Earth.Picture perfect
It’s clear we can
learn a lot from
these indirect
methods of planet
detection. Indeed,
for most systems
we have no choice
but to rely on these
techniques, where we
learn about the plan-
ets by studying stars. It
is, though, undoubtedly true
that direct imaging — seeing the planets
themselves — can tell us much more. The
problem isn’t so much that the planets
are intrinsically faint — especially after
formation, when they are still being heat-
ed by their gravitational contraction, they
will shine brightly — but that the glare of
light from the star makes them very dif-
ficult to detect.
The solution is to use an instrument
called a coronagraph to block the light
from the star. By placing an obstruction
in the field of view, and through some
very careful use of image processing algo-
rithms, planets can be revealed. It is easier
to see companions which are further fromthe star, and direct imaging has been used
to find worlds tens or even hundreds of
astronomical units from their star.
(One astronomical unit is the average
Earth-Sun distance.) There are only 50
or so planets which have been directly
imaged, each of them extremely
precious. They tend to be
young, and massive —
many are nearly large
enough to be stars.
Comparing each
of these exoplanet
systems to our own
raises many ques-
tions. Why are the
worlds of the solar
system on nearly cir-
cular orbits? Why don’t
we have a super-Earth?
Why did Jupiter remain
where it was, rather than plow-
ing through the inner solar system and
becoming a hot Jupiter? Today, scientists
look at the origins of our own solar sys-
tem in the light of these new discoveries,
learning more about our own home now
that we’ve looked outward at the stars.close — its eccentricity is 0.016, so the
difference in length between the longest
and shortest axes is not much more than
1 percent. A third of exoplanets have
eccentricities greater than 0.1, an order
of magnitude larger. This fact is a clue
that life in a forming solar system may
be even more complicated than we had
suspected until now.
We know that planets can move
through the protoplanetary disk, usually
by interacting with the material in the
disk itself. These large eccentricities were
caused by more dramatic interactions
between planets — so the fact that the
planets of our solar system have largely
circular orbits tells us it must have been
an unusually calm place when they were
forming.
Even more extreme interactions are
possible. When two large bodies come
close together, their spheres of gravita-
tional inf luence overlap. Anything
caught between them will become
dynamically excited — in other words,
once-stable orbits will be disrupted, and
material can be expelled from the system.
It’s even possible for one of the planets to
be expelled, spinning off into space.
Some interstellar wanderers have even
been detected, thanks to a technique
called gravitational lensing, originally
developed for looking at distant galaxies.
If a planet passes between us and a dis-
tant star, it acts as an otherwise unde-
tected lens, bending and amplifying the
star’s light and revealing its presence.
Distant planets — even on the other side
TOI-421 b GJ 1214 b
NEPTUNE
It turns out
that the mostcommon type of planet
in the Milky Way is one
that does not exist in ourown solar system, with a
radius between that
of Earth and
Neptune.Brian May is best known as the lead
guitarist for Queen and holds a Ph.D.
in astrophysics. Patrick Moore was a
beloved astronomer and presenter of the
BBC’s The Sky at Night. Chris Lintott is
an astrophysicist, author, and broadcaster
at the University of Oxford. Hannah
Wakeford is a lecturer in astrophysics
at the University of Bristol.