Sky & Telescope - September 2024 - 36

The Exoplanet Search
Ten Billion to One
Up to now, the exoplanets that astronomers have imaged have
been at most 1 million times dimmer than their host star
- the firefly around the lighthouse. When we try to directly
image an Earth-like exoplanet, though, we're actually looking
for something much fainter. We need to replace our firefly
with the infinitesimally dim glow of a single bioluminescent
alga cell - and even that is still a little too bright. Put
another way, any Earth analog will be at least 10 billion times
fainter than its sun.
Spotting such a puny signal requires extreme precision in
blocking out as much of the star's light as possible, much as
you would do by holding up your hand to block out the dazzle
from the lighthouse. This must be done without obstructing
Contrast Ratio
the precious photons coming from the planet.
As light from a planetary system enters an exoplanetimaging
telescope, it is first corrected by adaptive optics, a
continuous monitoring system linked to actuators that intentionally
warp the telescope's mirror to compensate for any
aberration in the captured light (S&T: May 2016, p. 30).
For space telescopes, this system corrects for things like
polishing errors and slow drifts in the system. For groundbased
telescopes, it also crucially corrects for turbulence in
Earth's atmosphere - the primary confounding factor when
conducting any astronomical studies from terra firma.
Once all the aberrations are removed and the light from
the star is exquisitely focused, the next step is (ironically) to
block most of that light out. This is done using an instrument
called a coronagraph.
" The name coronagraph harkens all the way back to the
first application of this technology, which was by Bernard
Lyot in the 1930s, who wanted to block out the disk of the
Sun to create an artificial eclipse, " explains Vanessa Bailey
(NASA Jet Propulsion Laboratory).
Lyot essentially installed a spot or " mask " inside his
1
Lighthouse
~10,000 candela
Sun
telescope to exactly cover the disk of the Sun. Combined with
other optical elements, the design enabled him to observe our
star's outer atmosphere, called the corona. " We can adapt
that technology from being applied to the Sun to a small,
point-like distant star and have the same goal of blocking the
bright starlight to see something faint around it. "
Amateur astronomers may have done something similar,
10-6
Firefly
~0.01 candela
Young exo-Jupiter
(10 million years old,
near-infrared heat glow)
using an occulting bar to hide bright stars in order to see a
faint planetary nebula or other deep-sky object in the same
field of view. But seeing a planet nestled up close to its star
requires something more. Although the principle remains
the same, today's professional coronagraphs are significantly
more sophisticated than a physical bar or disk, incorporating
various optical principles of interference using multiple
masks and deformable mirrors.
After passing through the instrument's adaptive optics
10-9
and coronagraph, the tiny amount of remaining light coming
from the planet (as well as the final dregs of stray starlight)
feeds into a spectrograph and image-processing equipment.
These extract every last piece of information from every last
photon, including which compounds may exist in the exoplanet's
atmosphere.
Bioluminescent algal cell
(dinoflagellate)
~0.00001 candela
Jupiter
(4.5 billion years old,
reflected visible light)
IF YOU SQUINT A young gas giant, still glowing with the heat of its
formation, is a millionth as bright as the Sun. A more mature gas giant,
like Jupiter, no longer glows brightly in infrared; its brightness primarily
comes from reflected starlight. This gleam is even fainter - a thousandth
as bright as the young planet and a billionth as bright as the Sun.
36 SEPTEMBER 2024 * SKY & TELESCOPE
Unwelcoming Worlds
In all, somewhere between 25 and 80 exoplanets have been
directly imaged using this process. The uncertainty comes
from how we define a planet: Some candidates are so massive
that they appear to be more akin to failed stars, known as
brown dwarfs, while other " rogue planets " wander the galaxy
without a host star.
Bruce Macintosh (University of California, Santa Cruz)
has had a hand in many of these exoplanet discoveries.
Among his favorites are the proverbial poster children for
direct imaging: the planets that form the multi-planet system
BEATRIZ INGLESSIS / S&T, SOURCE: KATHERINE B. FOLLETTE / PUBLICATIONS OF THE
ASTRONOMICAL SOCIETY OF THE PACIFIC 2023 / CC BY 3.0
Sky & Telescope - September 2024

Table of Contents for the Digital Edition of Sky & Telescope - September 2024
