Sky & Telescope - December 2018 - 20

Extremely Large Telescopes, Part II

Stars and Planets
Closer to home both in space and in time, astronomers
expect to learn much more about the evolution of galaxies
and the birth of stars. Here's where the combination of highresolution imaging and high-precision spectroscopy comes
into play. Studying star-forming regions in very early galaxies
in detail will reveal how the activity of central black holes
inﬂuences these processes, and how galaxies acquire their
ordered shapes and structures over time. In nearby galaxies,
such studies will hopefully shed light on the formation of the
most massive stars in the universe (S&T: Oct. 2015, p. 24), as
well as on the birth of low-mass brown dwarfs and free-ﬂoating planetary bodies.
Recent results by the SPHERE instrument at ESO's Very
Large Telescope have revealed protoplanetary disks around
newborn stars in unbelievable detail; the imaging integralﬁeld spectrometers of the new extremely large telescopes
will yield much more detail and provide astronomers with
stunning views of the birthplaces of planetary systems. These
measurements will be complemented by millimeter and submillimeter observations by the 66-dish ALMA observatory in
Chile: ALMA remains the instrument of choice to study the
chemical composition and the presence of prebiotic molecules
in protoplanetary disks, since most spectroscopic signatures
of molecules appear at those longer wavelengths.
And then there's exoplanet research. While most discoveries of new worlds will be made by dedicated instruments,
both in space and on the ground, the monster scopes of the
future are particularly suited to study their properties and
characteristics. Astronomers know of a handful of potentially
habitable Earth-size planets (including Proxima Centauri b,
which orbits the nearest star beyond the Sun), and NASA's

PROTOPLANETARY DISK The glowing dot in this image is a planet
forming around the red dwarf star PDS 70. (The central star's light is
blocked out.) The system lies about 360 light-years away.

DECE MBE R 2 018 * SK Y & TELESCOPE

u MOON UP CLOSE The Keck telescopes
cannot resolve surface details on Jupiter's volcanic moon Io (top). But with the
TMT's greater light-gathering power, the
telescope's Infrared Imaging Spectrograph
should detect small-scale features (center,
simulated image), not too far off from what
appears in a real image from the Galileo
spacecraft (bottom).

Keck adaptive optics

TESS spacecraft is expected to ﬁnd
TMT IRIS
dozens more in the solar neighborhood. "So the next key question is,
well, what are these worlds actually
like?" says exoplanet researcher
Tyler Robinson (Northern Arizona
University). "Do they have an atmosphere, like Earth? Oceans? Life?"
The European Extremely Large
Telescope in particular will play
Galileo
a key role in ﬁnding water on
Earth-like exoplanets. Because of
its huge light-gathering power and
the sophisticated coronagraphic
(starlight-blocking) capabilities of
its METIS instrument, it will be
able to directly image such planets
around the nearest stars, says Ignas
Snellen (Leiden University). Studying an exoplanet's thermal
emission and reﬂected starlight could reveal the spectroscopic
ﬁngerprints of water vapor - a feat never before accomplished
for an Earth-like exoplanet. For a planet within the right
temperature range, the existence of atmospheric water vapor
points to liquid surface water.
Finding atmospheric oxygen is going to be much more
difﬁcult, says Snellen, because molecular oxygen produces a
less conspicuous spectroscopic signature than water. Still, he
is hopeful that oxygen could be detected out to distances of a
few tens of light-years.
James Kasting (Penn State) is less optimistic. "The ELT may
be good for Proxima Centauri b," he says, "but probably not
much beyond that. The same is true for the James Webb Space
Telescope - it's not supposed to be able to see oxygen." If you
also want to detect other gases, like methane or nitrous oxide
- promising biomarkers, especially in combination with
oxygen - you may need a much larger telescope in space, says
Kasting. "Still, I hope to be alive when this happens."
According to Robinson, determining whether or not
habitable (water-bearing) worlds and life are common in the
universe is the most important next leap in the ﬁeld of exoplanet science. "It may be that we uncover a huge number of
nearby habitable worlds, but also ﬁnd that virtually none of
these show signs of life. That would be humbling - it would
say that life is a rare phenomenon. Alternatively, it may be
that nearly all of these ocean-bearing worlds show signs of
life. That would be extremely exciting. Large telescopes, both

IO: T M T IN TER N ATION A L OBSERVATORY;
PROTOPL A NE TA RY DISK: ESO / A . M ÜLLER E T A L.

between elementary particles - may have slightly increased
over cosmic history, although measurements so far have been
equivocal. These and other issues in the ﬁeld of fundamental
physics cannot be addressed by particle accelerators, which is
why theoretical physicists are as keen to use the instruments
of the future as astronomers are.

Sky & Telescope - December 2018

Table of Contents for the Digital Edition of Sky & Telescope - December 2018