WWW.ASTRONOMY.COM 45Delporte’s group finished
their study and presented their
report in 1928, and two years
later the book Délimitation
Scientifique des Constellations,
tables et cartes with the official
list and boundaries appeared.
As the commission per-
formed its duties, members
incorporated two general rules.
First, all of the constellation
borders followed lines of right
ascension (R.A.) and declina-
tion (Dec.), the celestial coordi-
nate system astronomers use. In
the 85 years since 1930, how-
ever, a long-term motion of our
planet called precession has
altered the boundaries so that
they no longer exactly follow
current R.A. and Dec. lines.
Second, the group tried to
accommodate the historical
shapes of the constellations
and not impose a square or
rectangular region where it
didn’t make sense. The legacy
we have, then, is 88 figures
whose borders follow lines of
celestial coordinates and which
still make sense historically.
Michael E. Bakich
Senior Editor
Q: WE OFTEN READ OF
DISCOVERING EXOPLAN-
ETS. HOW FAR AWAY FROM
EARTH WOULD WE BE “DIS-
COVERED” BY ANOTHER
CIVILIZATION WITH OUR
CURRENT DETECTION
TECHNOLOGY?
Tom Connelly, Chicago
A: Currently, the most common
ways to detect exoplanets are the
photometric transit technique,
which seeks dips in light due to
planets passing in front of their
host stars, and spectroscopic ob -
servations, which detect radial
velocity variations when a planet
pulls on its star. Measuring radi-
al velocity shifts for Earth-sized
planets with one-year orbits
around Sun-like stars is not pos-
sible with current technology.
Earth’s orbit around the Sun
causes a velocity change of only
about 1 centimeter per second,
whereas the best instruments
available today only can pro-
duce measured values of about
10 cm/s or larger. Photometric
work to search for exoplanet
transits from the ground
require the brightest stars and
can measure brightness varia-
tions less than about 1 percent
or so, enabling them to detect
exoplanets down to about
Neptune’s size.
Both of these techniques
suffer from having to observe
through Earth’s atmosphere
and, if trying to measure a long
orbital period such as Earth’s,
the need to keep tremendous
stability in the instrument over
extended time periods. Thus, at
present, neither of these tech-
niques could detect an Earth-
like planet orbiting a Sun-like
star at any distance with a
ground-based telescope.
NASA’s Kepler mission, how-
ever, was built just for this spe-
cific purpose — to find the
frequency of Earth-sized plan-
ets in approximately yearlong
orbits around Sun-like stars.
Kepler discovers exoplanets by
measuring the small drop in
brightness that occurs when a
planet transits the disk of its
host star. A planet as small as
Earth causes a 0.01 percent
drop in brightness while tran-
siting our Sun. This tiny change
is the reason these types of
measurements only can be made
in the stable conditions of space.
If we take Kepler as the best
current technology we have, we
could detect a planet similar to
Earth orbiting stars like our
Sun down to a visual magni-
tude of 12 to 13. Thus, the tech-
nology in a mission such as
Kepler would allow Earth’s
detection from as far away as
800 to 1,300 light-years.
Steve Howell
NASA’s Ames Research Center,
Moffett Field, CaliforniaQ: EVERYTHING AROUND
US IS SPINNING: PARTICLES,
PLANETS, STARS, GALAXIES.
WHY NOT THE UNIVERSE?
Cornel Halmaghi
Maple Ridge, British ColumbiaA: Spin is ubiquitous in the
cosmos. Planets rotate, as do
stars and galaxies. This comes
about simply from conserva-
tion of angular momentum.
When two ice skaters approach
each other and link arms, they
will start rotating — clockwise
if they link right arms and
counterclockwise if they link
left arms. If two stars approach
each other, gravity along with
other effects might cause their
mutual capture. The associated
matter may form planets and
other objects that share the
original angular momentum
and are all likely to rotate or
spin with an axis along its
original direction. This is a
random process, so we would
not expect the universe as a
whole to have a net angular
momentum, unless it had one
originally.
Astrophysicists believe the
universe started some 13.8 bil-
lion years ago in a “Big Bang”
that rapidly expanded into the
universe we see today. We are
confined within that universe,
and we can never see what, if
anything, is outside it. Still, we
can imagine seeing our uni-
verse from the outside.
It is possible to visualize
our universe spinning in this
larger space. Protons wereborn spinning, as were elec-
trons, neutrinos, etc. Why not
universes? If the universe was
born with an initial spin, as it
expanded from the Big Bang,
turbulence would cause the
initial angular momentum
to dissipate among smaller
and smaller objects. In other
words, we would not expect the
universe as a whole to be rotat-
ing now. Instead, the smaller
objects like galaxies would
“remember” the primordial
angular momentum and show
a preference for rotating about
the original spin axis.
This would show up in the
orientation of spiral galaxies as
we see them. In fact, there is
significant evidence that spiral
galaxies do exhibit a preferred
spin direction about an axis
close to the north pole of our
Milky Way. About 10 percent
more spiral galaxies are left-
handed spirals, spinning in the
same direction as our own.
Michael J. Longo
University of Michigan, Ann ArborSend us your
questions
Send your astronomy
questions via email to
[email protected],
or write to Ask Astro,
P. O. Box 1612, Waukesha,
WI 53187. Be sure to tell us
your full name and where
you live. Unfortunately, we
cannot answer all questions
submitted.Kepler-186f is the most likely habitable exoplanet candidate yet, but it’s
not another Earth. Nonetheless, astronomers say current human technol-
ogy could find our planet from as far as 1,300 light-years away. NASA/SETI/JPL