44 ASTRONOMY • SEPTEMBER 2017A: Finding something we’ve
never seen before is hard, so
naturally there are numerous
parameters for SETI searches.
Will a signal arrive today, next
year, did it happen during the
reign of Julius Caesar, or all
three? Will it last for less than
a billionth of a second or
remain continuously visible?
Will its wavelength be in radio,
optical, infrared, or not even a
photon? A signal could
“sound” like a simple tone or a
complex, information-rich hiss.
Could you see it with your
naked eye, or will it require an
enormous telescope?
A SETI search tries to cover
as much of this “parameter
space” as possible. Ideally, we’d
observe the whole sky all the
time, removing both time and
space from the equation. But
most of today’s high-powered
telescopes can’t even observe
the whole Sun or Moon at once,
never mind the entire sky. So,
you could choose to stare at
one tiny patch of sky all year
round, or you could look at half
a million patches for just a
minute each. But either way,
you’d cover only about five
millionths of the space dimen-
sion — and that’s assuming you
operated every day of the year!
To deal with this resource
limitation, SETI researchers
often bias toward targets
deemed more likely to yield a
detection, such as closer stars,
more densely packed areas like
the galactic plane, or stars with
known orbiting planets. One
technique is indeed to look at astar when one of its planets
passes in front of (a transit) or
behind it (an occultation)
Transits are how we’ve
found most exoplanets to date,
so maybe other civilizations
are using this to help us know
where and when to look for
their signal. Occultations are
useful because the parent star
suddenly hides the planet from
us, which might ironically illu-
minate a pattern in the radia-
tion that we hadn’t discerned
from the overall cacophony.
However, observing an occul-
tation requires preparation and
luck for the once per exo-year
event!
Eliot Gillum
Director of the Optical SETI
Program, SETI Institute,
Mountain View, CaliforniaQ: WHAT IS THE DIAMETER
OF A BODY WITH JUST
ENOUGH MASS TO PULL
ITSELF INTO A SPHERE?
HOW BIG ARE THE SMALL-
EST SPHERICAL BODIES
IN OUR SOLAR SYSTEM?
Paul Kendelman
Pennsville, New JerseyA: The solar system contains
hundreds of thousands of small
bodies larger than a few kilo-
meters in size. Many are in the
main asteroid belt, which con-
tains the fragmentary rem-
nants of planetesimals, small
bodies from which the planets
formed. A few have survived
nearly intact. Most asteroids
are irregular in shape; however,Astronomy’s experts from around the globe answer your cosmic questions.HOW SETI SEARCHES
Ceres, the main belt’s largest
body and the smallest dwarf
planet, is round. The smallest
round bodies may be found
among the icy moons of Saturn
or in the outer solar system.
Roundness requires hydro-
static equilibrium, which is one
of the criteria used by the
International Astronomical
Union to define a planet or
dwarf planet. When subjected
to self-gravity, a rotating f luid
will relax to a round (ellipsoi-
dal) shape. Solids (rigid bodies)
resist changes in shape, but
when stresses are large enough,
they deform, fracture, and
f low. For massive objects, grav-
itational and rotational forces
overcome rigid body forces,resulting in round bodies with
low topographic relief.
Planetesimals that formed
close to the Sun incorporated
little water and condensed
largely from silicate grains con-
taining short-lived radionu-
clides such as aluminum-26,
which originated from a nearby
supernova. The heat produced
by radioactive decay may have
been sufficient to melt bodies
as small as 62 miles (100 kilo-
meters) in diameter. Imagine a
massive, round blob of molten
rock — as heating abated, these
bodies solidified to form an
iron-rich core, silicate mantle,
and basaltic crust.
For these igneous planetesi-
mals, the diameter needed toASKASTR0
Q: WHAT ARE THE PARAMETERS FOR SEARCH FOR EXTRATERRES-
TRIAL INTELLIGENCE (SETI) SEARCHES AND THEIR DIRECTION
OF FOCUS? ASSUMING OTHER CIVILIZATIONS CAN SEE EARTH
TRANSITING THE SUN, DO WE SEARCH AREAS OF THE SKY IN THE
SAME PLANE AS OUR POTENTIAL OBSERVERS? Michael Martin, Gilbert, ArizonaThe Allen Telescope Array carries out
SETI searches at centimeter wave-
lengths. It currently uses 42 elements
in the Cascade Mountains near Lassen
Peak in California.The dwarf planet Ceres is round due to its size; Vesta once may have been
round, but it has since been deformed by impacts. Planetesimals smaller
than the size required to reach hydrostatic equilibrium were never round.
(Scales are approximate; shape models are available from NASA’s
Planetary Data System at http://sbn.pds.nasa.gov.)SETH SHOSTAKTHOMAS H. PRETTYMAN, PLANETARY SCIENCE INSTITUTE; DATA SOURCES: NASA, JAXA, ESACeresPhobos(4) Vesta(25143) Itokawa620 miles (1,000km)12 m i l e s (20 k m)326 miles (525km)1,6 40 feet (50 0m)