December 2012 Sky from the Keeble Observatory
Astronomers used to puzzle over whether our home planetary system is unique, or
if planets are commonly to be found orbiting other stars. We always suspected that
we were not alone; we now know that planets are a common byproduct of star formation.
Last month’s column identified just a few of the planetary discoveries recently
in the news, but you should know that our current list of suspected planets is in
the thousands, with the number confirmed at 852! For the current best catalog of
confirmed exoplanets, check out the site maintained by the Paris Observatory at
So, just how do we go about finding these planets? There are three key techniques,
two of which rely on how gravity works in particular, and how forces work in general.
Gravity is an attractive force. It’s what keeps the Earth orbiting the Sun, or the
Sun orbiting in the Galaxy, or you firmly planted on Terra Firma. Newton’s
so called 3rd Law of Motion tells us that when the Sun pulls on Earth,
Earth also pulls on the Sun with a force of equal magnitude but opposite in direction.
Newtons’ 2nd Law tells us that the acceleration of either Earth or Sun
is inversely proportional to their masses.
If the exoplanet orbits in or near the plane of the sky, the tug that planet exerts
on the star can cause its apparent proper motion (relative to the background “fixed
stars”) to wiggle with the same period as the planet’s orbit. Obviously, this is
easier for nearby stars since the actual deviations in the trajectory are small.
It should also be clear that a massive planet orbiting close to a low mass star
will give the best opportunity to find a planet. This is known as an observational
bias, a point we’ll return to shortly.
If the plane of the orbit is nearly along the line of sight, the planet’s tug will
cause periodic changes in the line of sight velocity which we can detect with a
spectrometer by looking for a Doppler shift in the wavelengths of light. The fractional
shift in wavelength is directly proportional to the radial velocity, and we can
use this information to determine the size of the orbit and in turn the mass of
the planet. Again, the observational bias favors detection of massive planets orbiting
close to the star.
This observational bias means that many of the planetary systems we discover don’t
look like ours, with smaller planets close to the star and the massive gas giants
far away. Rather they have very massive planets in orbits so close that their periods
are measured in days or even hours. But we are beginning to find, as our technical
skills are refined, that there are also a lot of systems which look more and more
like our home.
A third technique which lends itself to survey work is that employed by the Kepler
probe. It examines a small but rich region of the sky and monitors thousands of
stars for periodic dimming of the light we receive. This is taken as evidence for
a planet passing between the star and our telescope. From the amount of light blocked
and the period of the orbit we can also get an estimate of the mass of the planet
and its physical size. If we know mass and radius, we can calculate density and
determine whether the object is rocky or if it’s a low density gas or ice world.
Lunar phases for December: Last Quarter on the 6th, at 10:31 am;
New Moon on the 13th, at 3:42 am; First Quarter on the 20th,
at 12:19 am; Full Moon on the 28th, at 5:21 am.
Mercury returns to the predawn skies through mid-December, but will be low on the
east-southeast horizon and likely lost in ground clutter. Better opportunities are
there for seeing Venus and Saturn. Saturn rises about 3:00 am, and will be about
20 degrees above the horizon as the Sun rises. Venus is about two hours behind Saturn,
but will be much brighter and lower. If you manage to see both of them, extending
an imaginary line through them toward the horizon will help you find Mercury. Jupiter
graces the evening sky, and is visible all night, near the Pleiades in the constellation
As we noted above, planets are “wanderers” so we cannot expect them to repeat their
configurations from year to year. The “fixed stars” are predictable. Here’s how
the December constellations were described in 2007:
Looking overhead at mid-month, about three hours after sunset, finds the constellation
Andromeda at zenith. It’s not an impressive or terribly bright asterism, but is
noted for the presence of the Andromeda Galaxy. Two million years ago the light
you see from this relatively nearby galaxy, much like our own, left to come to Earth.
We see our own galaxy in cross section as the faintly luminous Milky Way, stretching
from east to west, with a distinct bow towards the north. The stars making up this
band are typically hundreds to thousands of light years away – a factor of over
a thousand smaller than the distance to Andromeda.
The constellation Cassiopeia is to the north, in the plane of the Milky Way. It
looks like a crooked M. To the west-northwest, Cygnus the Swan now looks like its
other namesake, the Northern Cross. Deneb is the bright star at the top of the cross,
about 40 degrees from the horizon. To the northwest is Vega, at about 18 degrees,
and Altair is a little lower and to the west. When these were high in the sky, we
called them the Summer Triangle. Now we bid them adieu until late spring.
Rising to the east is Orion, which will dominate the winter starscape. Above Orion
is the familiar cluster of the Pleiades, about 55 degrees from the horizon and climbing.
A special note, that the Geminid meteor shower should peak early in the morning
of the 14th. For several days around that date, you can expect to see
up to 75 “shooting stars” per hour. For best viewing, get to a dark site away from