The^
Sun’s^ pathTOP VIEWThe SunPlane of the galaxyNorth eclippotliec Ecliptic plane
The Sun
Earth’s orbitThe Sun’s path60 °The Sun and Earth’s orbit not to scaleThe Sun’s pathSIDE VIEWPlane of the galaxyNorth (^) ec
lipti
c (^) po
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Earth
Earth’s spin axis
60 °
23. 4 °
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times Earth’s mass — as the lower limit, while others
consider planets as massive as Uranus — about 14.5 times
Earth’s mass — super-Earths as well. It’s confusing!)
The leading theory for the formation of the Moon, the
Giant Impact Hypothesis, states that a Mars-sized object,
called Theia, collided with Earth to create the Moon. The
mass of Earth is about 1.32 x 10^25 pounds (5.97 x 10^24
kilograms). The mass of the Moon is about 0.016 x 10^25
pounds (0.073 x 10^24 kg), or roughly 1.2 percent Earth’s
mass. For added reference, the mass of Mars (and, thus,
Theia) is about 0.14 x 10^25 pounds (0.642 x 10^24 kg), or just
under 11 percent Earth’s mass. Both the masses of Mars
and the Moon are incredibly small compared with Earth.
So, even if you added the entire mass of the Moon back
to Earth, our planet would only grow 1.2 percent more
massive, or about 1.01 times its current mass.
Additionally, this might be an overestimate, as not all the
material in the Moon likely came from Earth, but some
also may have come from Theia as well.
Thus, if we considered a “pre-Moon Earth” as the
definition for the mass of Earth, we still wouldn’t really
need to adjust the definition of super-Earths. Because the
current definition of a super-Earth is somewhat arbi-
trary, I think that deciding on a universal definition
would likely be the best way to make sure everyone’s
talking about the same type of planet when they say
“super-Ea r t h.”
Alison Klesman
Senior Associate Editor
QI
IN WHICH DIRECTION, WITH RESPECT
TO ITS AXIS OF ROTATION, DOES THE
SUN MOVE THROUGH THE MILKY WAY?
Carlos Rivera
Tegucigalpa, Honduras
AI
If you imagine looking down on the Milky Way,
the Sun is located nearly 27,000 light-years from
the center, about halfway between the center and the
edge of our disk-shaped galaxy. Looking from the side,
the disk is relatively f lat and the Sun is currently located
about 55 light-years above the plane of the galaxy’s disk.
Over time, the Sun orbits the center of the galaxy, sketch-
ing out a roughly circular path (again, looking down from
above) that takes about 230 million years to complete at
a speed of about 137 miles (220 kilometers) per second.
With respect to its own axis of rotation, the Sun is
moving through the galaxy tipped at an angle of about
60° from the galactic plane. This also applies to the
planets orbiting the Sun — just like the disk of our
galaxy, if you were to look at our solar system from the
side, the planets orbit the Sun in a relatively f lat plane.
Essentially, the Sun and the plane in which the bodies
of the solar system orbit around it are both tilted for-
ward by 60° as they move through the galaxy.
It’s perhaps also worth noting that the Sun doesn’t
appear to trace a f lat circle — in one plane only — as it
moves around the galaxy. The Sun actually appears to
bob up and down through the disk (we are up right now,
above the plane of the disk) as it moves, with a period
of about 60 million years.
Alison Klesman
Senior Associate Editor
Super-Earth exoplanets are larger and more massive than Earth,
but smaller and less massive than the ice giant Neptune. This
artist’s concept shows the size of the super-Earth planet
CoRoT-7 b (middle), which weighs about 3.18 Earths and falls
between the mass and radius of Earth (left) and Neptune.
The Sun orbits the
center of our galaxy
tipped at an angle of
60 °. Over the course of
its orbit, our star also
bobs up and down
above and below the
galactic plane.
THE SUN’S
GALACTIC
MOTION
ALD
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ON
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