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By Juan Ivaldi, March 11, 2012 9:19 pm

In case you are wondering, I am referring to the planets Venus and Jupiter on the 11^th through the 15^th of March, 2012.   They are absolutely spectacular in the western sky just after the Sun has set. To see them, go outside after sunset but before the sky is fully dark and look up in the general direction of where the sun has set (west).  If skies are clear, you simply cannot miss the two bright planets near each other in the sky.

Venus is the brighter of the two.  The pair are so bright that they are easily spotted during twilight so don’t wait until dark. After sunset, Venus and Jupiter appear as brilliant diamonds against a velvet blue twilight sky.  They are putting on a dramatic show night after night as their positions can be seen to change.  This is one of those perfect times to share the sky with kids.

Over the next three days, Jupiter will appear to descend closer to the horizon as Venus rises higher above the horizon.  So, the two planets will pass each other.  The closest they get is about 3 degrees on the 12^th of March 2012.  This is about the same separation in the sky as 6 full moons end to end.  That may sound like a lot but when you see it for yourself, you may be amazed how close Venus and Jupiter appear.  If weather permits, keep observing the pair over successive nights to see the apparent motion.

Despite the fact that the pair are getting cozy in the sky, Venus and Jupiter are in fact hundreds of millions of miles apart.  Jupiter is the fifth planet from the Sun and Venus is the second.  Earth is the third planet so Jupiter is on an outside track (relative to us) and Venus is on the inside track as the planets orbit the Sun.  Jupiter is much farther away from us, but its huge size makes up for that and it still seems relatively bright.

In scientific terms, the alignment we see is called a planetary conjunction.  This happens when two planets have similar right ascension.  Alignments like these have happened before and will continue to happen because all the planets tend to stay close to a line in the sky called the ecliptic.  This is the line traced out by the motion of the Sun.  Eventually, two (and sometimes more) planets will appear in the same part of the sky.  As such, these conjunctions have no real significance other than the fact that they are marvelous to see.  This will be one of the better planetary conjunctions for some time so go out and see the dazzling pair in west.  All you will need are your eyes and clear skies to the west!

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By Juan Ivaldi, January 3, 2010 5:11 pm

Quick answer: It is still out there and continues to orbit the Sun as it has for eons.

Pluto is a distant and frozen world orbiting the Sun once every 249 years at an average distance of 3.7 billion miles away. To the best of our knowledge, it continues to be a frigid ball of rock and ice orbiting the Sun in the same way it has done for eons.   Since Pluto’s discovery in 1930, our understanding of the solar system has progressed significantly and many more outer solar system bodies have been discovered. Despite this increase in knowledge, much remains to be learned about the icy world we call Pluto.

Pluto’s largest moon Charon was discovered in 1978. The precise sizes of Pluto and Charon only became known in the late 1980’s.  In 2005, two additional and much smaller moons of Pluto were imaged with the Hubble Space Telescope.  These newly discovered moons received the names Nix and Hydra (respectively in order of their distance away from Pluto).

Pluto shares characteristics of other solar system objects orbiting in a broad flat ring of rock-and-ice bodies just beyond the orbit of Neptune known as the Kuiper Belt.  Named after astronomer Gerard Kuiper, the Kuiper Belt is home to the left over icy debris from an early time in the formation of the solar system.  Although Kuiper proposed the existence of this belt in the 1950s, it wasn’t until the 1990s that Kuiper Belt objects (KBOs) were discovered.  At the time of this writing, there are 1100 known KBOs.  For the vast majority of these, only the positions and orbits are known.

Because of their great distance away from the Sun, KBOs are very cold and retain the simple molecules of the solar system in the form of ices on their outer layers. Depending on the amount and type of ice on the surfaces, the objects can appear more or less bright in telescopes.  Pluto and the KBOs are so distant that the images appear as fuzzy dots or blobs even with the most powerful of telescopes. The best photographs of Pluto taken by the Hubble Space Telescope now show that the surface has light and dark markings which change seasonally.  Scientists think this is caused by ice which sublimates into a gas, leaving one location only to refreeze elsewhere on the surface.

Some astronomers believe that Pluto is the king of the KBOs, that is, it is among the largest, brightest, and nearest of them. Based on its density, Pluto is estimated to be composed of 70% rock and 30% water ice.  For comparison, water is much less than 1% of the total composition of Earth.

Another trait that Pluto has in common with other KBOs is a highly tilted and oblong orbit compared to the major planets. Size is yet another differentiator.  Although the largest KBOs are considered to have a significant amount of rock in their composition, they are generally much smaller in size compared to the rocky inner planets of the solar system, Mercury, Venus, Earth, and Mars.  The smallest of these four is Mercury.  Pluto’s diameter is slightly less than half that of Mercury.

In the past decade, planet-like objects similar in size to Pluto have been discovered in the Kuiper Belt. The largest of these, later named Eris, was discovered by the team of Michael Brown, Chad Trujillo, and David Rabinowitz in 2005.  Eris, with its single moon Dysnomia, lies further away from the Sun on average compared to Pluto. Measurements of the diameter of Eris show that it is slightly smaller than Pluto but not by much.  A handful of other objects have been found in a similar size range but no KBOs larger than Pluto have been discovered so far.

After its discovery in 2005, the community of astronomers and space scientists had great difficulty with the classification of Eris.  Astronomers became caught in a name game that turned out to be a long, controversial, and mostly empty debate about the definition of the word “planet”.  Does Eris become a tenth planet with potentially more planets joining the ranks or is it in a different class?  If Eris is in a different class then does this have implications for the classification of Pluto?

In 2006, the International Astronomical Union (IAU), an international society of astronomers, created a new classification called “dwarf planet”.  According to the new convention, the word “planet” is reserved for the 8 largest Sun-orbiting bodies of the solar system.  Pluto and Eris were assigned into this new dwarf planet classification by virtue of a vote by society members at an IAU meeting. Pluto was thereby downgraded and Eris never made it to full planet status.

The controversial vote received intense media attention and caused confusion in the general public. For now, many researchers have adopted the IAU nomenclature. Not surprisingly, the debate still rages on among astronomers and non-astronomers alike.  Perhaps it illustrates that we still have a long way to go in our understanding of the solar system.  Regardless of the naming convention, most would agree that Pluto and its cousins in the Kuiper Belt are important members of our solar system and greater study is needed.

Although no spacecraft has ever visited Pluto, one is on the way right now.  The NASA New Horizons mission spacecraft has already passed the half-way point and is on schedule to arrive at Pluto in 2015.  The purpose of the mission is to make measurements of the chemical composition of the surface and the extremely thin nitrogen atmosphere of Pluto.  NASA scientists also plan to take high quality photographs of the surface of Pluto and Charon to better understand the nature of the ice and the surface topography.  They will also look for other moons beyond the three that are currently known to orbit the remote icy world.  After visiting the Pluto system, New Horizons will move on to other targets in the Kuiper Belt.

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By Juan Ivaldi, December 3, 2009 11:01 pm

Quick answer: The planets all orbit around the Sun in the same direction because they retain the rotation of the original cloud of gas and dust from which they formed.

If you had a spaceship and were able to blast off from the North Pole of the Earth and rise a few hundred million miles above the plane of the solar system, you could look down on the planets.  You would be able to see that the planets are all orbiting in a counterclockwise direction around the Sun.  This is no coincidence and neither is the fact that the orbits of the planets lie in nearly the same plane.  Yet another clue is that the equator of our spinning Sun is well aligned with the plane of the orbiting planets.

These facts were known to scientists in the 1700’s.  In the later part of that century, Immanuel Kant and Pierre-Simon de Laplace began to conclude that the situation could not have arisen by chance.  They proposed that the planets evolved out of a primordial whirling disk.  This helps explain the observations.  Astronomers today agree that the basic idea of Kant and Laplace is correct although their ideas needed refinement.

The modern version of their idea is known as the nebular hypothesis.  It is generally accepted by astronomers as the preferred description for how the solar system formed.  According to the nebular hypothesis, the solar system began roughly 4.6 billion years ago as an immense rotating cloud of gas and dust called the solar nebula.    Our Sun was born from the gas in the core of this protostellar cloud.  As material was pulled inward by the gravity, the speed of rotation increased.  The increased rotation rate caused the cloud to begin to flatten out into a disk shape.

Every planet started out as a grain of dust within the disk.  Dust particles began to stick to one another by electrostatic attraction.  As the clumps grew bigger they started to have larger and larger gravitational fields.  Gravity attracted the larger chunks to one another causing collisions.  The collisions became more numerous.  The gentler collisions resulted in objects staying stuck together into bigger objects whereas the higher speed collisions caused destruction.  The bigger objects, called planetesimals tended to survive since these were harder to destroy depending on the intensity of the impacts.

Over time the planetesimals aggregated into protoplanets.  These were big enough to sweep out their orbits.  The protoplanets then underwent a period of massive and violent mergers.  Ultimately the greatest survivors of this process became the known planets of the solar system.  Getting to this point is believed to have taken many millions of years.  A large amount of leftover building materials was still flying around the solar system causing heavy bombardment of the planets.  Today we see the evidence of these bombardments as craters.  Some of that left over material is still around in the solar system.  Although impacts still occur today, the really big collision events are much more rare.

The nebular hypothesis explains why the planetary orbits are all going counterclockwise as viewed from the north and why the Sun has a counterclockwise rotation on its axis.  The original rotation direction of the solar nebula is still here today billions of years after the Sun began to shine and the planets emerged from their beginnings as tiny grains of dust in a whirling cloud.