What is a planet?

While many people can point to a picture of Jupiter or Saturn and call it a "planet," the definition of this word is much more subtle and has changed over time. Many astronomers decided on a new definition in 2006 after the discovery of several worlds at the fringes of the solar system — a decision that remains controversial.

The International Astronomical Union defined a planet as an object that:

• - orbits the sun
• - has sufficient mass to be round, or nearly round
• - is not a satellite (moon) of another object
• - has removed debris and small objects from the area around its orbit

The IAU also created a new classification, "dwarf planet," which is an object that meets planetary criteria except that it has not cleared debris from its orbital neighborhood. This definition meant that Pluto — considered a planet at the time — was demoted and reclassified as a dwarf planet. But not all scientists agree with this classification.

The term "planet" originally comes from the Greek word for "wanderer." Many ancient cultures observed these "moving stars," but it wasn't until the advent of the telescope in the 1600s that astronomers were able to look at them in more detail. Small telescopes revealed moons circling Jupiter — a big surprise to Galileo Galilei (the likely discoverer) and his opponents at the Catholic Church — as well as rings around Saturn and an ice cap on Mars.

Telescopes also revealed the existence of objects not known to the ancients, because they are too far away and small to be spotted with the naked eye. Uranus was found on March 13, 1781, by the prolific astronomer William Herschel. Ceres was discovered between Mars and Jupiter in 1801. It was originally classified as a planet, but it was later realized that Ceres was the first of a class of objects eventually called asteroids. Neptune was discovered in 1846.

Astronomers continued scouring the solar system's outer reaches in search of a large "Planet X" that was believed to be disturbing the orbits of Uranus and Neptune. While these irregularities were later discounted by further observations, Clyde Tombaugh did spot a smaller object in 1930 beyond the orbit of Neptune. Called Pluto, the object (then called a planet) was relatively small and had a highly eccentric orbit that sometimes even brought it closer to the sun than Neptune is.

Discovery of more worlds

Nothing close to Pluto's size was found in the solar system for more than two generations. That changed in the 2000s, when Mike Brown — a young astronomer at the California Institute of Technology — was in search of a defining research project and decided upon searches for objects in the outer solar system.

In quick succession, Brown and his team discovered several large "trans-Neptunian objects," or icy bodies beyond Neptune's orbit. While discovering icy objects that far away was not unexpected — the supposed Oort Cloud, the birthplace of comets, should have trillions of these things — it was the size that made other astronomers pay attention.

Some of Brown's notable discoveries included Quaoar; Sedna; Haumea; Eris and its moon, Dysnomia; and Makemake. All were found in a relatively short period of time, between 2001 and 2005. Eris (which was originally nicknamed "Xena" after a popular television show of the time) was large enough that some in the media were calling it the 10th planet.

Stars made from galactic recycling material

Ordinary galaxies such as our own Milky Way contain a plethora of gas and dust. Nevertheless, there is not nearly enough matter to explain how galaxies produce new stars at the observed rates for long. As a solution, a matter cycle on gigantic scales has been proposed, for which concrete traces exist in our local galactic neighbourhood. Now, a study led by Kate Rubin of the Max Planck Institute for Astronomy has found the first direct evidence of such a key part of "galactic recycling" also in distant galaxies gas flowing back into distant galaxies.

Star formation regions, such as the Orion nebula, create some of the most beautiful astronomical sights. It is estimated that in our home galaxy, the Milky Way, on average one solar mass's worth of matter per year is turned into stars. Yet a survey of the available raw material, clouds of gas and dust, shows that, using only its own resources, our galaxy could not keep up this rate of star formation for longer than a couple of billion years.

Our own Milky Way, however, is significantly older than this and still active. Why is this the case? Is our home galaxy currently undergoing a rather special, short-lived era of star formation? Both stellar age determinations and comparison with other spiral galaxies show that not to be the case. One solar mass per year is a typical star formation rate, and the problem of insufficient raw matter appears to be universal as well. 

Evidently, additional matter finds its way into galaxies. One possibility is an inflow from huge low-density gas reservoirs filling the intergalactic voids; there is, however, very little evidence that this is happening. Another possibility, closer to home, involves a gigantic cosmic matter cycle. Gas is observed to flow away from many galaxies, and may be pushed by several different mechanisms, including violent supernova explosions (which are how massive stars end their lives), and the sheer pressure exerted by light emitted by bright stars on gas in their cosmic neighborhood.

As this gas drifts away, it is pulled back by the galaxy's gravity, and could re-enter the same galaxy in time scales of one to several billion years. This process might solve the mystery: the gas we find inside galaxies may only be about half of the raw material that ends up as fuel for star formation. Large amounts of gas are caught in transit, but will re-enter the galaxy in due time. Add up the galaxy's gas and the gas currently undergoing cosmic recycling, and there is a sufficient amount of raw matter to account for the observed rates of star formation.

There was, however, uncertainty about the viability of this proposal for cosmic recycling. Would such gas indeed fall back, or would it more likely reach the galaxy's escape velocity, flying ever further out into space, never to return? For local galaxies out to a few hundred million light-years in distance, there had indeed been studies showing evidence for inflows of previously-expelled gas. But what about more distant galaxies, where outflows are known to be much more powerful – would gravity still be sufficient to pull the gas back? If no, astronomers might have been forced to radically rethink their models for how star formation is fueled on galactic scales.

Now, a team of astronomers led by Kate Rubin (MPIA) has used the Keck I telescope on Mauna Kea, Hawai'i, to examine gas associated with a hundred galaxies at distances between 5 and 8 billion light-years (z ~ 0.5 – 1), finding, in six of those galaxies, the first direct evidence that gas adrift in intergalactic space does indeed flow back into star-forming galaxies.

As the observed rate of inflow might well depend on a galaxy's orientation relative to the observer, and as Rubin and her team can only measure average gas motion, the real proportion of galaxies with this kind of inflow is likely to be higher than the 6% directly suggested by their data, and could be as high as 40%. This is a key piece of the puzzle and important evidence that cosmic recycling ("galactic fountains") could indeed solve the mystery of the missing raw matter.

The Big Bang theory

The Big Bang Theory is the leading explanation about how the universe began. At its simplest, it talks about the universe as we know it starting with a small singularity, then inflating over the next 13.8 billion years to the cosmos that we know today.

Because current instruments don't allow astronomers to peer back at the universe's birth, much of what we understand about the Big Bang Theory comes from mathematical theory and models. Astronomers can, however, see the "echo" of the expansion through a phenomenon known as the cosmic microwave background.

The phrase "Big Bang Theory" has been popular among astrophysicists for decades, but it hit the mainstream in 2007 when a comedy show with the same name premiered on CBS. The show follows the home and academic life of several researchers (including an astrophysicist). 

In the first second after the universe began, the surrounding temperature was about 10 billion degrees Fahrenheit (5.5 billion Celsius), according to NASA. The cosmos contained a vast array of fundamental particles such as neutrons, electrons and protons. These decayed or combined as the universe got cooler.

This early soup would have been impossible to look at, because light could not carry inside of it. "The free electrons would have caused light (photons) to scatter the way sunlight scatters from the water droplets in clouds," NASA stated. Over time, however, the free electrons met up with nuclei and created neutral atoms. This allowed light to shine through about 380,000 years after the Big Bang.

This early light — sometimes called the "afterglow" of the Big Bang — is more properly known as the cosmic microwave background (CMB). It was first predicted by Ralph Alpher and other scientists in 1948, but was found only by accident almost 20 years later.

Arno Penzias and Robert Wilson, both of Bell Telephone Laboratories in Murray Hill, New Jersey, were building a radio receiver in 1965 and picking up higher-than-expected temperatures, according to NASA. At first, they thought the anomaly was due to pigeons and their dung, but even after cleaning up the mess and killing pigeons that tried to roost inside the antenna, the anomaly persisted.

Simultaneously, a Princeton University team (led by Robert Dicke) was trying to find evidence of the CMB, and realized that Penzias and Wilson had stumbled upon it. The teams each published papers in the Astrophysical Journal in 1965.

Determining the age of the universe

The cosmic microwave background has been observed on many missions. One of the most famous space-faring missions was NASA's Cosmic Background Explorer (COBE) satellite, which mapped the sky in the 1990s.

Several other missions have followed in COBE's footsteps, such as the BOOMERanG experiment (Balloon Observations of Millimetric Extragalactic Radiation and Geophysics), NASA's Wilkinson Microwave Anisotropy Probe (WMAP) and the European Space Agency's Planck satellite.

Planck's observations, released in 2013, mapped the background in unprecedented detail and revealed that the universe was older than previously thought: 13.82 billion years old, rather than 13.7 billion years old.

The maps give rise to new mysteries, however, such as why the Southern Hemisphere appears slightly redder (warmer) than the Northern Hemisphere. The Big Bang Theory says that the CMB would be mostly the same, no matter where you look.

Examining the CMB also gives astronomers clues as to the composition of the universe. Researchers think most of the cosmos is made up of matter and energy that cannot be "sensed" with conventional instruments, leading to the names dark matter and dark energy. Only 5 percent of the universe is made up of matter such as planets, stars and galaxies.

Our place in the universe

Recently, NASA scientists combined data from the Spitzer and Hubble Space Telescopes to discover the most distant galaxy known to date. The galaxy, named Abell2744 Y1, was formed around 13.2 billion years ago when the universe was extremely young. As the universe is expanding, Abell2744 Y1 is currently closer to 40 billion light years away from us, an astounding distance.

But what does that really mean?

Most of us have trouble visualizing the height of buildings, or the distance it takes to get home from work, let alone things on an intergalactic scale. 

This is far as anything on Earth can see. The age of the universe is 14 billion years, so in theory we are unable to observe anything further than 14 billion light-years away. Due to the expansion of space, these objects are now around 46 billion light years away — the limit of the observable universe.