Planets Outside the Solar System

For years, scientists were searching for a hypothetical planet that would lie just outside of the Solar System, a planet responsible for apparent discrepancies in the orbit of the gas giants such as Neptune and Uranus. Dubbed “Planet X”, the search ended in the 1990s when modern measurement solved the discrepancies and it was therefore determined there was no “Planet X.” There are, however, plenty of other planets far outside of our Solar System.

What is a Planet?

From the origin of the word in ancient Greece, based on the words for “wandering star” used to describe the points of light in the night sky that seemed to journey though the season, “planet” has been used to refer to Mercury, Venus, Earth, Mars and the outer planets. Since the discovery of a “tenth planet” near Pluto, however, the term has been under close scrutiny for a re-definition, one that ended up declassifying Pluto as a planet and renaming it a “dwarf planet.”

To be a planet the current nomenclature requires a body to have sufficient mass to have achieved a spherical shape, to have gained control of other objects in its orbit, and to orbit the Sun itself—in our Solar System. Moving outside our Solar System the scientific community reverts back to the loose, comfortable definition of a planet, which is, roughly, a large enough body in orbit around a star.

A Look at Planets Outside the Solar System

The current confusion over “what is a planet” does not stop at the boundaries of our Solar System. A star is considered a body large enough to sustain fusion of hydrogen, but dwarf stars have stopped sustained fusion; does this, then, make them planets? What of planets that have been ejected from their solar system by a decayed orbit- if a planetary requirement is to orbit a star, does this body stop being a planet? One thought on the matter is to say a planet must have originated as a body orbiting a star, but this could very well turn Neptune’s captured moon Triton into a planet.

There are 248 currently known Exosolar Planets, which is what planets outside our Solar System are called. While the theory of exoplanets has been around for a very long time, it was not until the 1990s that the first such planet was discovered, after which they are being found at a rate of around 20 every year.

With current technology actually seeing an exoplanet is a very rare event, as the brightness of the object is far, far less intense than the brightness of its parent star. To detect an exoplanet six methods are used; Astrometry, Radial Velocity, Pulsar timing, Transit method, Gravitational microlensing, and Circumstellar disks. All of these methods take what we understand about the motion of and composition of stars and measures variances and discrepancies that would be cased by a planet.

Final Thoughts

Most of the exoplanets discovered are thought to be gas giants, with very few terrestrial objects in the mix. Of those terrestrial exoplanets, only one comes close to being a likely candidate for supporting life, and is some 20 light years away.

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• ISS Assembly Sequence
• Jupiter
• Mars
• Neptune
• Planet Mercury
• Planet Venus
• Pluto
• Saturn
• Uranus